Interactions between myostatin and hormones 

Myostatin levels and muscle size play a significant role because it might contribute to individual differences in muscularity or muscular atrophy in response to altered physical activity.

Numerous studies have shown that resistance training reduces myostatin expression in the 20 to 50 percent range. Recent studies focused on the interactions between myostatin and hormones such as growth hormone and testosterone suggesting that myostatin counteracts the anabolic effects of growth hormone and IGF-1.

The researchers suggested that anabolic effects resulting due to a combination of growth hormone and testosterone or anabolic steroids would be because of the steroid use. 

Another study that used humans as subjects found no relationship between body mass and myostatin in men, younger or older. However, when men are given testosterone, they found that myostatin increases after 56 days, and returns to the baseline after 20 weeks of testosterone treatment. Researchers suggest that myostatin rises with testosterone treatment due to the anabolic effects of testosterone that triggers the release of myostatin to stop excessive muscle growth, which is seen in younger men prone to muscle buildup compared to older men. In older men, the body reduces myostatin to counter loss of muscle.

Combined with large amounts of amino acids in the blood, insulin triggers muscle protein synthesis. Insulin on its own inhibits muscle protein breakdown and counters the catabolic effects of cortisol. Insulin resistance adversely affects these mechanisms. 

In extremely obese women, muscle cells produce a nearly threefold increase in myostatin. Insulin resistance boosts myostatin due to inhibited insulin activity, leading to cellular starvation. Loss of muscle mass is common in people with Type 2 diabetes.

Weight training and myostatin synthesis 

Regular weight training reduces myostatin activity in the body, which dictates the anabolic response necessary for muscle repair. The drop in myostatin levels sets off the process to repair muscles. This results in subsequent increase in muscle size in anticipation of another weight training session. The greater the level of workout, the lower the myostatin. Smaller decrease in myostatin will result in small changes in muscle growth, and bigger change will result in correspondingly bigger changes.

Myostatin behavior in humans is reportedly affected by race. Studies have reported that black people have higher muscular density, muscular mass, and strength compared to Caucasians. In a study that compared ethnic groups, it was found that black people showed gene activity that is consistent with lower myostatin than other ethnic groups. The group exhibited greater upper arm gains after a 12-week training.