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Anna Wiik 1 , Mats Holmberg 1 , Tommy Lundberg 1 , Mats Lilja 1 , Daniel Andersson 1 , Stefan Arver 1 , Thomas Gustafsson 1 1. Karolinska Institutet 

"Background 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 and mass remains poorly described. The aim of the current study was to assess the effects of an altered sex hormone pattern on muscle strength and cross-sectional area. 

Methods Twelve transgender individuals, 6 trans women and 6 trans men 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. The cross-sectional area and radiological density of the thigh muscles were assessed by CT scans performed bilaterally at the midpoint of femur of each subject at baseline and after 11 months of cross-sex hormone treatment. 

Results and Conclusions Muscle area increased 17% in trans men (p<0.001) with an 8% increase in radiology density after eleven month of cross-sex hormone treatment. No change was seen in trans women. There were significant (P<0.05) group x time interactions at each angular velocity. Thus, while the trans men increased their strength over the four time points, strength levels were generally maintained in the trans women. When averaging the three strength tests, knee extension (16%) and knee flexion (34%) strength increased from T1 to T4 in trans men. The corresponding changes in the trans women group were -6% and 0%, respectively. CONCLUSIONS: These results show that ~1 year of cross-sex hormone treatment results in increased muscle strength in trans men. Cross-sectional area and radiological density is also increased after testosterone treatment. However, trans women maintain their strength levels as well as cross-sectional area and radiological density throughout the treatment period. We conclude that the altered sex hormone pattern induced by gender-affirming treatment deferentially affect muscle strength in trans men vs. trans women."

*Trans Women are men who self identify as women*