Gender-affirming hormone therapy (GAHT) represents a significant physiological change, and its effects on the body’s musculoskeletal system are profound. These therapies involve either testosterone or estrogen, causing changes in muscle mass, strength, and overall body composition. These hormones directly influence how the body builds and maintains muscle tissue, redistributes fat, and modifies physical performance capacity. Understanding how these hormones function at a cellular level and the specific timelines of shifts is important. The impact of this transition extends beyond appearance, affecting daily physical function, exercise capacity, and overall metabolic health.
The Baseline Influence of Sex Hormones on Muscle Tissue
Testosterone and estrogen are steroid hormones that exert their influence on muscle cells by binding to specific receptors. Testosterone is the primary anabolic hormone, promoting tissue growth and determining skeletal muscle mass and strength. It stimulates protein synthesis while simultaneously inhibiting muscle protein breakdown, actions primarily mediated through the androgen receptor (AR) found in muscle tissue.
Estrogen also plays a role in muscle health, though its effects are more focused on maintenance and metabolism. It has been shown to have anabolic effects, helping to preserve muscle tissue and mitochondrial function. Estrogen’s influence is also evident in fat storage patterns, directing fat to accumulate in subcutaneous areas like the hips, thighs, and buttocks. By contrast, testosterone tends to suppress this subcutaneous fat storage and encourages a more visceral, or abdominal, fat distribution.
Muscle and Strength Changes During Testosterone Therapy
Masculinizing hormone therapy involves administering testosterone, which leads to rapid physiological changes in muscle and body composition. The increase in circulating testosterone levels stimulates protein synthesis and muscle growth, resulting in a notable increase in lean body mass (LBM). Studies show trans men typically experience an increase in LBM of approximately 12% within the first year of therapy.
This gain in muscle mass correlates with an increase in physical strength. Hand grip strength, a common measure of overall muscle function, has been observed to increase by as much as 18% over the first 12 months of therapy. These strength increases continue, with physical performance measures often improving to match the range of cisgender men by the two-to-three-year mark.
Body composition also shifts dramatically as testosterone encourages the redistribution of fat away from the hips and thighs toward the abdomen. While the magnitude of change is dose-dependent, low-dose testosterone is often sufficient to achieve long-term, sustained muscle development. The most substantial changes generally occur within the first year, after which the rate of change slows down as hormone levels stabilize.
Muscle and Strength Changes During Estrogen Therapy
Feminizing hormone therapy, which typically involves estrogen paired with anti-androgens, suppresses endogenous testosterone, leading to a reduction in muscle mass and strength. The loss of testosterone’s anabolic effects, combined with anti-androgens blocking androgen receptors, drives these changes. This results in a decrease in total lean body mass, often observed as a modest loss of about 3% to 5% after 12 months of therapy.
The decrease in muscle size is accompanied by a reduction in strength. The timeline for strength loss is often slower than the speed of strength gain seen with testosterone therapy. While significant decreases are observed after the first year, studies indicate that strength values may still remain higher than those of cisgender women even after 36 months of therapy. For example, grip strength has been shown to decrease by an average of 1.8 kilograms after 12 months of treatment.
The most prominent change in body composition is the redistribution of fat, which increases significantly—by approximately 28% to 30% after 12 months—and moves toward a more feminine pattern. Estrogen promotes fat deposition subcutaneously around the hips, thighs, and buttocks. These changes can take up to six years to reach their maximum effect.
Tailoring Exercise and Training During Transition
Individuals undergoing testosterone therapy can optimize their physical changes by focusing on strength and hypertrophy training. Since testosterone enhances the body’s ability to build muscle, a structured program that incorporates progressive overload and resistance exercises will maximize gains in lean mass and strength. Consistency in training during the first year is beneficial to take advantage of the most rapid period of muscle growth.
For those undergoing estrogen therapy, the focus of an exercise routine shifts toward maintaining core strength, mobility, and cardiovascular health. As strength naturally decreases, incorporating low-impact activities such as walking, swimming, or yoga helps support the body without excessive strain. Managing body weight is also a consideration, as hormone therapy can sometimes lead to weight gain, making a consistent exercise routine important for metabolic health.