Written by Robbie J. Durand, MA
11 September 2006

Bodybuilding Science

By Robbie J. Durand, MA


High-intensity Exercise Increases GH & Testosterone


"I can assure you that if I was not on testosterone, I would not be what I am today. I was a great athlete as a kid, but now most of the day I'm forced to be sedentary. I take testosterone to maintain my muscle strength, which is something I could not do at my age with exercise alone. My energy level is up, my musculature is up, and my fat is down because of testosterone. I feel the same way sexually as I did in my 40s. Maybe I should accept the fact that at age 73, things change and I should just let it happen, but I don't want to and with testosterone I don't have to." -Dr. Norman Orentreich, Pioneering Researcher on Aging


Samson was a legendary warrior and is something of a Herculean figure, using massive strength to combat his enemies and perform heroic feats unachievable by ordinary men: wrestling a lion, slaying an entire army with nothing more than a donkey's jawbone and tearing down an entire building. Samson would have definitely been a hardcore reader of MD with feats of strength like that.

Eventually Samson tells his wife Delilah that he'll lose his strength at the loss of his hair. Delilah (who's not exactly on Sampson's side) calls for a servant to shave Samson's head as he sleeps. Sampson wakes up an ordinary man, having lost all his superhuman strength. But it may not have been the loss of Samson's hair that caused his strength loss...maybe it was his testosterone levels!


Zero Strength Gains

The research on testosterone and strength training is complex and isn't as clear-cut as one would expect. For example, examinations of elite Olympic weightlifters have shown no changes in testosterone over a one-year training period.5 Others have shown that during a two-week heavy weight-training session there was a 12 percent reduction in resting testosterone concentration.6

There's a fine line between training hard enough to present an overloading stimulus and prompt new muscle growth and training too hard to where the body can't adapt quickly and efficiently enough before the next training session. When one pushes the body past its limits it's termed "overreaching." Being involved in a heavy resistance exercise "overreaching" program can result in reductions in testosterone during strength training as well.32 Furthermore, women can have large increases in strength from a resistance exercise strength training protocol, yet have low endogenous testosterone levels.4

Just how important is testosterone for increasing muscle strength? In a recent issue of the American Journal of Physiology Endocrinology and Metabolism, a research study documented that without testosterone, you can expect zero strength gains from a heavy resistance training protocol. Testosterone is just like Sampson's magic hair...you need it to be strong, but just like Sampson cutting his hair, loss of testosterone will impede strength gains.

In the study, 22 young men with minor experience with strength training participated in this double-blind intervention study. It was important to use relatively untrained subjects because when untrained subjects start a strict weightlifting program, they make huge gains in strength. Some subjects were randomized to treatment with a medication called goserelin, which is a gonadotropin-releasing hormone agonist. Goserelin acts on the pituitary gland in the brain and affects leutinizing hormone (LH) release, which stimulates testosterone release. Chronic administration of goserelin desensitizes the pituitary gland. This means that the pituitary gland produces less LH, which in turn stops the production of testosterone. Other subjects received a placebo subcutaneously every four weeks for a period of 12 weeks.

The strength-training period of eight weeks included exercises for all major muscles (three to four sets per exercise x six to 10 repetitions) and one-minute rest periods between sets. The protocol was designed to cause acute increases in testosterone, which has been validated by previous investigations.2 The subjects who received goserelin had a decrease in testosterone that was 10 percent lower than that of normal males, whereas testosterone remained constant in the placebo group. So here's where the importance of testosterone becomes clear for muscle strength and weight loss.

The goserelin group showed no changes in isometric knee extension strength after training, whereas the placebo group had increased strength gains. Body fat mass increased by 3 pounds in the goserelin group while it decreased by 1.3 pounds in the placebo group. Interestingly, even though the goserelin group had below-normal testosterone, they still were able to gain small increases in lean mass, but not as much as the placebo group.

The study demonstrates that testosterone is one of many growth factors controlling muscle growth (i.e., GH, IGF-1, intramuscular growth factors). The researchers concluded that endogenous testosterone is of paramount importance for the adaptation to strength training.

So, how can subjects not have changes in resting testosterone, but have increases in strength and lean mass? Researchers in the study suggested that the acute increases in GH and testosterone produced during the high-intensity weight-training program might have been a key regulator of those subjects gaining strength and size. The research findings are in agreement with other scientists who have shown that acute elevations in circulating anabolic hormones are potent stimulators of strength.

For example, Hakkinen et al.,3 reported that when subjects were divided into an arm-only training group and an arm-plus-leg training group, the arm and leg group had greater increases in testosterone and growth hormones than arm training alone that resulted in greater increases in isometric arm strength compared to arm training alone. It's interesting that the even though both groups trained arms, the group that trained both arms and legs had greater increases in arm strength. Additionally, the larger gains in strength from arm and leg training were related to the larger increases in GH and testosterone produced during the exercise sessions.


Resistance Exercise and Strength Gains

The gym is filled with all sorts of people who can sabotage your workout. The guy who does a set and talks to his friends for five minutes isn't going to have significant increases in testosterone or GH from his workout. All readers of MD know that in order to stimulate testosterone during a workout, training sessions must have short rest periods (30 seconds or less) with multiple sets, a training volume with 65 percent to 80 percent of a 1 RM, repetitions between 10 and 12, and use large muscle mass exercises such as deadlifts, squats and jump squats, which have been shown to produce large increases in testosterone and GH compared to small muscle mass exercises such as arm curls.18,22

Back in 1990, researcher Bill Kraemer reported findings that changed the way men needed to exercise with weights. In that study, men trained with an equal weight-training volume, except one group trained with moderate exercise (using a weight they could lift for 10 reps) and short rest periods (one minute) while the other group used heavy exercise (five reps) with three-minute rest periods. The results were that men using high reps with short rest periods had increases in both GH and testosterone, whereas the group using heavy weights with long rest periods had no changes in either testosterone or GH.2


Acute Metabolic Stress Increases Muscle Hypertrophy

In life nothing comes easy...especially when you're trying to put on muscle. Ever notice how some people go to the gym day after day but never make gains? It's not because they're "hard gainers" or have "bad genetics" as they would like to believe, it's because they have zero training intensity! Researchers have determined that strength and muscle mass gains can come from mechanical stress (intramuscular IGF-1) and hormonal mediated (GH, testosterone, IGF-1) muscle hypertrophy. Though mechanical factors are essential to resistance training adaptations, metabolic factors have also been shown to play a role in hypertrophy. The feeling of the "pump" or "burn" is associated with the buildup of these metabolic products (hydrogen ions, lactate and potassium) in the muscle; the higher the number of reps in a set, the greater their accumulation and effect. Although many people may think that in order to pack on muscle you have to lift heavy weights, this isn't always the case. For example, Olympic weightlifters lift considerably more weight than a bodybuilder or weight trainer, but they don't have nearly the same amount of muscle mass. Muscular hypertrophy and strength gains following a resistance-training program are thought to be due to an intensity level of 65 percent of a 1 RM to achieve an effect.2

Powerlifters and Olympic weightlifters use considerably more weight with longer rest periods than bodybuilders, but bodybuilders have much more muscle mass than both powerlifters and Olympic weightlifters.

Let's look at the different types of training: powerlifters and Olympic weightlifters generally use 5 RM with training percentages of 80 percent to 95 percent with prolonged rest periods (five minutes). Bodybuilders use high repetition (10 to 12 repetitions) with short rest periods (30 to 60 seconds). Bodybuilders produce much more metabolic stress during workouts than either powerlifters or Olympic weightlifters. If you were to make a powerlifter perform a high-intensity bodybuilding program, more than likely he would be puking from the extreme metabolic stress from the short rest periods. In fact, an older study by Kraemer et al., documented nausea from the extreme metabolic stress of a high-intensity bodybuilding program that powerlifters just could not withstand.48

Bodybuilders and powerlifters were matched for age, size and experience. Each performed a 10-station, heavy-resistance exercise protocol. Each subject performed three sets of 10-repetition maximum with a 10-second rest between sets and alternated 30-second and 60-second rest periods between exercises. No significant differences were observed between bodybuilders and powerlifters for any of the physiologic responses (lactate and adrenaline) measured. However, powerlifters exhibited a higher incidence of clinical symptoms of dizziness and nausea compared to bodybuilders...get out the barf bag!


Is a 30-second Rest Period too Long?

Most bodybuilders rest 30 seconds or less between sets, but what happens if you don't rest at all? According to one study, a no-rest period bodybuilding routine produced greater increases in GH, testosterone, adrenaline and lactate that led to larger increases in strength and muscle mass.38 The researchers compared the acute and long-term effects of a "no-rest regimen" and those of a regimen with "rest period within a set."

The no-rest regimen consisted of three to five sets of 10 repetitions at 10-repetition maximum with a one-minute rest period between exercises (lat pulldowns, shoulder presses and bilateral knee extensions).

In the rest-period regimen, subjects completed the same protocol as the no-rest regimen, but took a 30-second rest period at the midpoint of each set of exercises in order to reduce exercise-induced metabolic stress. So, at the end of the study, the results were impressive. The subjects who performed the high-intensity, no rest period exercise protocol had a 13 percent increase in muscle cross-sectional area while the rest-period group had no increase in muscle mass. Interestingly, both groups performed the same exercise volume, the only difference was the training intensity. No wonder why the guy who spends more time talking than working out next to you looks the same every year!

Another study examined metabolic adaptations in muscle to intermittent (prolonged rest periods) and continuous muscle (short rest periods) contractions. Subjects trained both their right and left legs with a protocol that used the same weight; the only difference was the rest period duration. The right leg (intermittent contractions) was trained using four sets of 10 contractions, each lasting three seconds with a two-second rest period between each contraction and two minutes between each set. The left leg (continuous contractions) was trained using four 30-second contractions with a one-minute rest period between each. Both protocols involved isometric contractions at 70 percent of a maximum voluntary isometric contraction. The increase in isometric strength was significantly greater for the continuous leg training than for the intermittent training. In addition, the leg that was trained with the short rest periods gained more muscle mass than the leg that was trained with prolonged rest periods.

These findings suggest that factors related to the greater metabolite changes during high-intensity, high-lactate, high-GH and testosterone-producing exercise training results in greater increases in isometric strength and muscle mass.37 Furthermore, when acute and chronic hormonal responses to resistance training were evaluated in 11 college men who completed 12 weeks (33 sessions) of high-volume resistance training, only the acute increases in GH during exercise were associated with increases in muscle hypertrophy. No differences in resting concentrations of growth hormone, insulin-like growth factor-1, testosterone or sex hormone-binding globulin occurred from pre- and post-training. Significant correlations existed only between absolute mean GH increases that occured during each exercise session and the degree of muscle fiber hypertrophy for type I and type II fibers.43 It may be that the combined acute metabolic stress and hormonal responses to short-rest exercises lead to greater acute increases in GH and testosterone.


How to Increase GH During Exercise...Go for the Burn!

So, what's the key to getting a good increase in GH in response to your exercise session? The key is to use high-intensity, short rest periods and multiple sets. The "one-set to failure is all you need" approach isn't going to increase GH optimally. For example, Gotshalk et al.,39 reported that subjects who performed a heavy resistance training protocol, which consisted of eight exercises with a training volume of three sets at 10 RM, produced greater GH secretion than the same exercises with one set at 10 RM. Results of the study demonstrated that higher volumes of total work produced significantly greater increases in circulating GH during exercise and upon recovery from exercise.

Exercise intensity should also be performed above 65 percent of a 1 RM for optimal gains in muscle mass and strength. Pyka et al.,40 reported that young men who performed a circuit training routine consisting of 13 stations with 30-second rest periods had minimal GH responses at 60 percent of 1 RM, while GH increased progressively using the same protocol at 70 percent and 85 percent of 1 RM. Minimal GH responses from the 60 percent protocol were attributed to the lack of training intensity.

Training intensity must rise above a certain threshold in order to cause augmented GH responses. Both the duration of rest periods between sets and training volume seem to influence the magnitude of plasma GH increases during exercise. In order to get a large increase in GH, the resistance-exercise protocol needs to produce large increases in lactate. Weltman et al.,41 reported a dose-dependent response between lactate and GH levels to low- and high-intensity running protocols. In that study, five treadmill-running intensities were studied at various percentages of the subjects' lactate threshold (LT; 0.25 LT, 0.75 LT, 1.25 LT, and 1.75 LT). Lactate threshold is the exercise intensity at which lactate (lactic acid) starts to accumulate in the bloodstream. This happens when it's produced faster than it can be removed (metabolized). GH responses were the lowest with low lactates being produced, while GH responses were the highest with increasing production of lactate being produced.

Additionally, a study by Gordon et al.,42 reported that even if the subject exercised at high thresholds without a large increase in lactate production, GH responses are blunted. In this interesting study, researchers administered sodium bicarbonate (sodium bicarbonate reduces lactate accumulation in the blood. It increases the pH in the blood and makes the blood pH less acidic) to athletes and had them perform a cycle exercise test to exhaustion. Ingestion of sodium bicarbonate resulted in a blunted GH secretion in response to exercise. Moderate-volume training programs, which have reduced rest periods (less than one minute) and produce high lactate levels during exercise, result in greater GH responses than those with high volume and protracted rest periods (greater than five minutes).


A New Era of Metabolic Stress Training: Occlusion Training

When resistance-training protocols of equal volume yet different training intensities are performed, the higher intensity protocol, which produces higher lactate levels, results in the greatest GH secretion. Earlier, it was mentioned that muscular hypertrophy and strength gains following a resistance-training program are thought to be due to an intensity level of 65 percent of a 1 RM to achieve an effect.2 Well, this is true for traditional weight training routines, but a new type of training called occlusion training has yielded increases in muscle mass and strength with training intensity as low as 20 percent of a 1 RM.44

Occlusion training or blocking blood flow may revolutionize bodybuilding training with huge increases in strength and size, although the exact mechanisms of how occlusion training increases muscle mass‑‑ an increase in metabolic by-products‑‑ may only be a partial answer. It has been reported that five sets of leg extensions with 30-second rest periods performed at 20 percent of a 1 RM with vascular occlusion resulted in a 290 percent increase in GH above baseline. The increased lactate resulting from tissue metabolic stress elevated GH significantly, although training volume was low.45

Let's examine the endocrine responses of occlusion training to regular resistance training. A recent study in the Journal of Applied Physiology compared light-resistance exercise with partial occlusion to a moderate-resistance exercise protocol with no occlusion, and finally occlusion alone. Three sets of single-arm biceps curls and single-leg calf presses were completed to failure with one-minute rest periods between sets. Workloads for light-resistance exercise with occlusion were performed at 30 percent of a 1 RM, while at 70 percent 1 RM for moderate-resistance exercise. Lactate increased significantly in the light resistance exercise with occlusion and moderate-resistance exercise trials and wasn't significantly different from each other at any point. GH increased significantly by four-fold from pre- to post-exercise in the light-resistance exercise with the occlusion session, but didn't change significantly during this time period in the moderate-resistance exercise and occlusion alone. It was interesting that occlusion alone- without any voluntary muscular activity- tended to cause a rise in GH to a similar extent as that of the moderate-resistance exercise protocol.46


High Metabolic Stress in Muscle Increases Muscle Hypertrophy at Low Workloads

Restricted blood flow training is so powerful that even walking programs result in significant increases in strength and muscle hypertrophy. Yes, let me repeat that. Walking with restricted blood flow increases muscle hypertrophy in the legs! Researchers at the University of Tokyo had men walk on a treadmill with automated cuffs (which restricted blood flow) during exercise. Training was conducted two times a day, six days per week for three weeks using five sets of two-minute bouts, with a one-minute rest between bouts. Compared to the control group who did the same protocol without restricted blood flow, the occlusion group had more than doubled the GH levels during exercise, while the walking group with no occlusion had no increase in GH. Additionally, the increase in cross-sectional area of muscle increased approximately 2 percent per week.47

Occlusion training is rapidly being investigated as a possible means to treating sarcopenia. Sarcopenia is the degenerative loss of skeletal muscle mass and strength in old age. This loss of mass reduces the performance of muscles. Due to the increasing number of elderly people, sarcopenia is an up-and-coming health issue in the developed world. The level of sarcopenia can be so severe that it prevents an elderly person from living an independent life and requiring constant assistance and care. You can't just have grandma start squatting to increase her muscle mass, but low-intensity walking with occlusion may be the new treatment for sarcopenia in the future.

After all the research presented on growth hormone and testosterone it would seem that it's a no-brainer for increasing muscle mass and strength, however the data on the effects of testosterone on muscle performance are contradictory.


The "Intramuscular Growth Factor" Camp

Elevated testosterone levels have been reported to occur in some studies,2,7,8,9 whereas several studies have shown no difference10,11,12,13 or even reductions.6,15 Not all studies in aging older men have shown that testosterone replacement therapy increases muscle strength.33,34 It should be mentioned that some studies have used low-dose testosterone gels and creams, which aren't as effective as shots for increasing testosterone and which may negatively obscure results. Additionally, different dosages have been used in different studies. Some researchers have even gone on to believe that the gains in strength and size are all related to "intramuscular growth factors" that are independent of testosterone.

A paper about the relationship between muscle growth and testosterone presented by Dr. Goldberg in 1975 stunned scientists. In his research, he castrated rats so that they couldn't produce testosterone and put their leg muscles on tension overload. Surprisingly, the rats' leg muscles grew in size, suggesting that mechanical overload increases muscle hypertrophy independent of testosterone.17 Research scientists are now discovering the signaling pathway by which mechanical stimulation of contracting muscle and intramuscular growth factors such as IGF-1 activity leads to changes in satellite cells, muscle DNA content, increased muscle protein synthesis, increased muscle mass and strength.

Other recent research has demonstrated that IGF-1 increases intracellular calcium ion concentrations, leading to the activation of the muscle growth signaling pathway and subsequent muscle fiber hypertrophy.15,16 For example, in one study, 10 healthy men completed eight sets of maximal eccentric squats. The intramuscular IGF-1 mRNA concentration increased 62 percent, but serum testosterone showed little change.35 The results suggest that mechanical stress leads to hypertrophy in the absence of testosterone. This very well may have been a reason why the subjects in the study mentioned earlier, who had been taking a medication to reduce testosterone, still had small increases in muscle mass despite having low testosterone. An increase in muscle hypertrophy can lead to greater increases in muscle strength. Reports in the literature have suggested that the insulin-like growth factor-1 protein plays a major role in strength training-induced skeletal muscle hypertrophy and strength improvements. Due to differences in genetics, some people express higher levels of IGF-1 than others- that's what we call "lucky bastards."

One study compared those who expressed high levels of IGF-1 levels to those who didn't. After 10 weeks of training with a single-leg knee-extension strength training program, one-repetition maximum, muscle volume and muscle quality increased significantly for all exercising groups. Subjects whom expressed higher natural levels of IGF-1 gained significantly more strength than those who didn't. Thus, the data suggests that IGF-1 may influence the strength response to strength training. Hold on...before I lose you to the side of the "intramuscular growth factor" group, there are some important considerations about testosterone and strength that need to be addressed.


Acute Increases in Testosterone after Exercise are Essential!

The magnitude of the increase in testosterone can be affected by the amount of muscle mass used during exercise,18,19 intensity and volume,2 training experience20 and nutritional intake.21 There are a lot of variables influencing testosterone's secretion after exercise, but it seems that acute elevations in testosterone are necessary components for strength gains. Hansen et al.23 measured muscle strength changes in muscle flexion in the arms following nine weeks of resistance exercise. One group performed muscle flexion exercises only, whereas the second group performed a lower-body exercise prior to doing elbow flexion. Performing elbow flexion exercises only failed to elevate testosterone after exercise, however testosterone was significantly elevated when lower-body exercises were performed before elbow flexion exercises.

Muscle strength increased to a greater extent in the arms when the lower and upper-body exercises that were combined produced increases in testosterone compared to training arms only, which yielded no changes in testosterone.

Another study reported that the acute increase in both testosterone and free testosterone correlated with the individual changes in strength that occurred during a six-month training period.12 Although some studies have reported no long-term changes in resting testosterone after resistance exercise, acute increases in testosterone, which may only last for 15 minutes after exercise, are essential for muscle strength and hypertrophy gains. It appears that the acute response to resistance exercise is more important to muscle growth and remodeling than chronic changes in resting hormonal changes, as many researchers haven't shown a significant change in resting hormonal levels despite increases in muscle strength and hypertrophy.


Testosterone Dose-dependently Increases Muscle Strength

During adult life, the average male produces about 7 milligrams of testosterone daily. The normal range of plasma testosterone in males is 300-1,000ng/dl, but the average value declines by age 80 to approximately 50 percent of that at age 20.26 The low levels of the anabolic hormone testosterone may be a limiting factor as to why older adults tend to have lower gains in strength compared to younger men. The impressive gains in strength from testosterone can be demonstrated in one study where hypogonadal men receiving testosterone increased muscle strength on the bench press by 22 percent without exercise.27 Testosterone is a wonderful hormone for men, but it can't perform miracles if you lay on your ass!

NASA investigated the usefulness of using testosterone to prevent muscle atrophy and strength loss during spaceflight. Men were assigned to bed-rest while using low supraphysiological-range testosterone. Interestingly, a significant anabolic response was achieved, as both whole-body nitrogen balance and leucine kinetics were improved by testosterone treatment, but there was still a decline in muscle strength. These results suggest that in the absence of daily physical activity, testosterone administration won't increase or, in the case of this bed-rest model, preserve muscle strength.28 The natural changes in testosterone throughout the day make measuring testosterone complex, as there are several studies that have shown no change in testosterone over several weeks of training, but when pharmacological testosterone is administered in combination with resistance exercise there are clear increases in muscle strength. When supraphysiological dosages of testosterone are administered to healthy young men, there was a significant increase in muscle size and strength without exercise.24 When exercise is added to supraphysiological dosages of testosterone an even greater effect on strength is achieved.

The same research group conducted an interesting study on different dosages of testosterone and how it affected muscle strength. They first gave all the healthy young men in the study a gonadotropin-releasing hormone agonist (GnRH antagonist) to suppress natural testosterone similar to the earlier study mentioned. The men then received 25, 50, 125, 300 or 600 milligrams of testosterone for 20 weeks. The researchers discovered that testosterone was associated with a dose-dependent increase in leg press strength and power. Meaning that the changes in leg press strength and power was highest for the 600-milligram group and lowest for the 25-milligram group. Another interesting finding was that the strength and muscle power were not correlated with serum IGF-1 levels.


How Testosterone Enhances Muscle Power

            Theoretically, testosterone should improve strength by increasing muscle mass (via increased protein synthesis, nitrogen retention and anti-glucocorticoid actions), as well as by increasing exercise motivation. Moreover, recent studies in humans indicate that testosterone may increase muscle protein synthesis, possibly through stimulation of intramuscular insulin-like growth factor-1 (IGF-1) gene expression.31 Testosterone may also enhance strength by altering the way neurotransmitters are produced for muscle contraction. Contraction of muscle is a duty of the central nervous system comprised of brain and spinal cord. The connection between a motor neuron (a neuron that conveys impulses from the central nervous system to a muscle) and muscle fiber is a specialized synapse called the neuromuscular junction. With adequate stimulation, the motoneuron releases a flood of neurotransmitters that bind to receptors and trigger muscle contractions.

The first step in the sequence of events causing contraction of a muscle is the chemical messenger from a nerve (in the form of the neurotransmitter molecules, acetylcholine) to the muscle. Testosterone has been shown to alter the neurotransmitter acetylcholine within motor neurons, which could result in a more efficient muscle contraction. For example, castration causes a decrease in the activity of an enzyme called choline acetyltransferase, resulting in less acetylcholine, which technically, could result in lower force output. However, if the rats are supplemented with testosterone, the levels of acetylcholine return to normal. Furthermore, if the levels are supraphysiologically elevated- as with testosterone abuse- the levels of the neurotransmitters are increased even further.30

Both motor neurons and muscles have androgen receptors located on them and are, therefore, potential sites of androgen action. Patients with Kennedy's disease, a degenerative disease in which androgen receptors are defective, can exhibit the significance of how androgen receptors affect muscle strength. The disease is characterized by severe muscle weakness. How testosterone interacts with the receptor is like a lock and key system. If you have lots of locks (testosterone) and no keys (receptors), it's not going to do you a whole lot of good.

A study in the Journal of Steroid Biochemistry and Molecular Biology reported that when resistance-trained men had muscle biopsies taken, the greatest predictor of males' 1 RM strength wasn't testosterone, but the androgen receptor content in the thigh muscle.36 It appeared that the quantity of androgen content in muscle was a better predictor of muscle strength than circulating androgens in the men studied. Both motor neurons and skeletal muscle have androgen receptors. Testosterone causes enlargement of motor neurons, causing greater force enhancement. Males have larger motor neurons than females due to the actions of testosterone. Research in rats has shown that testosterone administration causes an increase in motor size while castration causes a decrease in the motor size of neurons.29 The data suggest that testosterone may increase strength by increasing motor neuron size.

            The research suggests that both mechanical stress from muscle overload and acute anabolic hormone (GH and testosterone) responses are key mediators of muscle growth and strength gains. Although some studies have shown no changes in resting testosterone after resistance exercise, more recent research has demonstrated the importance of acute testosterone responses and mechanical stimuli from resistance exercise acting as upregulators of the androgen receptor, which influence muscle strength and size. Other factors such as nutrition, overtraining, intensity and rest duration all influence the secretion of testosterone during exercise. The acute increases in testosterone are associated with changes in lean muscle mass and strength.


Key Points:

  • Increased metabolic stress (short rest periods) results in greater increases in strength and muscle mass than prolonged rest periods.
  • Lactate increase GH responses in a dose-dependent fashion.
  • A decrease in testosterone as little as 10 percent below normal will impede strength gains in response to a strength-training program.
  • Acute elevations in testosterone are a potent stimulator of muscle mass and strength.
  • Testosterone produces a dose-dependent effect on muscle strength.
  • Motor neurons contain androgen receptors and increase in size in response to testosterone, which may mediate muscle strength.
  • Testosterone increases the neurotransmitters' firing potential of muscle, which may enhance force production.



•1.       McDonagh MJ, Davies CT. Adaptive response of mammalian skeletal muscle to exercise with high loads. Eur J Appl Physiol Occup Physiol, 1984;52(2):139-55. Review. 
•2.       Kraemer WJ, Marchitelli L, Gordon SE, Harman E, Dziados JE, Mello R, Frykman P, McCurry D, Fleck SJ. Hormonal and growth factor responses to heavy resistance exercise protocols. J Appl Physio, 1990 Oct;69(4):1442-50. 
•3.       Hakkinen K, Pakarinen A, Alen M, Kauhanen H, Komi PV. Neuromuscular and hormonal adaptations in athletes to strength training in two years. J Appl Physiol, 1988 Dec;65(6):2406-12.
•4.       Aizawa K, Akimoto T, Inoue H, Kimura F, Joo M, Murai F, Mesaki N. Resting serum dehydroepiandrosterone sulfate level increases after 8-week resistance training among young females. Eur J Appl Physiol, 2003 Nov;90(5-6):575-80.
•5.       Galloway GP (1997) Anabolic-androgenic steroids. In: Substance abuse: a comprehensive textbook (Lowinson JH, Ruiz P, Millman RB, Langrod JG, eds), pp 308-318. MD: Williams & Wilkins).
•6.       Raastad T, Glomsheller T, Bjoro T, Hallen J. Changes in human skeletal muscle contractility and hormone status during 2 weeks of heavy strength training. Eur J Appl Physiol, 2001 Jan-Feb;84(1-2):54-63. 
•7.       Kraemer WJ, Hakkinen K, Newton RU, Nindl BC, Volek JS, McCormick M, Gotshalk LA, Gordon SE, Fleck SJ, Campbell WW, Putukian M, Evans WJ. Effects of heavy-resistance training on hormonal response patterns in younger vs. older men. J Appl Physiol, 1999 Sep;87(3):982-92. 
•8.       Ahtiainen JP, Pakarinen A, Alen M, Kraemer WJ, Hakkinen K. Muscle hypertrophy, hormonal adaptations and strength development during strength training in strength-trained and untrained men. Eur J Appl Physiol, 2003 Aug;89(6):555-63.
•9.       Staron RS, Karapondo DL, Kraemer WJ, Fry AC, Gordon SE, Falkel JE, Hagerman FC, Hikida RS. Skeletal muscle adaptations during early phase of heavy-resistance training in men and women. J Appl Physiol, 1994 Mar;76(3):1247-55. 
•10.   Hickson RC, Hidaka K, Foster C, Falduto MT, Chatterton RT Jr. Successive time courses of strength development and steroid hormone responses to heavy-resistance training. J Appl Physiol, 1994 Feb;76(2):663-70. 
•11.   Hakkinen K, Pakarinen A, Alen M, Kauhanen H, Komi PV. Neuromuscular and hormonal adaptations in athletes to strength training in two years. J Appl Physiol, 1988 Dec;65(6):2406-12. 
•12.   Hakkinen K, Pakarinen A, Kraemer WJ, Newton RU, Alen M. Basal concentrations and acute responses of serum hormones and strength development during heavy resistance training in middle-aged and elderly men and women. J Gerontol A Biol Sci Med Sci, 2000 Feb;55(2):B95-105. 
•13.   Alen M, Pakarinen A, Hakkinen K, Komi PV. Responses of serum androgenic-anabolic and catabolic hormones to prolonged strength training. Int J Sports Med, 1988 Jun;9(3):229-33. 
•14.   Mujika I, Padilla S, Pyne D, Busso T. Physiological changes associated with the pre-event taper in athletes. Sports Med, 2004;34(13):891-927. Review. 
•15.   Semsarian C, Wu MJ, Ju YK, Marciniec T, et al. Skeletal muscle hypertrophy is mediated by a Ca2+-dependent calcineurin signaling pathway. Nature, 1999 Aug 5;400 (6744) :576-81
•16.   Musaro A, McCullagh KJ, Naya FJ, Olson EN, Rosenthal N. IGF-1 induces skeletal myocyte hypertrophy through calcineurin in association with GATA-2 and NF-ATc1. Nature, 1999 Aug 5;400(6744):581-5
•17.   Goldberg AL, Etlinger JD, Goldspink DF, Jablecki C. Mechanism of work-induced hypertrophy of skeletal muscle. Med Sci Sports, 1975 Fall;7(3):185-98. 
•18.   Volek JS, Kraemer WJ, Bush JA, Incledon T, Boetes M.Testosterone and cortisol in relationship to dietary nutrients and resistance exercise. J Appl Physiol, 1997 Jan;82(1):49-54. 
•19.   Hansen S, Kvorning T, Kjaer M, Sjogaard G. The effect of short-term strength training on human skeletal muscle: the importance of physiologically elevated hormone levels. Scand J Med Sci Sports, 2001 Dec;11(6):347-54. 
•20.   Tremblay MS, Copeland JL, Van Helder W. Effect of training status and exercise mode on endogenous steroid hormones in men. J Appl Physiol, 2004 Feb;96(2):531-9. Epub 2003 Sep 26. 
•21.   Kraemer WJ, Volek JS, Bush JA, Putukian M, Sebastianelli WJ. Hormonal responses to consecutive days of heavy-resistance exercise with or without nutritional supplementation. J Appl Physiol, 1998 Oct;85(4):1544-55. 
•22.   Kraemer WJ, Fry AC, Warren BJ, Stone MH, Fleck SJ, Kearney JT, Conroy BP, Maresh CM, Weseman CA, Triplett NT, et al. Acute hormonal responses in elite junior weightlifters. Int J Sports Med, Hansen S, Kvorning T, Kjaer M, Sjogaard G. 1992 Feb;13(2):103-9. 
•23.   The effect of short-term strength training on human skeletal muscle: the importance of physiologically elevated hormone levels. Scand J Med Sci Sports, 2001 Dec;11(6):347-54. 
•24.   Bhasin S, Woodhouse L, Casaburi R, Singh AB, Bhasin D, Berman N, Chen X, Yarasheski KE, Magliano L, Dzekov C, Dzekov J, Bross R, Phillips J, Sinha-Hikim I, Shen R, Storer TW. Testosterone dose-response relationships in healthy young men. Am J Physiol Endocrinol Metab, 2001 Dec;281(6):E1172-81. 
•25.   Bhasin S, Storer TW, Berman N, Callegari C, Clevenger B, Phillips J, Bunnell TJ, Tricker R, Shirazi A, Casaburi R. The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men. N Engl J Med, 1996 Jul 4;335(1):1-7.
•26.   Bardin CW. The anabolic action of testosterone. N Engl J Med, 1996 Jul 4;335(1):52-3.
•27.   Bhasin S, Storer TW, Berman N, et al. 1997 Testosterone replacement increases fat-free mass and muscle size in hypogonadal men. J Clin Endocrinol Metab, 82:407-413.
•28.   Zachwieja JJ, Smith SR, Lovejoy JC, Rood JC, Windhauser MM, Bray GA. Testosterone administration preserves protein balance but not muscle strength during 28 days of bed rest. J Clin Endocrinol Metab, 1999 Jan;84(1):207-12. 
•29.   Fraley GS, Ulibarri CM. Long-term castration effects motoneuron size but not number in the spinal nucleus of the bulbocavernosus in the adult male Mongolian gerbil. Brain Res, 2002 Oct 25;953(1-2):265-71. 
•30.   Blanco CE, Popper P, Micevych P. Anabolic-androgenic steroid induced alterations in choline acetyltransferase messenger RNA levels of spinal cord motoneurons in the male rat. Neuroscience, 1997 Jun;78(3):873-82. 
•31.   Urban RJ, Bodenburg YH, Gilkison C, et al. Testosterone administration to elderly men increases skeletal muscle strength and protein synthesis. Am J Physiol, 1995; 269: E820-E826.
•32.   Kraemer WJ, Ratamess NA, Volek JS, Hakkinen K, Rubin MR, French DN, Gomez AL, McGuigan MR, Scheett TP, Newton RU, Spiering BA, Izquierdo M, Dioguardi FS. The effects of amino acid supplementation on hormonal responses to resistance training overreaching. Metabolism, 2006 Mar;55(3):282-91. 
•33.   Snyder PJ, Peachey H, Hannoush P, Berlin JA, Loh L, Lenrow DA, Holmes JH, Dlewati A, Santanna J, Rosen CJ, Strom BL. Effect of testosterone treatment on body composition and muscle strength in men over 65 years of age. J Clin Endocrinol Metab, 1999 Aug;84(8):2647-53. 
•34.   Wittert GA, Chapman IM, Haren MT, Mackintosh S, Coates P, Morley JE. Oral testosterone supplementation increases muscle and decreases fat mass in healthy elderly males with low-normal gonadal status. J Gerontol A Biol Sci Med Sci, 2003 Jul;58(7):618-25. 
•35.   Bamman MM, Shipp JR, Jiang J, Gower BA, Hunter GR, Goodman A, McLafferty CL Jr, Urban RJ. Mechanical load increases muscle IGF-1 and androgen receptor mRNA concentrations in humans. Am J Physiol Endocrinol Metab, 2001 Mar;280(3):E383-90. 
•36.   Deschenes MR, Maresh CM, Armstrong LE, Covault J, Kraemer WJ, Crivello JF. Endurance and resistance exercise induce muscle fiber type specific responses in androgen binding capacity. J Steroid Biochem Mol Biol, 1994 Aug;50(3-4):175-9. 
•37.   Schott J, McCully K, Rutherford OM. The role of metabolites in strength training. II. Short versus long isometric contractions. Eur J Appl Physiol Occup Physiol, 1995;71(4):337-41. 
•38.   Goto K, Ishii N, Kizuka T, Takamatsu K. The impact of metabolic stress on hormonal responses and muscular adaptations. Med Sci Sports Exerc, 2005 Jun;37(6):955-63. 
•39.   Gotshalk LA, Loebel CC, Nindl BC, Putukian M, Sebastianelli WJ, Newton RU, Hakkinen K, Kraemer WJ. Hormonal responses of multiset versus single-set heavy-resistance exercise protocols. Can J Appl Physiol, Jun;22(3):244-55, 1997.         
•40.   Pyka G, Wiswell RA, Marcus R Age-Dependent Effect of Resistance Exercise on Growth Hormone Secretion in People. J Clin Endocrinol Metab, 75: 404-407, 1992.
•41.   Weltman A, Pritzlaff CJ, Wideman L, Weltman JY, Blumer JL, Abbott RD, Hartman ML, Veldhuis JD. Exercise-dependent growth hormone release is linked to markers of heightened central adrenergic outflow. J Appl Physiol, Aug;89(2):629-35, 2000.             
•42.   Gordon SE, Kraemer WJ, Vos NH, Lynch JM and Knuttgen HG. Effect of acid-base balance on the growth hormone response to acute, high-intensity cycle ergometer. J Appl Physiol,  76: 821-829, 1994.
•43.   McCall GE, Byrnes WC, Fleck SJ, Dickinson A, Kraemer WJ. Acute and chronic hormonal responses to resistance training designed to promote muscle hypertrophy. Can J Appl Physiol, 1999 Feb;24(1):96-107.
•44.   Takarada Y, Sato Y, Ishii N. Effects of resistance exercise combined with vascular occlusion on muscle function in athletes. Eur J Appl Physiol, 2002 Feb;86(4):308-14. 
•45.   Takarada Y, Nakamura Y, Aruga S, Onda T, Miyazaki S, Ishii N. Rapid increase in plasma growth hormone after low-intensity resistance exercise with vascular occlusion. J Appl Physiol, Jan;88(1):61-5, 2000.
•46.   Reeves GV, Kraemer RR, Hollander DB, Clavier J, Thomas C, Francois M, Castracane VD. Comparison of Hormone Responses Following Light Resistance Exercise with Partial Vascular Occlusion and Moderate Resistance Exercise Without Occlusion. J Appl Physiol, 2006 Aug 10.
•47.   Abe T, Kearns CF, Sato Y. Muscle size and strength are increased following walk training with restricted venous blood flow from the leg muscle, Kaatsu-walk training. J Appl Physiol, 2006 May;100(5):1460-6. Epub 2005 Dec 8.
•48.   Kraemer WJ, Noble BJ, Clark MJ, Culver BW. Physiologic responses to heavy-resistance exercise with very short rest periods. Int J Sports Med, 1987 Aug;8(4):247-52. 

Testosterone Research Update: Low Testosterone is Associated With Increased Mortality

Not only will low testosterone completely blunt strength gains in response to a workout and increase your waistline...even more frightening is that a low testosterone level is associated with increased death rates in men. Several studies have noted that low testosterone levels are associated with multiple risk factors for heart disease, including hypertension, central obesity, blood clots and increased markers of inflammation such as C-reactive protein.

A report published in the August issue of Archives of Internal Medicine revealed a correlation between reduced levels of testosterone and an increased risk of mortality during up to eight years of follow-up. Researchers analyzed the association between testosterone levels and death in 858 male veterans over the age of 40. The participants' testosterone levels were measured at least twice between 1994 and 1999. Subjects were followed through 2002, and any deaths among the group were noted. Participants were placed in either low total testosterone or normal testosterone levels. After adjusting for age, illness and other factors, men whose testosterone levels were classified as low experienced an 88 percent adjusted increased risk of dying over the course of the follow-up compared to those with normal levels. To reduce the effects of acute illness on the findings, the researchers re-examined the data excluding men who died within the first year of follow-up, yet they still found an increase of 68 percent in the risk of dying among men with low testosterone.