Testosterone’s Modulation of Muscular Strength

Testosterone’s Modulation of Muscular Strength

A Review

Abraham Campbell

A Major Research Paper for MSc Kinesiology Integrative Biosciences

December 8, 2021

Introduction

Testosterone (T) is generally associated with masculinizing characteristics such as sex drive, bone mass, muscle mass, body hair growth, fat distribution, and hematocrit levels (Leon et al., 2021). However, less commonly associated with testosterone is its role in emotion, social behavior, development of brain structures, and neural protection (Derntl at el., 2009; Filová et al., 2013; Montoya., 2012; Oki et al., 2015). Although, testosterone and its modulation of muscular strength has been debated in the academic community for over five decades (Bhasin et al., 2001). However, testosterone’s influence on muscular strength is unequivocal (Bhasin et al., 1996; Bhasin et al., 2001; Blazevich et al., 2001; Giorgi et al., 1999; Kvorning et al., 2006; Storer et al., 2003).  It is clearly demonstrated that testosterone deficiency results in low muscle mass and consequently decreases in functional strength (Grinspoon et al., 1996; Mauras et al., 1998). However, upon the restoration of testosterone to normal biological values (264 to 916 ng/dl or 9.16 nmol/l to 31.7 nmol/l) (Travison et al., 2017) subsequent losses in muscle mass and strength are restored in both the young and elderly male hypogonadal populations (Morley et al., 1993; Urban et al., 1995; Bhasin et al., 1997; Wang et al., 2000). However, the administration of supraphysiological exogenous testosterone (elevating serum levels of T > 916 ng/dl or 31.7 nmol/l) and its derivatives Anabolic-Androgenic Steroids (AAS) to increase muscle mass and strength has been widely criticized by the academic community citing the lack of evidence (Bhasin et al., 2001). This is despite AAS’ ubiquitous use among athletes for multiple decades dating as far back as to the 1936 Olympics (Bhasin et al., 2001; Yesalis et al., 2002). The lack of association between testosterone/AAS and muscular strength is mostly due to poor methodology, inadequate dosing protocols, inaccurate evaluative methods, and lack of control of confounding variables which resulted in inconclusive or contradictory research findings (Bhasin et al., 2001). Therefore, the purpose of this review is to examine the evidence concerning testosterone’s indispensable role in muscular strength adaptations and to elucidate the mechanistic link between testosterone and strength. This will be accomplished by examining and critically analyzing the current body of literature on this topic.

Testosterone’s Biosynthesis: A Brief Summary

Testosterone (T) is a 19-carbon-based steroid hormone derived from a series of pathways biosynthesized from cholesterol (León et al., 2021). Testosterone is also converted into dihydrotestosterone (DHT) via 5a-reductase in peripheral tissue, both androgens are responsible for the development and maintenance of primary sexual characteristics (external genitalia and spermatogenesis), and secondary androgenic characteristics (vocal changes, hair growth, and voice deepening) (Nassar & Leslie, 2018). Additionally, T and DHT are also responsible for the masculinizing effects of males during the onset of puberty (growth spurts, increases in muscle mass/bone density, and stimulation of erythropoiesis) (Nassar & Leslie, 2018). At the onset of puberty and throughout the adult male lifespan, testosterone is regulated by the hypothalamic-pituitary-gonadal-axis (HPGA) via negative feedback (Nassar & Leslie, 2018). Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus via the hypothalamohypophyseal portal system which stimulates the anterior pituitary to release luteinizing hormone (LH) and follicle stimulating hormone (FSH) into the bloodstream (Nassar & Leslie, 2018). Luteinizing hormone (LH) stimulates the Leydig cells of the testes to produce testosterone derived from cholesterol (Nassar & Leslie, 2018), whereas FSH and testosterone work in concert to regulate spermatogenesis maturation (Oduwole et al., 2018). Furthermore, most of testosterone is bound to plasma proteins; sex-hormone-binding-globulins (SHBG) and albumin. This permits small amounts of bioavailable free testosterone to be delivered to a variety of tissues and cells, while a greater amount of testosterone remains bound to plasma proteins as a surplus (Nassar & Leslie, 2018). A small amount of testosterone is also produced by the hypothalamic-pituitary-adrenal-axis (Nassar & Leslie, 2018).

Dysfunction can occur in the HPGA resulting in suppression of endogenous testosterone production, this is termed hypogonadism. Hypogonadism is characterized by inadequate levels of testosterone (< 264 to 916 ng/dL or 9.16 nmol/l to 31.7 nmol/l) (Travison et al., 2017). Testosterone suppression results in a variety of systemic issues corresponding to sexual health both; libido & fertility, cognitive deficits, metabolic disease/disorders, changes in vascular function, increases in fat mass, reductions of lean body mass, and other physiological abnormalities (Kumar et al., 2010). Mechanistically, hypogonadism can be attributed to primary dysfunction ie., testicular failure; the inability to produce testosterone from the testes and/or secondary dysfunction ie., an issue with the hypothalamus or pituitary signaling of LH or FSH to the testes to produce testosterone (Kumar et al., 2010). Both types of hypogonadism can be congenital, associated with age-related decline, and/or attributed to an injury or infection (Kumar et al., 2010). Therefore, Testosterone Replacement Therapy (TRT) is the primary treatment option for hypogonadism (Kumar et al., 2010). There are a variety of therapeutic routes of testosterone administration including gels, oral, nasal, and intramuscular injections (IM) (Shoskes et al., 2016). Intramuscular injections of testosterone have been used since the 1950s and are the most favorable route of administration (Shoskes et al., 2016). Testosterone Enanthate (TE) and Testosterone Cypionate (TC) are the most frequently used in therapeutic administration of T IMs (Shoskes et al., 2016). Whereas the former, TE is most commonly used in TRT, and appears across many research publications evaluating the effects of supraphysiological administration of testosterone on muscular strength (Blazevich et al., 2001; Bhasin et al., 1996, 2001; Giorgi et al., 1999; Storer et al., 2003). The differences between the IM preparations of TE and TC (which are similar) is the synthesis through the esterification of the 17β carbon of natural testosterone (Shoskes et al., 2016). This permits the solubility of testosterone in an oil-based suspension (usually sesame or cottonseed oil) and prolongs the duration of biological activity (Shoskes et al., 2016). Unesterified testosterone has an approximate half-life of 10 minutes whereas the TE and TC have a slower release after injection due to the longer esterified carbon side chain permitting a longer duration of action, only required to be injected once every 2-3 weeks in TRT (Nieschlag, 2006; Shoskes et al., 2016). Testosterone Propionate (TP) administration also appears in the testosterone and muscular strength literature (Blanco et al., 1996; Egner et al., 2013). However, its biological activity is very rapid due to the shorter esterified carbon chain and must be administered once every 2-3 days making TE or TC a better candidate for TRT (Meikle, et al. 2007). However, all IM preparations of testosterone elicit the same biological activity once the ester group is cleaved off (Shoskes et al., 2016). The rate of delivery is more rapid or prolonged dependent on ester formulation ie., TP vs. TE.

Testosterone’s Modulation of Cellular Activity: A Brief Review

Testosterone and DHT are mediated via a steroid hormone nuclear receptor known as the Androgen Receptor (AR) (Davey et al., 2016). These receptors are a ligand-dependent nuclear transcriptional factor found ubiquitously in cells and tissues that modulate a variety of biological functions associated with both development and maintenance of musculoskeletal, neural, cardiovascular, and reproductive systems among other body systems (Davey et al., 2016). Steroid hormones such as testosterone can regulate cell activity by two pathways; “the classical” ie., DNA binding-dependent actions or “non-classical” ie., non-DNA binding-dependent actions of the AR (Davey et al., 2016). In DNA binding dependent pathways, androgen binding causes conformational change of the androgen/AR complex which translocates to the nucleus where it interacts with Androgen Response Elements (AREs) in target genes which regulate transcription (Davey et al., 2016). The interaction of AR and protein co-regulator sub-types: co-activators and co-repressors, activate or suppress target genes (Bevan et al., 1999). The “non-classical” AR pathways are rapid, non-genomic, and thought to signal through a variety of secondary messenger systems, and intracellular signaling pathways (Davey et al., 2016; Kadi et al., 2008; Wehling et al., 1994). Additionally, ARs are also suggested to have a direct influence on ion gate channels (Wehling et al., 1994 as cited in Alonso-Solis et al., 1996). As a result, Alonso et al., (1996) proposed that non-genomic AR pathways also modulate neural properties. Androgen receptors are well distributed throughout the nervous system such as the spinal cord, limbic system, brain stem, and cerebellum (Sar et al., 1977). This is supported by Sar et al., (1977) research that demonstrated after the administration of DHT, a testosterone metabolite (with a high affinity for ARs) a high concentration was found in the nuclei of purkinje cells, motor neurons of the seventh nerve, neurons of the reticular formation, alpha motor neurons in lamina IX and the thoracic segment of the spinal cord. Sar et al., (1977) proposed that DHT must be implicated in motor systems because of the high concentration found in the lower brainstem and spinal cord. DHT is thought to be an activator of certain motor systems via transport through cerebrospinal fluid (Sar et al., 1977). However, there are a very limited number of studies in humans that examine the interaction between testosterone and motor systems (Oki et al., 2015). Although despite this limitation, some basic evidence suggests that testosterone can improve conduction velocity in peripheral nerve neuropathy (Izumi et al., 2000). Additional research has demonstrated a link between testosterone’s modulation of the threshold of corticospinal motor neurons in humans after intramuscular injections of human chorionic gonadotropin (hCG) (which elevates serum testosterone) (Bonifazi et al., 2004). This elevation of testosterone resulted in the lowering of the motor threshold indicating an increase in membrane excitability (Oki et al., 2015). Other research suggesting testosterone’s interaction with motor systems is demonstrated by the correlation between low levels of free and total testosterone and sarcopenia (skeletal muscle mass loss and function) in males (Szulc et al., 2004). This suggests some sort of connection between age related loss of motor units and testosterone resulting in sarcopenia and subsequent reductions in muscular strength (Drey et al., 2014; Shin et al., 2018). Overall, despite limited studies of testosterone’s modulation of motor systems some basic evidence suggests an interconnection.  

A Review of Testosterone and Strength: The History, Debate, and Controversy

Anabolic-Androgenic steroid use is well documented in strength sports such as weightlifting, powerlifting, throwing, and explosive-strength sports like football, sprinting, and other track events (Bhasin et al., 2001). In fact, the World Anti-Doping Agency cites AAS as the most commonly used performance enhancing drug with a 63% test positivity rate and the highest prevalence of use among all PEDs (WADA, 2013). Although, the association of strength and testosterone dates as far back as 1889 with documented reports of a reputable physiologist known as Charles Edouard Brown-Seguard who self-administered intramuscular injections of a “testicular fluid extract” (considered to be testosterone) derived from the testes of dogs and guinea pigs. Brown-Sequard noted his improvement in muscular strength, energy, and mental abilities (Freeman, et al., 2001; Yesalis et al. 2002). Although this was never substantiated, it facilitated a great deal of curiosity and further research (Freeman, et al., 2001). This newly discovered “extract” coined as an elixir, was endorsed, and marketed by over 12,000 physicians and chemists to treat a variety of illnesses (Freeman, et al., 2001). By 1935, the newly discovered hormone was termed, testosterone (testo = testes, ster = sterol, one = ketone) and synthesized in the same year (Freeman, et al., 2001). In 1948, initially testosterone was used medicinally as an anti-depressive agent (Altschule & Tillotson, 1948). This neuroendocrine integrative role was later verified as testosterone has been shown to interact with neurotransmitters (serotonin, dopamine, noradrenaline, vasopressin, and cortisol) which are also implicated in depression (Ebinger et al., 2009).

Although, in a performance context documentation of German athletes administering exogenous testosterone dates as far back as the 1936 Olympics in Berlin (Yesalis et al., 2002). However, by the 1950s the Soviet Union weightlifting teams’ dominance facilitated the widespread use of AAS and its association with strength (Yesalis et al., 2002). The soviet weightlifting coach disclosed the usage of exogenous testosterone among their athletes to American physician Dr. John Ziegler (Yesalis et al., 2002). Ziegler began to experiment with testosterone on himself and other weightlifters. Ziegler also worked alongside CIBA pharmaceuticals to develop a more anabolic agent and derivative of testosterone known as Dianabol (methandrostenolone) (Yesalis et al., 2002). Testosterone administration resulted in championship status among strength athletes working with Ziegler (Yesalis et al., 2002). Word spread rapidly about the effectiveness of AAS on the attributes of strength and muscle mass. By the 1960s, Anabolic-Androgenic steroids were widely adopted in strength and explosive strength sports, in addition to more aesthetic orientated sport such as bodybuilding (Yesalis et al., 2002). Although controversial, Wilson (1988) contended that the general population and athlete use of AAS is a result of misunderstanding and the misconception of the physiological mechanisms of action of AAS. Wilson (1988) states that even at normal physiological ranges of testosterone androgen receptors are saturated or downregulated limiting any anabolic actions of exogenous AAS. Wilson (1988) also asserted that use of AAS effects is purely psychological in nature. However, it is now postulated that supraphysiological doses of testosterone are likely to work independent of androgen receptor mediated pathways (Kadi et al., 2008). Although, a generally accepted mechanism is the increased rate of protein synthesis elicited via exogenous testosterone resulting in greater increases in muscle mass (Griggs et al., 1989, Kadi et al., 2008; Storer et al. 2003). Although, the functional consequences of increases in muscle mass remains unclear and how this exactly translates to athletic performance is unknown (Bhasin et al., 2001; Storer et al. 2003). Even in more recent times testosterone effects on athletic performance are disputed. A 2020 randomized double blind placebo-controlled study evaluated exogenous testosterone’s effect on strength and power (Solheim et al., 2020). Solheim et al., (2020) concluded that testosterone has no ergogenic benefit in recreational young males after an acute dose of a mixed esters (fast and slow acting) of testosterone (Sustanon 250) after a 24-hour period (Solheim et al., 2020). However, this result is likely due to insufficient increases in testosterone levels to facilitate any increases in strength and power due to the dosage, acute administration, and subsequent testing time period (24 hours) (Solheim et al., 2020). The pharmacokinetics of Sustanon 250 absorption parameters occur 40-60 hours after administration in the short acting esters and 72 hours in the longer acting esters (Kumar et al., 2010; National Center for Biotechnology Information, 2021). Therefore, serum testosterone levels may have failed to reach supraphysiological levels in some of the participants due to an already elevated level of basal testosterone which was present in some of the individuals (12.2–37.1 nmol/L) (Solheim et al., 2020). Furthermore, after exogenous administration of testosterone, serum T levels are then verified to evaluate an individual’s physiological response (Shoskes et al., 2016). However, interindividual variability in physiological response of exogenous T administration was not assessed (Solheim et al., 2020). These results are similar to Crist et al., (1983) study’s results which demonstrated 100 mg/wk of Testosterone Cypionate or Nandrolone Decanoate versus placebo for 3 weeks had no significant effect on increases in upper body strength or changes in body composition in participants. However, failure to reach supraphysiological levels of testosterone is likely the explanation for the lack of increases in strength output. An additional consideration is that AAS physiological strength adaptations take several weeks to months (Blazevich et al., 2001; Bhasin et al., 1996, 2001; Giorgi et al., 1999; Storer et al., 2003). Researchers wishing to examine acute responses of testosterone on muscular strength or power may consider the non-esterified versions of T to examine short-term strength adaptations. Non-esterified versions of testosterone would be more valid due to its extremely rapid pharmacological delivery (Shoskes et al., 2016). Nevertheless, evidence suggests that testosterone induced strength adaptations occur over a duration of time and perhaps non-acutely (Blazevich et al., 2001; Bhasin et al., 1996; 2001; Giorgi et al., 1999; Storer et al., 2003).

Although despite many inconclusive results, this contradicts decades of athlete’s anecdotal reports of AAS eliciting increases in muscle mass and subsequent strength resulting in a performance benefit (Bhasin et al., 2001). However, it wasn’t until the year 1996 that anecdotal reports were validated by the scientific community mainly because of AAS’ anabolic effects were measured by nitrogen balance which resulted in low specificity, sensitivity and lacked precision (Bardin, 1996). Other limitations cited by Bhasin et al., (2001) indicated poor methodological considerations such as a lack of standardized nutritional intake impacting nitrogen balance, inadequate training protocols, administration of therapeutic doses of testosterone, and in some studies participants would exercise at their own discretion leading to inconclusive results. Although, with advancements in technology, Magnetic Resonance Imaging (MRI) can detect changes of the muscle cross sectional area up to 6% (Bardin, 1996). Thus, permitting improvements in evaluative methods to examine the effects of exogenous testosterone on muscle physiology (Bardin, 1996; Bhasin et al., 2001). However, much of Testosterone’s effect on athletic performance remain debated particularly in endurance sport and many of the mechanisms are not completely understood (Bhasin et al., 1996; Bhasin et al., 2001; Storer et al. 2003; Kvorning et al., 2006; Solheim et al., 2020).

An additional area of concern and controversy is the health concerns associated with exogenous testosterone use both medicinally and recreationally (Hoffman & Ratamess, 2006). However, the cause-and-effect relationship between AAS with myocardial infarction, cancer, and suicide is inconclusive and lacking sufficient evidence (Hoffman & Ratamess, 2006). AAS’ correlation with adverse health outcomes is based on a variety of confounding variables such as the combination of multiple drug use simultaneously ie., thyroid hormone, diuretics, insulin, anti-estrogens, and recreational drugs making it difficult to isolate a cause-and-effect relationship (Hoffman & Ratamess, 2006). This is in combination with other extraneous variables such as dietary practice and genetic predisposition to disease states (Hoffman & Ratamess, 2006). The medical community asserts that long-term use of testosterone and its derivatives (AAS) is associated with numerous adverse health outcomes; cardiovascular; lipid changes, elevations in blood pressure, and decreases in myocardial function; endocrine risks; gynecomastia, fertility issues, testicular atrophy, and sexual dysfunction; musculoskeletal; increased risk of tendon tears and intramuscular abscesses; psychological issues; mania, depression, and aggression. Additionally, female AAS use can result in reproductive system abnormalities, menstrual irregularities, clitoromegaly, and systemic masculinization. Although, this is mostly attributed to chronic supraphysiological doses without doctor supervision (Hoffman & Ratamess, 2006). It is commonplace that athletes in physique and strength sports will often use multiple AAS compounds simultaneously and at excessive dosages (Bhasin et al., 2001). Although, the long-term consequences of the administration of one and/or several AAS compounds simultaneously at supraphysiological doses remain unclear (Hoffman, & Ratamess, 2006). However, acute supraphysiological dosages not exceeding 600 mg/wk seem to be well tolerated in male subjects (Bhasin et al., 1996; Bhasin et al., 2001; Giorgi et al., 1999; Storer et al., 2013). It is also well demonstrated that AAS have multiple applications in medicine for a variety of catabolic disease states, hormonal disorders/dysfunction, and age-related decline (Tauchen et al., 2021). For example, testosterone replacement therapy (TRT) can improve strength and physical function (ie., walk test) in elderly males (Bhasin et al., 2001; Nam et al., 2018).

Testosterone’s Modulation of Muscular Strength: The Evidence

Giorgi et al., (1999) examined the influence of testosterone enanthate’s effect on muscular strength, body composition, and health. Twenty-one previous weight trained males (at least 2 years lifting experience) were recruited for the study between the ages of 19 and 45. The experimental group (n=11) were administered intramuscular injections of testosterone enanthate (TE) (3.5 mg/kg) for a period of 12 weeks (double blinded). Both the experimental and control (n=10) (placebo injection) group participated in a full body resistance training (RT) program, 4 days of the week for the duration of 12 weeks. The experimental group was also required to continue to train for an additional 12 weeks after cessation of TE, and this was also a requirement for participants in the control group. In the outcomes of upper body strength (1RM of bench press) both TE and placebo showed improvements from baseline to post test at 12 weeks. However, the TE group showed significant improvement at both the midpoint testing (6 weeks: 14% vs. 7%) and post-test (12 weeks: 22% vs 9%) as compared to the non-TE and RT group. Although, after the cessation of TE (experimental group; week 12) at week 24 no differences were present between the groups (experimental versus control) in upper body strength. Giorgi et al., (1999) research demonstrated a “doubling” of upper body strength (1RM of the bench press) at the 6- and 12-week evaluation in participants after administration of TE. This demonstrated that TE (3.5mg/kg) in conjunction with RT can improve strength at twice the rate of RT alone. However, after cessation of TE from week 12 to the follow-up (week 24) all muscular size and strength gains were not maintained despite concurrent RT. It should be noted that TE (3.4 mg/kg) although a moderate dose, exceeds the normal testosterone replacement therapeutic range (~125 mg per week of TE generally used) (Borst et al., 2007). Similar dosages (>3.4 mg/kg) have been reported in the literature as being well tolerated (Bhasin et al., 1996). The participants experienced minimal adverse side effects except for minor increases in systolic blood pressure (average 10 mmHg) (Giorgi et al., 1999). Giorgi et al., (1999) did not examine nor propose any mechanistic determinants of exogenous testosterone and RT in relation to strength related outcomes. However, it is important to note TE’s “doubling” effect on the rate of strength as compared to RT alone. Therefore, testosterone must be a primary mediator of muscular strength related adaptations.

Testosterone’s influence on muscle fibers

In female mice, Egner et al., (2013) demonstrated similar findings as Giorgi et al., (1999) after administration of testosterone propionate (TP). Testosterone induced a 77% increase in fiber cross sectional area (CSA) as compared to control. The additive effect of testosterone and overload led to an 118% increase in muscle hypertrophy. Additionally, increases in muscle myonuclei (46 to 56 nuclei per millimeter) were reported by the researchers. A greater number of muscle nuclei have been associated with increases in hypertrophic growth of skeletal muscle (Bruusgaard et al., 2013). Similar to the results by Giorgi et al (1999) in humans, the researchers also reported reductions in muscle size to baseline levels as compared to the control group 3 weeks after cessation of testosterone. However, there were some distinct epigenetic changes within the muscle syncytium upon re-introduction of overload exercise after the 3-week cessation of testosterone. In the former TP administered group, muscle fiber diameters in the CSA increased by 42% as compared to control (21%) in 14 days. These results indicate a muscle “memory” and an enhanced rate of growth even after a brief exposure to exogenous testosterone. As a follow-up to the acute effects of TP exposure, Egner et al., (2013) investigated the long-term effect of prior testosterone administration after 3 months in the same female mice. The 3-month follow-up would constitute about ~12% of the mice’s lifespan (~10-year equivalency in humans). Despite no difference in CSA or fiber type, muscle nuclei remained 28% greater in the TP groups versus control. Within 6 days of the introduction of exercise overload CSA increased by 31% in the treatment group (TP) versus the control group (6%). After, 14 days CSA remained 20% greater than the control group. This study by Egner et al., (2013) demonstrated that even after cessation of exogenous testosterone, AAS users may have a greater capacity to increase muscle mass via resistance training than non-users, even in the absence of subsequent Anabolic-Androgenic Steroid use. Additionally, Egner et al., (2013) suggested that even acute use of testosterone may possibly have a permanent effect on muscle nuclei eliciting a greater capacity for increases in muscle hypertrophy. Maden-Wilkinson et al., (2020) demonstrated that increases in muscle mass are often accompanied by increases in muscular strength. Therefore, it maybe plausible for acute short-term use of testosterone to also impact long-term muscular strength adaptations in humans.

Testosterone’s Modulation of Architectural Adaptations

Blazevich et al., (2001) did a follow-up study to Giorgi et al., (1999) by examining changes in muscle architecture induced by TE and RT. It was documented that individuals using supraphysiological doses of Anabolic-Androgenic Steroids (AAS) like TE and perform heavy resistance training (high force and low velocity) gain muscle mass and strength more rapidly than with therapeutic doses of testosterone (Blazevich et al., 2001). Therefore, it is proposed that muscle architectural adaptations may explain some of the rapid increases in muscle strength and size associated with supraphysiological AAS and “heavy” RT. Blazevich et al., (2001) propose that changes in muscle architecture may be a primary mechanism as to the RT and TE strength induced increases reported by Giorgi et al., (1999) study. Ten male participants (age 22.4 +/- 3.8 years) from Giorgi et al., (1999) were selected to participate in a similar RT protocol for 12 weeks.  Five randomized participants were administered TE (3.5 mg/kg) over a 12-week duration while the control group received a saline injection. Primary outcomes included the upper body strength evaluation (1RM bench press) and ultrasound imaging of the participants’ triceps brachii was used to determine muscle architectural adaptations of TE in conjunction with RT from baseline. The researchers evaluated variation in muscle thickness, fascicle length, and pennation angle. Results indicated that participants in the TE (24.6+/- 6.3 mm to 31.8 +/- 4.8 mm) and non-TE (28.0 +/- 7.2 mm to 31.8 +/- 6.8 mm) group showed no significant differences in triceps brachii muscle thickness after 12 weeks despite the TE group showing significant improvements in 1RM of the bench press. However, TE subjects increased muscle pennation angles as compared to control (3.4° vs. 1.9°), a larger pennation angle is associated with high-force contractions and maximal force generating capacity (Aagaard et al., 2001; Van Eijden et al., 1997). Blazevich et al., (2001) propose that greater strength development in TE subjects is due to muscle pennation as both length and velocity of fiber shortening is less in musculature with greater pennation angle. Fibers would produce more force due to the length-tension relationship and force-velocity properties (Blazevich et al., 2001). A greater pennation angle results in more contractile tissue attachment to a specific area of the tendon (Kawakami et al., 1993). Muhl (1982) propose that a larger angle between the fascicles and tendon results in greater tendon excursion. This is due to pennate muscles not only shortening but also rotating upon contraction (Blazevich et al., 2001). It is now generally accepted that muscle hypertrophy elicits increases in pennation angle, although pennation angles above 45° cause a detrimental effect on muscle force (Kawakami, 2005). Thus, Blazevich et al., (2001) propose that it is by this mechanism that TE and RT influences the increases in strength. Additionally, Blazevich et al., (1999) propose that fascicle length has been associated with muscle contractile properties as muscles with longer fibers are recruited during high velocity contractions (Burkholder et al., 1994) and muscles that produce high-force contractions have shorter fibers (Van Eijden et al., 1997). Subjects administered TE had fascicle lengths that were estimated to be less than non-TE subjects. Blazevich et al., (2001) suggests that participants administered supraphysiological doses of TE in conjunction with high intensity RT become better adapted to produce high force muscular contractions. Therefore, supraphysiological doses of AAS in conjunction with high intensity RT have improved adaptations in muscle architecture that are more conducive to the development of high-force output than non-TE users (Blazevich et al., 2001). However, architectural changes induced via TE and high intensity RT remain unclear as increases in pennation angle are generally associated with hypertrophied musculature (Blazevich et al., 2001; Kawakami et al., 1993). The Blazevich et al., (2001) study reported no significant differences in muscle thickness in TE and non-TE users. Thus, Blazevich et al., (2001) propose that alternate factors must modulate pennation angle such as TE induced retention of fluid and sodium into the muscle resulting in enlargement of muscle fibers, and the subsequent increases in muscle pennation angle. Additionally, TE participants also increased the training loads more rapidly than non-TE trainees therefore increases in strength may be the result of adaptation to increased loading parameters, enhanced recovery, adaptation in the nervous system, and other factors such as hypertrophy of the synergists of the bench press (ie., pectoralis major/minor and deltoids) (Blazevich et al., 2001). Blazevich et al., (2001) concluded that it is unclear from their study which event preceded the other (ie., changes in muscle thickness, pennation, and increases in strength). However, results would suggest that muscular architectural changes (ie., reduced fascicle length; shorter fibers, and larger pennation) in TE versus non-TE trainees in combination with high intensity RT account for the differences in force production and strength outcomes in participants (Blazevich et al., 2001).

Storer et al., (2003) study examined the effect of TE dose dependency on muscular strength in 54 healthy eugonadal males (ages 18-35 years). Participants were randomized into 5 separate groups administered 25, 50, 125, 300 or 600 mg of testosterone weekly for 20 weeks (with a 16-week recovery period). Additionally, all participants received a GnRH agonist to suppress endogenous testosterone. This would control for testosterone plasma levels by negating any endogenous contribution. Subjects had previous RT experience; however, they were not currently participating in any RT during the treatment or control phase of the study. Participants were asked to refrain from all RT or any endurance related exercise during the duration of the study (Storer et al., 2003). Researchers assessed maximal voluntary muscle strength (1RM of the leg press), muscle fatiguability (75% of 1RM of the leg press until total number of repetitions to failure), muscle power (leg power instrument to measure peak power (watts)), specific tension (ratio of maximal voluntary strength in leg press to muscle’s cross-sectional area derived from magnetic resonance imaging (MRI)). Secondary outcomes included hormone assays: serum testosterone, free testosterone, and serum IGF-1. Results indicated that maximal voluntary strength increases of the leg press (1RM) were dose dependent and significantly correlated with free (p=0.0005) and serum testosterone (0.0006) independent of IGF-1 concentrations (Storer et al., 2003). Groups administered 300 and 600 mg/wk of testosterone had the greatest changes in leg press strength from baseline, however there were no significant differences between these groups (300 mg vs 600 mg). Baseline measures of strength in the leg press demonstrated significant change in the 50, 300, 600 mg/wk groups while the 25 and 125 mg/wk showed no significant changes from baseline. Changes in muscle fatiguability were not correlated with serum total (p= 0.17) or free testosterone (p= 0.35) nor IGF-1 (p= 0.35). Additionally, there were no correlations between specific tension and testosterone, free or total serum concentrations or a dose dependency effect. Overall, results indicate that testosterone has very specific characteristics that influence muscular strength and this effect also has a dose-dependency, whereas greater doses elicit greater increases in strength up to a biological plateau (Storer et al., 2003).

Storer et al., (2003) research indicates that improvements in maximal strength (1 RM) are proportional to increases in muscle mass (MRI-determined) elicited by testosterone. Although, RT and testosterone are likely to share some similar pathways modulating increases in muscular strength, there are also distinct separate pathways (Storer et al., 2003). For example, the researchers stated that RT improves specific tension of muscle, while testosterone only improves maximal voluntary contraction (Storer et al., 2003). Resistance training induced improvements in muscular specific tension was demonstrated by Welle et al., (1996) study which showed significant increases in muscle specific tension (38% and 32%) after a 3 month of resistance training in both young (ages=22-31 years) and older (ages=62-72 years) subjects. This contrasts with Storer et al., (2003) research demonstrating that testosterone administration did not affect specific tension. However independent of RT, testosterone induces increases in maximal strength (1 RM) thus demonstrating testosterone’s direct influence, selectivity of strength attributes, and modulation of maximal strength.

Storer et al. (2003) speculate on the mechanisms in which testosterone induced increases in muscle mass and subsequent strength increases independent of RT. Although, much remains to be determined on exactly how testosterone elicits its anabolic effects on muscle function (Storer et al., 2003). The researchers suggest testosterone is likely to influence muscle growth regulators like myostatin and IGF-1. Wu et al., (2010) suggest that hypertrophy induced IGF-1 expression is mediated by Erk and mammalian target of rapamycin (mTOR) signaling. Storer et al., (2003) also suggest other AR independent pathways elicit increases in muscle mass, postulating an anti-glucocorticoid effect that results in enhanced rates of protein synthesis. This seems plausible as Saartok et al., (1984) suggested that androgen receptors can be saturated even at low ranges of testosterone concentrations. Therefore, supraphysiological doses of testosterone may operate on a separate mechanism independent of AR mediation (Storer et al., 2003). The AAS antiglucocorticoid effect is supported by Hickson et al., (1990) research which shows that AAS work oppositional to glucocorticoids and enhance protein synthesis. This promotes full body skeletal muscle growth suggesting an additional potential hypertrophic mechanism (Hickson et al., 1990). Interestingly, as a closing statement; Storer et al., (2003) also note testosterone’s modulatory role in neuromuscular transmission. However, this was not examined.

Testosterone’s Modulation of Neural Factors

Testosterone’s effect on neuromuscular transmission is supported by Blanco et al., (1996) whose research demonstrated testosterone’s modulation of neurotransmission is elicited by the alteration of choline acetyltransferase (ChAT) messenger ribonucleic acid (mRNA). This alteration in ChAT mRNA levels was demonstrated in male rats where chronic elevation of serum testosterone via exogenous testosterone propionate (TP) administration for 28 days resulted in increases in motorneuron ChAT mRNA levels in the spinal nucleus of the bulbocavernosus (SNB), the retrodorsal lateral nucleus (RDLN) of the lumbar spinal cord, and the lateral columns of the cervical and lumbar spinal cord. This demonstrated testosterone’s modulation of motorneuron ChAt mRNA levels in motor columns. Specifically, ChAT gene expression in motorneurons is thought to be directly regulated by testosterone through androgen receptor activation by ChAT gene enhancer regions that contain androgen response elements (AREs) (Beato et al., 1987). Consequently, testosterone’s modulation of ChAT mRNA may influence the properties of neurotransmission. The primary source of ChAT in skeletal muscle is at the presynaptic terminal of the neuromuscular junction, increased ChAT induced by TP would provide an increased availability of acetylcholine (ACh) for release at the neuromuscular junction (Blanco et al., 1996). The implication of increased ACh availability may mitigate neurotransmission failure and consequently muscle fatigue. Neurotransmission failure is due to diminished neurotransmitter release, failure of axonal action potential propagation, and reduction of endplate excitability (Blanco et al., 1996). However, there may be a motor unit dependent differences in neurotransmission failure as demonstrated by Eggington et al., (1986) who demonstrated AAS increased fatigue resistance in the fast-twitch muscle of the extensor digitorum longus. Blanco et al., (1996) state that TP treatment may mitigate neurotransmission failure in musculature innervated by a larger proportion of fast-fatigable (FF) motor units (MUs). Potvin et al., (2017) demonstrated that there is a close association between force generating capacity and fatiguability of high threshold FF MUs. The rapid decrement in maximal force in the FF MUs at 100% of maximal voluntary contraction is due to a greater fatiguability of these MUs resulting in total overall drop in muscle force capacity (Potvin et al., 2017). Larger more powerful motor units are recruited when high force or power outputs are required according to Henneman’s size principle (Haff et al., 2001). Therefore, it would seem plausible that TP induced increases in fatigue resistance of FF MUs, and the attenuation of reduction of muscle force generating capacity may potentially contribute to the increases in maximal strength (1RM) as seen in participants administered supraphysiological exogenous testosterone (Blazevich et al., 1999; Giorgi et al., 1999; Storer et al., 2003).

Strength adaptations are also dependent on endogenous testosterone whereas low serum testosterone (2.0 +/- 0.5 and 1.1 +/- 0.6 nmol/l) attenuates increases in muscular strength (Kvorning et al., 2006). This was demonstrated in a double-blinded intervention study of 22 male participants, subjects were randomized to a goserelin group (n=12) (age= 25 +/-5 years) or placebo (saline) (n=10) (age=23+/-2 years) (Kvorning et al., 2006). The goserelin group received 3.6 mg of the GnRH agonist every four weeks resulting in reductions of testicular production of testosterone via inhibition of LH (Kvorning et al., 2006). Participants completed an 8-week full body strength training program for a total of 24 training sessions. Results indicated that the control group (non-goserelin group) improved isometric strength significantly after 8 weeks of RT (240.2 +/- 41.3 to 264.1+/-35.3 Nm) (P<0.05), however no significant changes occurred in the goserelin group. Interestingly, lean body mass increased significantly in both groups (placebo= 56.8 +/- 5.7 to 58.1 +/- kg) and (goserelin 57.5 +/- 5.7 to 59.8 +/- 6.3 kg) (both P<0.05). However, lean leg mass increased significantly more in the control group versus goserelin. Fat mass increased significantly in the goserelin group (15.6+/- 8.4 to 17.0+/-8.3kg) (P<0.05) while the placebo group had significant reductions in fat mass (16.8 +/- 5.7 to 16.2 +/- 5.4 kg) (P<0.05). Interestingly, GH remained stable in both groups throughout the intervention, while all other hormones remained stable in the placebo group (Kvorning et al., 2006). Overall, results demonstrated that suppression of both free and total testosterone resulted in attenuation of strength training adaptations (Kvorning et al., 2006). Although, the results of the study are particularly interesting as the goserelin group maintained similar RT load progressions as the control group. However, this did not result in the same increases in strength by laboratory testing procedures as the control group. As a plausible explanation; Kvorning et al., (2006) suggested that testosterone likely elicits its action on neural tissue through different mechanisms independent of direct anabolic effects via increased rates of protein synthesis resulting in increases in lean body mass. This is supported by Alonso-Solís et al., (1996) who proposed that steroid hormones such as testosterone modulate various neuroendocrine related functions; including neurotransmitter synthesis and release, modulation of post-synaptic receptors, remodeling of synapses, and through molecular mechanisms steroid hormones participate in co-ordination of behavior. Although, neuroendocrine integration and modulation of testosterone on neural tissue needs to be investigated further; Kvorning et al., (2006) suggest that any variations in steroid hormones levels like testosterone are likely to impair neural function thus impacting muscular strength. However, despite low levels of endogenous testosterone in the goserelin group; similar training loads were used in both groups. Kvorning et al., (2006) attribute this to exercise specificity whereas load progression improvements were likely due to the co-ordination of muscular synergists indicating some degree of neural adaptions of strength. Other contributing factors independent of testosterone may also explain the similarities in load progression such as growth regulators; myogenin (myogenesis), myostatin (growth and differentiation), and myoD (muscle differentiation) (Kvorning et al., 2006). Additionally, in both groups GH levels remained stable, GH causes downstream release of IGF-1 from the liver, increasing rates of protein synthesis however GH nor IGF-1 does not seem to translate into increases in muscle mass or strength but may offer some anti-catabolic benefit (Velloso et al., 2008). Overall, Kvorning et al., (2006) research demonstrates the indispensable role of testosterone in muscular strength training adaptations.

The Testosterone Dichotomy

However cautioned AAS represents a dichotomy whereas upon discontinuation of AAS, users may experience significant physiological and psychological distress due to supraphysiological doses of androgen levels causing feedback inhibition of the hypothalamic-pituitary-gonadal-axis (HPGA) (Rahnema et al., 2014). Consequently, this results in AAS induced hypogonadism symptoms following cessation of supraphysiological AAS administration (Kanayama et al., 2015). Hypogonadism is characterized by major depressive disorder, suicidal ideation/attempts, decreased libido, decreases in lean body mass, and increases in fat mass (Kanayama et al., 2015; Rivas et al., 2014). Former AAS users may self-medicate to mitigate hypogonadal symptoms creating a perpetual cycle of dependency on AAS (Kanayama et al., 2015). Both therapeutic or AAS users wishing to completely discontinue AAS will self-administer or be prescribed one or several hormonal pharmacological modulators such as: human chorionic gonadotropin (hCG), selective estrogen receptor modulators (SERMs), and/or other aromatase inhibitors (anastrozole and letrozole) to combat withdrawal symptoms and aid in the restoration/stimulation of endogenous serum testosterone levels (Rahnema et al., 2014). However, some sources state that restoration of HPGA function and serum testosterone levels will gradually be restored upon cessation of AAS in the duration of weeks, months and up to a year without additional treatment (Kanayama et al., 2015). Other research states that reversal of testicular function may take up to 6 to 18 months even with treatment (HCG), while some former AAS users will be prescribed TRT indefinitely (Shankara-Narayana et al., 2020). Even with hormone augmentation (HCG, TRT, GnRH agonists ie., triptorelin) and the restoration of testosterone levels permanent side-effects remain present in many former AAS users suggesting Leydig cell damage and down-regulation of AR and/or AR signalling mechanisms (Kanayama et al., 2015). Although, the exact mechanisms of permanent AAS induced hypogonadism symptoms and dysfunction are not well understood (Kanayama et al., 2015).  The extent and duration of the suppression of HPGA is AAS dose-dependent, based upon duration of use, and other interindividual variability factors (Kanayama et al., 2015).

The Future of Testosterone Augmentation

Research efforts to synthesize chemical analogs, testosterone pro-hormones, “Designer Steroids,” Selective Androgen Receptor Modulators (SARMS), and the use of plant extracts that facilitate increases in endogenous testosterone, mimic exogenous testosterone/AAS and/or are involved in testosterone biosynthesis and/or signalling have become the new frontier in testosterone augmentation (Tauchen et al., 2021). The ideology behind many of these research chemicals is to provide a safer, effective, and more selective means of elevations or restoration of normal values of biological testosterone levels while mitigating any negative side-effects or total HPGA suppression for both clinical and performance-based applications (Tauchen et al., 2021). However, any elevations in serum testosterone also leads to increases in muscle strength and attributes of muscle performance (Bhasin et al., 2001). Therefore, any chemical analog that mimics AAS positive effects (ie., increases in strength) is likely to be widely adopted and abused amongst the athletic population in a performance context; as is the case with Selective Androgen Receptor Modulators (SARMS) (Vasilev et al., 2021). SARMS are a relatively new class of molecules that work similarly to their AAS counterparts without the alleged negative potential androgenic side-effects of the heart, liver, hematocrit, reproductive organs, masculinization (in females), and AAS associated psychological disorders (Vasilev et al., 2021). SARMS can be divided into two classes: steroidal and non-steroidal. The steroidal SARMS bind to ARs causing conformational changes followed by the formation of the AR receptor complex which then migrates to the cell nucleus whereas AR will cause changes in transcriptional activity via interactions with two different protein co-regulators subtypes; co-activators or co-suppressors, which essentially activate or suppress target genes (Vasilev et al., 2021). This is similar to AAS mechanisms; however, SARMS and AAS interact with different co-regulators resulting in alternate variations of the activation or suppression of target genes (Vasilev et al., 2021). Non-steroidal SARMS elicits their effects on a non-genomic level via AR binding and the phosphorylation of a variety of kinase pathways; MEK, ERK, p38 MAPK among others (Vasilev et al., 2021). SARMS are hypothesized to not interact with the enzymes; 5-a reductase or aromatase unlike AAS which are thought to explain some of their tissue selectivity (Vasilev et al., 2021). The mechanistic action of both signalling pathways; non-steroidal or steroidal SARMS minimizes androgenic effects while eliciting maximal anabolic selectivity at the level of muscle and bone (Tauchen et al., 2021; Vasilev et al., 2021). However, exactly how SARMS selectivity elicits its effect on muscle and bone is not completely understood (Tauchen et al., 2021; Vasilev et al., 2021). Despite SARMS’ high anabolic activity, SARMS still obtains minimal androgenic properties as these components are extremely difficult to remove completely (Tauchen et al., 2021). Although, widely used and available on the black market, no SARMS analogs have been approved for therapeutic use (Tauchen et al., 2021). Additionally, both the safety and long-term side effects is unknown in humans (Tauchen et al., 2021; Vasilev et al., 2021).

Only one SARM; GTx-024 has reached adequate levels of safety in human clinical trials (Dalton et al., 2012). In a double-blind placebo-controlled study consisting of 120 healthy elderly men and post-menopausal women (mean age 63.3 +/- 5.6 years) participants were randomized into one of five different groups and would receive 0.1, 0.3, 1, and 3 mg of GTx-024 SARM or placebo. SARMS; GTx-024 treatment groups resulted in significant reductions in fat mass, increases in lean body mass, and improvement in physical function (power). These results are typical to observations in TRT or AAS hypogonadal therapies however, GTx-024 did not result in androgenic side-effects and no significant adverse events were reported (Dalton et al., 2012). Dalton et al., (2012) research demonstrated that the SARM; GTx-024 is well tolerated in the elderly, seemingly safe, and obtains specific anabolic selectivity in the absence of androgenic side effects. Thus, mimicking the favorable properties of AAS while mitigating adverse side-effects. Interestingly, Dalton et al., (2012) reported serum free testosterone levels remained unaltered at any dosage of GTx-024 in both men and women; perhaps indicating only incomplete endogenous testosterone suppression. However, total testosterone and SHBG were reduced in males (-61%) and females (-80%) at 3 mg of GTX-024 more profoundly than 600 mg of intramuscular TE induced reductions of total testosterone (TT) and SHBG (-31%) (Dalton et al., 2012). Other research examining SARMS analog “S-4” (10 mg/kg for 12 weeks) effects on castrated Sprague-Dewey rats demonstrated improvements in both muscle-strength, bone density and body composition this was similar to the DHT (3mg/kg) treatment group (Gao et al., 2005). However, results indicated that SARMS; S-4 stimulated prostate and seminal vesicle weight 2-fold less as compared to DHT, this demonstrates S-4 specific tissue selectivity targeting skeletal muscle and bone while mitigating negative aspects of androgenic activity and tissue interaction (Gao et al., 2005). Although, current research on the 1st generations of SARMS suggest that they are nowhere near as effective of supraphysiological dosages of testosterone on muscle mass and presumably strength (Bhasin & Jasuja, 2009).  However, Bhasin & Jasuja (2009) suggest that more research is needed both to further understand molecular mechanism of action and develop enhanced SARMS with improved properties of both potency and selectivity.

Research investigating a Malaysian shrub’s active extract, Eurycoma longifolia, or commonly know as Tongkat Ali (TA) have demonstrated increases in endogenous testosterone levels, muscle strength, lean body mass, and improvements in body composition (Chan et al., 2021; Hamzah, S., & Yusof; 2003 Henkel et al., 2014). Henken et al., (2014) TA research in physically active elderly males (n=13) and females (n=12) aged 52-72 resulted in increases in muscle force in both males and females determined by handgrip testing after 5 weeks of administration of 400 mg of TA orally. Although, the improvements in grip strength were greater in male subjects than female (Henkel et al., 2014). TA increased free and total testosterone by 48.6% and 122% (still within physiological norms) in males and females after the 5-week trial. Henkel et al., (2014) suggest TA is both a safe and effective supplement to augment seniors exercise performance through enhancement of muscular strength. The researchers also propose that TA elicits its elevation in total and free testosterone via serum reductions of SHBG (Henkel et al., 2014). The improvement of muscular strength has also been demonstrated in young male participants engaging in a RT program (4x RT/week) while supplementing with TA (n=7; 100 mg/day for 5 weeks) as compared to placebo (n=7). The TA group displayed significant improvements in 1RM of RT exercises, increases in lean body mass, and reduction of fat mass (Hamzah & Yusof, 2003). Although, the researchers did not examine TA mechanisms; Ulbricht et al., (2013) suggests that TA elicits elevations in testosterone via stimulation of the Leydig cells of the testes and frees bound testosterone. However, Chan et al., (2021) research investigating the mechanisms of TA’s modulation of testosterone propose an alternate mechanism of action. In a double-blind placebo-controlled study of thirty-two males (24.4 +/- 4.7 years) subjects received 600 mg/day of TA or placebo for the duration of 2 weeks. Interestingly, total, and free testosterone increased while there was no difference in LH, FSH or SHBG. This suggested that TA likely increases testosterone via the hypothalamic-pituitary-adrenal-axis versus HPGA (Chan et al., 2021). The authors suggest that TA induced elevations of testosterone could be beneficial to younger individuals seeking to augment increases in both muscular strength and size (Chan et al., 2021). Although TA appears to be well tolerated and possibly safe with few adverse events reported, there are still many unknown variables with TA dosing, duration, and long-term use (Ulbricht et al., 2013). Although TA is unlikely to produce AAS like effects, TA can be implemented as a “restorer” or help to maintain normal ranges of testosterone levels during times of stress, age-related decline, and overtraining (Talbot et al., 2013).

Conclusion

Summary

Testosterone’s modulation of muscular strength is unequivocal (Bhasin et al., 1996; Blazevich et al., 2001; Giorgi et al., 1999; Kvorning et al., 2006; Storer et al., 2003). Acute supraphysiological dosages not exceeding 600 mg/wk seem to be well tolerated in male subjects (Bhasin et al., 1996; Bhasin et al., 2001; Giorgi et al., 1999; Storer et al., 2013). However, Supraphysiological (>300 mg of T/wk) doses of testosterone demonstrated a “doubling of strength” when combined with RT (Blazevich et al., 2001; Giorgi et al., 1999), testosterone exhibits a linear dose dependency whereas greater doses result in more profound effects in muscular strength (Storer et al., 2003). It is postulated that acute exposure to supraphysiological doses of testosterone cause profound changes in the muscle syncytium resulting in an increased number of myonuclei and subsequent increases in muscle hypertrophy for up to ~10 years upon RT re-introduction (Egner et al., 2013). Even in the absence of RT, testosterone can improve maximal strength (1 RM) indicating its very selective and specific properties (Storer et al., 2003). The attenuation of endogenous testosterone production results in maladaptation to strength training, this is accompanied by increases in fat mass and reduction in lean body mass (Kvorning et al., 2006). Although, the mechanisms of testosterone’s modulation of strength are not completely understood (Bhasin et al., 2001). Evidence suggests that adaptations in muscle architecture, enhanced rates of protein synthesis/muscle hypertrophy, influence on neuromuscular properties and transmission rates, direct AR modulation of growth regulators myostatin, myoD and myogenin, or AR independent pathways (anti-glucocorticoid effect), or secondary downstream signaling cascades of anabolic hormones ie., GH, IGF-1, and other growth factors are likely determinants (Alonso-Solís et al., 1996; Bhasin et al., 1996; Blanco et al., 1996; Blazevich et al., 2001; Kvorning et al., 2006; Storer et al., 2003). Although, testosterone and its derivatives (AAS) are highly effective at increasing muscular strength, long-term supraphysiological use has been associated with a variety of adverse health outcomes (Kanayama et al., 2015; Shankara-Narayana et al., 2020). However, the link between AAS and adverse health outcomes remain correlational and lack sufficient evidence to definitively conclude a cause-and-effect relationship (Hoffman & Ratamess, 2006). Although, the suppression of the HPGA is a well-established consequence of AAS use (Kanayama et al., 2015; Shankara-Narayana et al., 2020). However, the extent and duration of suppression is based upon a variety of variables such as age, dose, duration of use, and genetic determinants (Hoffman & Ratamess, 2006). Therefore, research efforts have been directed at optimizing analogs with favorable AAS properties while mitigating the negative characteristics such as SARMs (Vasilev et al., 2021). Other more “natural” extracts such as Tongkat Ali have demonstrated effectiveness of augmenting testosterone levels, but much remains unknown (Chan et al., 2021; Ulbricht et al., 2013).

Limitations

Testosterone use has been documented since the 1936 Olympics and observational correlations with muscular strength as early as 1889 (Yesalis et al., 2002). Many athletes have been using AAS for decades reporting its undeniable effects on muscle mass and strength (Bhasin et al., 2001; Yesalis et al., 2002). Although some AAS/testosterone research has demonstrated ambiguous results of its mediation of muscular strength (Bhasin et al., 2001; Crist et al., 1983; Wilson, 1988; Solheim et al., 2020). Most of the equivocal research findings have been due to either inadequate dosing protocols and/or misinterpretation of the pharmacokinetic biological actions of the various forms of testosterone esters. For example, administration of exogenous testosterone/AAS was not sufficient to raise serum levels of testosterone to supraphysiological levels, and doses were more consistent with Testosterone Replacement Therapy (Bhasin et al., 2001; Crist et al., 1983; Solheim et al., 2020). Additionally, some studies did not allow for an adequate duration of time for testosterone’s physiological adaptations to occur (Solheim et al., 2020; Crist et al., 1983). Other research lacked insufficient control of variables such as dietary protocols, training programs, and other methodological considerations like non-randomization and crude evaluative assessment tools providing unreliable results (Bardin, 1996; Bhasin et al., 2001).

Furthermore, significantly greater improvements in maximal strength (1RM) and/or inconclusive research findings may have been ameliorated with a more appropriate selection of resistance training parameters. For example, strength training is often used as an “umbrella” term for resistance training neglecting the distinct physiological stimulus and adaptations that must occur for improvements in maximal strength (1RM). Muscular hypertrophy training does not constitute muscular strength training (although it is an attribute of strength) in fact distinct training parameters must be implemented for improvements in 1RM to occur optimally. This is supported by Morrissey et al., (1995) research that demonstrated participants training at 3-5 repetition ranges versus 9-11 repetition range resulted in significantly better outcomes in 1RM. Many of the researchers implemented hypertrophy-based resistance training parameters and not muscular strength when investigating the testosterone and muscular strength relationship (Solheim et al., 2020; Crist et al., 1983; Giorgi et al., 1999; Blazevich et al., 2001; Kvorning et al., 2006). Research must implement specificity into training protocols to ascertain conclusions. For example, appropriate strength training (1-5 reps) must be prescribed when evaluating absolute or maximal strength (1 RM) and not hypertrophy training-based parameters.

Additionally, studies reported participants strength and muscle size reverted to baseline levels) upon cessation of AAS (Giorgi et al., 1999; Egner et al., 2013). However due to negative feedback on HPGA, serum testosterone would be supressed for several weeks to months placing subjects in a hypogonadal state whereas reductions in muscle size and strength would occur regardless of AAS use (Kanayama et al., 2015). However, it is unlikely that reductions in muscle mass and strength would have been so profound with proper pharmacological intervention of HCG and other ancillary drugs to stimulate endogenous testosterone production and control other hormonal fluctuations (Rahnema et al., 2014). This “post-cycle therapy” was not implemented in any of the studies upon cessation of AAS (Giorgi et al., 1999; Blazevich et al., 2001; Kvorning et al., 2006; Storer et al., 2003) Additionally, it is unclear how the “doubling of strength” and the retention or loss of strength and muscle mass would occur in well trained, experienced (10+ years of resistance training), and elite level athletes versus beginner trainees (Giorgi et al., 1999; Kvorning et al., 2006). It is also uncertain how these results can be extrapolated to larger more diverse populations rather than young to middle aged males. Additionally, very little research has been completed in AAS and females although they are prone to masculinizing irreversible side-effects (Havnes et al., 2021). However, examining the literature on testosterone’s profound effect on muscle physiology, architectural adaptations, and neural properties supraphysiological doses of AAS are likely to induce drastic positive alterations in strength regardless of age and training experience.

Future Directions    

Future research may potentially examine how to isolate and accentuate the favorable properties of AAS and mitigate the negative side-effects for the health, safety, and longevity of individuals to effectively increase muscle mass and strength. Future work may also investigate testosterone’s neuromodulatory role in muscular strength as there is limited research in this area in humans. However, testosterone’s role in human physiology far extends past the gym, strength, and athletic performance. In neurodegenerative disease states, testosterone has been shown to modulate both neuroprotection and neurodegenerative properties of motor neurons (Oki et al., 2015). There is also correlational evidence linking Amyotrophic Lateral Sclerosis disease to low levels of testosterone (Sane et al., 2019). For example, testosterone has been demonstrated to stimulate the formation of new myelin and has been reported to reverse myelin damage in brain lesions (Hussain et al., 2013). In Parkinson’s disease testosterone has been shown to improve fine motor skills (Mitchell et al., 2006). Additionally, it is postulated that age-related decline in testosterone and motor unit loss are related which results in sarcopenia and subsequent reductions in muscular strength (Drey et al., 2014; Shin et al., 2018). Therefore, testosterone may have potential therapeutic efficacy in the treatment of sarcopenia (Shin et al., 2018). Furthermore, low levels of testosterone are also implicated in atherosclerosis, coronary artery disease, and cardiac events (Kloner et al., 2016).  In mental health disorders, testosterone administration has been shown to improve depression symptoms as testosterone interacts with a variety of neurotransmitters also implicated in depression (Khera et al., 2013; Ebinger et al., 2009). Clearly the implications of testosterone research are broad and numerous far extending to many facets of life. It is evident that testosterone is indispensable in the development of muscular strength and a variety of systemic biological health promoting processes. Perhaps an often-misunderstood hormone, testosterone complexity is clearly implicated in the optimization of human health. Testosterone is an integral hormone with a broad range of yet undiscovered modulatory properties in human physiology. 

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