Exercise induced testosterone (EIT) and its effects on muscle growth are not well known among the general public. However, there is enough evidence to support that it does exist. EIT is a naturally occurring hormone produced by the testes and adrenal glands which plays an important role in male sexual development. Testosterone levels rise during puberty and remain elevated until menopause. During this time, testosterone production decreases due to decreased aromatase activity in the ovaries. 
Testosterone is converted into dihydrotestosterone (DHT), which is then converted into estrogen via 5α-reductase enzyme in the skin. Estrogen stimulates protein synthesis and inhibits breakdown of muscle tissue. 
The most common side effects of testosterone include: acne, hair loss, increased libido, erectile dysfunction, fatigue, depression, heart disease, high blood pressure, low bone density, osteoporosis, prostate cancer, weight gain, and weight loss.. Due to the potential for serious side effects, testosterone supplementation is contraindicated in individuals with prostate or kidney cancer.
Testosterone increases nitrogen retention and protein synthesis, which are both essential components of muscle growth. The ability to build skeletal muscle and increase strength is hampered with low testosterone levels. In addition, testosterone influences erythropoiesis (red blood cell production) and increases the number of oxygen carrying red blood cells.
These effects can increase physical stamina, energy levels, endurance, and reaction time.
Bodybuilders have been using anabolic steroids to increase testosterone levels and muscle mass for a long time. In fact, some bodybuilders inject anabolic steroids directly into their muscles or take testosterone pills. However, these practices are extremely dangerous and can lead to serious health problems later in life.
Pumping iron is known to raise testosterone levels as well as build muscle tissue.
Exercise and Testosterone
The relationship between exercise and testosterone is not as well known to the general public. The relationship between EIT and muscle growth is a fairly new concept that has been recently gaining momentum in the strength training community.
EIT does not cause an acute increase in muscle size or performance, but rather it increases the amount of lean mass one can hold. A positive relationship exists between the amount of muscle one can hold and basal (resting) testosterone levels. In other words, as the amount of muscles a person has increases, so will their testosterone levels.
There is a limit to this relationship, however. As one gains more muscle than their testosterone level can support, further gains in muscle size begin to decline.
Testosterone and Muscle Growth
Testosterone has a very strong influence on skeletal muscle size and strength. It is well established that a positive relationship exists between testosterone levels and skeletal muscle size. However, the question remains as to whether or not this relationship is causal.
In other words, does testosterone increase muscle mass or do people with more muscle produce more testosterone?
Most everyone will agree that there is a direct relationship between testosterone levels and muscle growth. In fact, most people will tell you that it’s steroids that cause the growth (they don’t know any better). This is not the case, however. Although steroids will obviously increase muscle growth over and above what natural testosterone production can accomplish, the relationship between testosterone and growth exists even in people that have normal testosterone levels (e.g., in children).
Even in the elderly, who have significantly lower testosterone levels than young adults, muscle mass still decreases at a much slower rate. This is why some elderly people can still be fairly strong even though they have very little muscle mass.
So, does testosterone increase muscle mass or do people with more muscle produce more testosterone?
The answer is both. Muscle is an androgen-sensitive tissue, meaning that it has the ability to increase or decrease its own receptor density. Androgens (such as testosterone) bind to receptors in muscle and cause a chain reaction that results in either an increase or decrease in receptor numbers, which in turn causes an increase or decrease in the muscular capacity.
Research has shown that the number of androgen receptors present in muscle is directly related to the production of testosterone. In other words, the more androgen receptors present, the more testosterone can enter the cell. And since steroids increase the number of androgen receptors in muscle, they can directly increase muscle size.
However, this relationship isn’t entirely one-sided. Muscle tissue is also androgen sensitive and can affect its own receptor density just like any other androgen-sensitive tissue. Heavy resistance training has been shown to increase androgen receptor density in muscle, as well as the amount of testosterone that can be bound to each receptor.
So does this mean that a heavy-training bodybuilder with a lot of muscle is producing more testosterone than a smaller bodybuilder with less muscle?
Not exactly. The muscles of the bodybuilder are already fully developed, so there is little need for them to get any bigger. In fact, if his androgen receptors get any higher, then testosterone levels may actually start to have a negative effect on muscle growth as testosterone can only be bound to a certain amount of receptors.
This explains why steroid-using bodybuilders often stop growing when they reach a certain size. They might keep pumping more and more steroids into their bodies, but they won’t grow any more because of the limitations on receptor density. This also means that even if a natural bodybuilder were to increase his androgen receptor density through heavy-resistance training, he still wouldn’t be able to get any bigger than his current maximum unless his androgen receptor density became so high that it started to have a negative effect by not allowing enough free testosterone into the cell.
In short, the relationship is bidirectional.
However, heavy-resistance training can still have a positive effect on muscle growth by increasing the amount of free testosterone in the blood that can then be bound to receptors or increase the activity of the cells without bound to receptors. This means that a bodybuilder who trains heavy and also takes steroids will grow more than a bodybuilder who only trains heavy but doesn’t take steroids.
So, there you have it. Heavy-resistance training increases maximum muscle size, but not by increasing the amount of testosterone in the body, rather by increasing the efficiency of the muscles and the amount of free testosterone in the blood. This means that natural bodybuilders can increase their maximum muscular potential through weight training, contrary to popular belief.
Sources & references used in this article:
Anabolic processes in human skeletal muscle: restoring the identities of growth hormone and testosterone by DWD West, SM Phillips – The Physician and sportsmedicine, 2010 – Taylor & Francis
Resistance exercise‐induced increases in putative anabolic hormones do not enhance muscle protein synthesis or intracellular signalling in young men by DWD West, GW Kujbida, DR Moore… – The Journal of …, 2009 – Wiley Online Library
Associations of exercise-induced hormone profiles and gains in strength and hypertrophy in a large cohort after weight training by DWD West, SM Phillips – European journal of applied physiology, 2012 – Springer
Growth hormone, IGF-I, and testosterone responses to resistive exercise. by RR Kraemer, JL Kilgore, GR Kraemer… – … in Sports and Exercise, 1992 – europepmc.org
Elevations in ostensibly anabolic hormones with resistance exercise enhance neither training-induced muscle hypertrophy nor strength of the elbow flexors by DWD West, NA Burd, JE Tang… – Journal of applied …, 2010 – journals.physiology.org
Resistance exercise-induced hormonal responses in men, women, and pubescent boys by T Pullinen, A Mero, P Huttunen… – … in sports and exercise, 2002 – academia.edu
Interventions for sarcopenia and muscle weakness in older people by SE Borst – Age and ageing, 2004 – academic.oup.com
Effects of gender on exercise-induced growth hormone release by L Wideman, JY Weltman, N Shah… – Journal of applied …, 1999 – journals.physiology.org