Gender transition involves significant bodily changes, and understanding how muscles adapt is a common area of interest. This article explores the biological mechanisms behind muscle changes in trans individuals undergoing hormone therapy. It delves into the scientific basis of sex-linked muscle differences and the specific effects of masculinizing and feminizing hormones on muscle mass, strength, and fat distribution. General considerations for tailoring exercise routines to align with transition goals are also provided.
Understanding Sex-Linked Muscle Differences
Before puberty, males and females have similar muscle strength, speed, and movement mechanics. Once puberty begins, sex hormones influence distinct differences in muscle composition and body structure. Testosterone in males leads to increased lean muscle mass, while estrogen in females shifts body composition and increases fat storage. These hormonal influences contribute to males developing more fast-twitch muscle fibers, which are beneficial for power-based activities like sprinting and jumping.
Females have a higher percentage of slow-twitch muscle fibers, which are better suited for endurance activities due to their improved oxygen consumption and resilience to oxidative stress. Males possess a greater proportion of upper body muscle mass (around 42.9%) compared to females (around 39.7%), while females have a slightly higher percentage of lower body muscle mass. Boys are stronger than girls, a difference more evident in upper-body muscles.
Muscle Changes with Masculinizing Hormones
Masculinizing hormone therapy, primarily involving testosterone, induces notable changes in muscle development for transgender men. Testosterone increases muscle protein synthesis, the process by which muscle cells build new proteins, leading to an increase in muscle mass and strength. Studies show that transgender men on testosterone therapy experience an increase in muscle volume, with one study reporting an average increase of 21% within six years.
This increase in muscle mass is accompanied by a reduction in fat mass and a redistribution of fat. Testosterone promotes a more masculine fat distribution pattern, reducing fat in the hips and thighs while increasing abdominal fat. Research indicates that muscle mass and strength in transgender men increase to levels comparable to cisgender men within one to three years of hormone therapy. Handgrip strength, for example, has been observed to increase by a mean of 18% over 12 months in transgender men.
Muscle Changes with Feminizing Hormones
Feminizing hormone therapy, involving estrogen and anti-androgens, leads to a reduction in muscle mass and strength for transgender women. Estrogen supplementation lowers testosterone concentrations, contributing to these changes. These hormones promote a more feminine fat redistribution, increasing adipose tissue on the hips and less in the abdominal region, and softening the skin.
Studies show that transgender women on estrogen therapy experience a decrease in muscle volume, with an average reduction of 7% over five years, while muscle strength remains stable or decreases. After 12 months of feminizing hormone therapy, muscle mass decreases by approximately 5%, with this decline continuing beyond three years. While absolute muscle mass may remain higher than cisgender women, their relative muscle and fat mass percentages, and muscle strength corrected for lean mass, become comparable to cisgender women over time.
Tailoring Exercise for Transition Goals
Transgender individuals adjust their exercise routines to align with the physiological changes brought about by hormone therapy and their personal transition goals. For transgender men, focusing on strength training and hypertrophy (muscle growth) amplifies the effects of testosterone. Incorporating compound movements like squats, deadlifts, and bench presses helps build overall muscle mass and strength. Regular resistance training, aiming for three times per week with 3-4 sets of 8-12 repetitions, supports these goals.
For transgender women, exercise focuses on maintaining muscle tone without excessive bulk and sculpting a more feminine physique. This involves prioritizing lower body movements like hip thrusts, lunges, and squats to encourage a fuller appearance in the hips and glutes. Instead of heavy lifting for maximal strength, a moderate approach with higher repetitions (12-15 reps per set) supports muscle endurance and a leaner look. Bodyweight exercises, Pilates, and resistance bands are also effective for shaping the physique without promoting a hyper-masculine training pattern.