Muscle Changes in Trans Individuals: A Biological Look

Gender-affirming hormone therapy (GAHT) reshapes skeletal muscle in measurable ways, but the changes are neither instant nor symmetrical between trans women and trans men. Trans women on feminizing hormones lose lean mass and grip strength modestly over the first year or two, while trans men on testosterone gain lean mass and strength more rapidly and to a greater degree. The biology behind these shifts involves more than just hormone levels, and the timeline, completeness, and real-world significance of the changes remain active areas of research.

What Feminizing Hormones Do to Muscle

For trans women, feminizing GAHT typically involves estrogen combined with an anti-androgen to suppress testosterone. The muscle-related effects are real but surprisingly moderate in the first year. Longitudinal studies show that total lean body mass drops by roughly three to five percent in the first twelve months of therapy, while fat mass climbs by about 28 percent over the same period.¹ That fat gain tends to redistribute toward the hips, thighs, and breasts, following a typically female pattern, while the muscle loss is distributed more evenly.

A two-year study tracked the shift in relative body composition more closely: lean mass as a percentage of total body weight went from about 77.5 percent at baseline to 72.5 percent at one year and 71.7 percent at two years, with fat mass rising from 19 percent to roughly 25 percent over the same window.¹ Those numbers tell an interesting story: the biggest swing happens in the first year, then the changes slow considerably.

When researchers zoom in on specific muscle groups, the picture sharpens. Cross-sectional thigh muscle area in one three-year study of 19 trans women dropped about nine percent by year one and twelve percent by year three, but the additional loss between years one and three was not statistically meaningful.¹ In other words, most of the muscle shrinkage that feminizing hormones will produce seems to happen early, and then the trajectory levels off.

How Strength Changes in Trans Women

Losing muscle mass does not translate one-to-one into losing strength, and the grip strength data from trans women illustrate that disconnect. A systematic review of prospective studies found that about half of the studies showed a six to eight percent drop in handgrip strength after six to twelve months, while others showed grip strength holding steady.² One study noted that the decrease was only statistically significant in the dominant hand. Another found that 67 percent of the grip strength decline happened in the final three months of the first year, suggesting the timeline is not linear.

Lower body strength tells an even more nuanced story. Isometric torque for knee extension and knee flexion remained essentially unchanged after twelve months and even after five years of feminizing GAHT in some studies.² The reasons are not entirely clear. Part of the answer may be that leg muscles respond differently to hormonal shifts than upper body muscles, and part may be that participants who remain physically active preserve strength even as their hormone profile changes.

The Residual Gap With Cisgender Women

Perhaps the most discussed finding in this area is that trans women on feminizing hormones, even after several years of therapy, tend to retain muscle mass and strength values above those of cisgender women. A systematic review focused on sport participation concluded that while significant decreases in strength, lean body mass, and muscle area occur by twelve months, those values remain above cisgender female averages even at the 36-month mark.³ The effects of longer therapy beyond three years in closing that gap further are unclear, largely because so few studies have followed participants that long.

This residual difference has fueled debate about athletic eligibility, but it is worth understanding what the gap actually looks like in practice. Total lean body mass decreased by somewhere between three and five percent from baseline, and cross-sectional muscle area decreased by roughly 1.5 to nearly 12 percent depending on the muscle group and study duration.³ Whether that remaining difference is meaningful depends heavily on the activity. In everyday life, a trans woman several years into hormone therapy would not notice the residual lean mass difference. In elite-level sport, even small absolute differences can matter at the margins.

What Testosterone Does for Trans Men

The masculinizing side of hormone therapy tends to produce faster and more dramatic muscle changes. Testosterone drives muscle protein synthesis, expands satellite cell populations within muscle fibers, and increases myonuclear number, all of which contribute to fiber hypertrophy.⁴ In practical terms, trans men on testosterone notice early changes in muscle fullness and strength that begin within the first few months.

A review of testosterone therapy timelines found that changes in fat mass, lean body mass, and muscle strength typically begin within 12 to 16 weeks, stabilize at six to twelve months, and can continue marginally for years afterward.⁵ The desired physical effects include increased lean mass, decreased fat mass, and greater strength, alongside other masculinizing changes like deeper voice and increased body hair.⁶

The strength gains are substantial. In prospective studies, handgrip strength increased by about 18 percent within twelve months of starting testosterone, with nearly half of that gain arriving in the first three months.² Lower body strength saw even larger relative gains in some studies: knee extension torque increased 12 percent and knee flexion torque jumped 26 percent over twelve months. Longer follow-up suggests the gains continue: one study found knee extension strength increased a total of 18 percent from baseline to six years post-GAHT, with an additional six percent of that coming between years one and five.

Age Matters for Testosterone’s Muscle Effects

Not everyone responds to testosterone equally, and age appears to be a significant modifier. A one-year observational study found that handgrip strength increased across all age groups on masculinizing GAHT, but the size of the gain dropped steadily with age. Participants aged 25 to 29 gained an average of 4.35 kg in grip strength, while those aged 20 to 24 gained about 2.14 kg. In participants over 30, the average gains were smaller and did not reach statistical significance.⁷

This gradient is not surprising if you consider what aging does to muscle biology in general. Older adults have fewer satellite cells, a blunted anabolic response to hormones, and often lower baseline activity levels. For trans men starting testosterone later in life, the muscle and strength changes will still happen, but the ceiling may be lower than for someone starting in their twenties.

Adolescent Trajectories

Trans adolescents who begin puberty suppression followed by hormone therapy face a somewhat different biological scenario, because their baseline body composition has not yet been fully shaped by endogenous puberty. A longitudinal study of transgender adolescents found that lean body mass z-scores increased by 0.93 during the first year of testosterone treatment and remained stable through three years.⁸ Fat mass z-scores decreased by about 0.43 in the first year, with a total decrease of 0.58 after three years. Those are large shifts in terms of body composition, particularly for a growing adolescent.

On the connective tissue side, an animal model studying adolescent masculinizing therapy found that testosterone increased the maximum load the Achilles tendon could withstand before failure, while puberty suppression on its own improved overall tendon load capacity and increased cell density at the point where tendon meets bone.⁹ This is just one mouse study, so it would be premature to draw firm conclusions for humans. But it raises interesting questions about whether tendons and ligaments adapt alongside muscle during hormone therapy, or whether the connective tissue remodeling follows a separate schedule.

Blood Changes That Affect Muscle Performance

Hormones do not just change muscle tissue directly. They also change the blood that feeds it, and those shifts have real consequences for endurance and recovery. Hemoglobin and hematocrit, two measures of the blood’s oxygen-carrying capacity, move in opposite directions depending on the type of hormone therapy.

Trans women on feminizing hormones see a downward trend in both hemoglobin and hematocrit, while trans men on testosterone see both rise.¹⁰ The speed of these changes is striking. In trans women, hemoglobin falls to match typical female ranges within about three to four months of starting GAHT and stays there for as long as therapy continues.¹¹ That rapid decline in oxygen-carrying capacity may contribute to reduced endurance performance even before significant muscle mass has been lost.

On the masculinizing side, the blood changes carry a clinical watch point. Trans men experienced a seven-fold higher rate of erythrocytosis, an excess of red blood cells, compared to cisgender males, and an 83-fold higher rate compared to cisgender females.¹⁰ This is one reason why regular blood monitoring is a standard part of masculinizing hormone therapy. Excessively high hematocrit raises the risk of blood clots and cardiovascular events, so clinicians typically adjust testosterone dosing or add therapeutic measures if levels climb too high.

Aerobic Capacity in Trans Athletes

A cross-sectional study comparing transgender athletes to cisgender athletes found some counterintuitive results. Trans women athletes actually had lower relative VO₂ max (a standard measure of aerobic fitness) than cisgender women athletes, not higher.¹² Trans men had lower absolute VO₂ max than cisgender men. These findings complicate the assumption that prior testosterone exposure always translates into a lasting performance edge, though it is worth noting that cross-sectional studies cannot separate the effects of hormones from differences in training history, motivation, and self-selection into athletics.

Muscle Quality and Fat Infiltration

Beyond sheer size and strength, the internal composition of muscle tissue itself changes during hormone therapy. Researchers use a measure called muscle fat infiltration (MFI) to assess how much fat has seeped into the muscle, which affects the tissue’s contractile quality. A five-to-six-year longitudinal study found that MFI decreased by eight percent in trans men and increased by four percent in trans women.¹³ In both groups, however, the change was driven entirely by the shift from baseline to twelve months. After that first year, MFI stayed stable for the remaining four to five years of follow-up.

This pattern of early change followed by a plateau appears repeatedly across different muscle measures. It suggests that the hormonal environment reshapes muscle composition relatively quickly, and then the new equilibrium holds. Whether exercise, nutrition, or other interventions can push beyond that plateau is an open question.

For organ-level fat deposits, a six-month study using magnetic resonance imaging found that pancreatic, liver, and heart muscle lipid contents did not significantly change in either trans men or trans women after half a year of GAHT.¹⁴ That is reassuring from a metabolic health standpoint, though longer-term data would be useful.

Why Measuring Body Composition in Trans People Is Tricky

One underappreciated wrinkle in this whole area is the challenge of measuring body composition accurately. Bioelectrical impedance analysis (BIA), the technology used in many clinical settings and consumer body composition scales, uses algorithms calibrated to either “male” or “female” body types. Neither setting fits a person whose body composition is actively transitioning between those norms.

A study comparing BIA to DXA scans (the gold standard) in trans men found that BIA underestimated fat mass regardless of which sex setting was used, by about five to seven percentage points.¹⁵ BIA also overestimated muscle mass, especially for the trunk, by as much as 6.6 kg when set to “female” and 5.7 kg when set to “male.” For fat measurement in the extremities, the “female” setting came closer to DXA results, while for trunk muscle mass, the “male” setting was more accurate.

What this means practically is that if you are a trans person tracking your body composition at home or even in a clinic using BIA, the numbers may be systematically off. The direction of the error depends on which setting your device uses, and neither option is calibrated for a body in hormonal transition. DXA scanning provides more reliable data, but it is less accessible and more expensive. Researchers in this field increasingly acknowledge that BIA results in trans populations need to be interpreted with caution.

The Molecular Level

Testosterone and estradiol do not just make muscles bigger or smaller in a generic way. Research on primary human muscle cells has shown that the two hormones have “profound but distinct effects” on amino acid incorporation into multiple individual proteins with specific functions.¹⁶ In other words, testosterone does not just turn up the volume on protein synthesis across the board. It preferentially drives the production of certain structural and contractile proteins, while estradiol favors a different protein profile. This means the qualitative nature of the muscle, not just its quantity, shifts during hormone therapy.

Testosterone also acts at the cellular infrastructure level. Higher doses increase the number of satellite cells, which are the muscle stem cells responsible for repair and growth, as well as the number of nuclei within each fiber.⁴ In studies of cisgender men, the increase in satellite cell percentage roughly correlated with changes in testosterone concentration. This cellular expansion is part of what makes testosterone such a powerful driver of hypertrophy and why the effects, once established, may be somewhat durable even if hormone levels later change. The concept sometimes called “muscle memory” in exercise science has cellular roots here: once new myonuclei are added to a fiber, they may persist long after the stimulus that created them fades.

What Happens If Hormones Are Stopped

This is a question many trans individuals and their clinicians think about but that researchers have barely studied directly. Most of the longitudinal data covers people on continuous hormone therapy. There is very little published evidence on what happens to muscle mass and strength if someone discontinues GAHT, whether by choice, for medical reasons, or due to access issues.

What can be inferred from the broader endocrinology literature is that muscle tissue will gradually adapt to whatever hormonal environment it finds itself in. A trans man who stops testosterone would be expected to lose some of the gained lean mass and strength over time, though the rate and completeness of that reversal are unknown. A trans woman who stops estrogen and anti-androgens would likely see some recovery of testosterone-driven muscle characteristics, assuming the gonads are still functional. For individuals who have had gonadectomy, stopping exogenous hormones would mean a very low-hormone state that could accelerate muscle and bone loss, making the clinical picture more urgent.

The satellite cell and myonuclear data add a layer of complexity. If testosterone exposure truly creates lasting increases in myonuclear number, then some structural foundation for muscle regrowth could persist even after testosterone levels drop. Whether this is clinically meaningful in trans individuals or primarily a theoretical consideration from animal and cisgender male studies remains unclear.

Exercise as a Wild Card

Nearly all the studies discussed here tracked changes during hormone therapy without controlling rigorously for physical activity. That is a major limitation, because exercise is one of the most powerful independent drivers of muscle mass and strength. A trans woman on feminizing hormones who trains with progressive resistance may maintain or even gain strength despite the hormonal push toward muscle loss. A sedentary trans man on testosterone will gain lean mass, but less than one who trains consistently.

The systematic review on strength changes during GAHT explicitly noted that physical activity was inconsistently reported and rarely controlled for across studies.² This means the average changes described in the literature are an aggregate of people with very different lifestyles. Your individual trajectory during hormone therapy will be shaped by what you do with your body as much as by what hormones are circulating through it. For anyone starting GAHT who is concerned about muscle changes going too far in either direction, resistance training is the most controllable lever available.

1
PubMed Central. The Impact of Gender-Affirming Hormone Therapy on Physical Performance
2
PubMed Central. Muscle strength changes and physical activity during gender-affirming hormone therapy: A systematic review
3
PubMed Central. How does hormone transition in transgender women change body composition, muscle strength and haemoglobin? Systematic review with a focus on the implications for sport participation
4
PubMed. Testosterone-induced muscle hypertrophy is associated with an increase in satellite cell number in healthy, young men
5
PubMed Central. Onset of effects of testosterone treatment and time span until maximum effects are achieved
6
PubMed. Testosterone therapy for transgender men
7
PubMed. Age-Dependent Muscular Response to Testosterone-Based Gender-Affirming Therapy: Evidence From a 1-Year Observational Study
8
The Journal of Clinical Endocrinology & Metabolism. Time Course of Body Composition Changes in Transgender Adolescents During Puberty Suppression and Sex Hormone Treatment
9
PubMed Central. Functional Changes to Achilles Tendon and Enthesis in a Mouse Model of an Adolescent Masculine Gender-Affirming Hormone Treatment
10
PubMed Central. Longitudinal Changes in Hematologic Parameters Among Transgender People Receiving Hormone Therapy
11
British Journal of Sports Medicine. How does hormone transition in transgender women change body composition, muscle strength and haemoglobin? Systematic review with a focus on the implications for sport participation
12
PubMed Central. Strength, power and aerobic capacity of transgender athletes: a cross-sectional study
13
PubMed Central. Longitudinal changes in regional fat and muscle composition and cardiometabolic biomarkers over 5 years of hormone therapy in transgender individuals
14
PubMed Central. The effects of gender-affirming hormone therapy on myocardial, hepatic, pancreatic lipid content, body fat distribution and other cardiometabolic risk factors
15
Clinical Nutrition Open Science. Is bioimpedance analysis feasible in transgender men?
16
PubMed Central. Impact of biological sex and sex hormones on molecular signatures of skeletal muscle at rest and in response to distinct exercise training modes