Many biological differences seen in men and women are driven by relative differences in estrogen and testosterone levels. In transgender individuals, gender-affirming treatment includes inhibition of endogenous sex hormones and subsequent replacement with the cross-sex hormones. Yet, the effect of this treatment on functional muscle strength remains poorly described.
PURPOSE: The aim of the current study was to assess the effects of an altered sex hormone pattern on muscle strength.
METHODS: Twelve transgender individuals, 6 male to female (tranwomen) and 6 female to male (transmen) who had been accepted to start gender-affirming medical intervention, were recruited. Knee extensor and flexor muscle strength was assessed using isokinetic dynamometry at three different angular velocities (0, 60 and 90 °/s). The assessments were made at four time points: (T1) before treatment initiation, (T2) four weeks after initiated gonadal hormonal down regulation but before hormone replacement, (T3) three months after hormone replacement therapy and (T4) eleven months after hormone replacement therapy.
RESULTS: There were significant (P<0.05) group x time interactions at each angular velocity. Thus, while the transmen increased their strength over the four time points, strength levels were generally maintained in the transwomen. When averaging the three strength tests, knee extension (16%) and knee flexion (34%) strength increased from T1 to T4 in transmen. The corresponding changes in the transwomen group were -6% and 0%, respectively.
CONCLUSIONS: These results show that ~1 year of cross-sex hormone treatment results in increased muscle strength in transmen. However, transwomen maintain their strength levels throughout the treatment period. We conclude that the altered sex hormone pattern induced by gender-affirming treatment differentially affect muscle strength in transmen vs. transwomen.