• Anna Wiik, Tommy R Lundberg, Eric Rullman, Daniel P Andersson, Mats Holmberg, Mirko Mandić, Torkel B Brismar, Dahlqvist Leinhard Olof O AMRA Medical AB, Linköping, Sweden. Department of Medical and Health Sciences,, Setareh Chanpen, John N Flanagan, Stefan Arver, and Thomas Gustafsson.
• Department of Laboratory Medicine, Division of Clinical Physiology, Karolinska Institutet, and Unit of Clinical Physiology, Karolinska University Hospital, Stockholm, Sweden.
• J. Clin. Endocrinol. Metab. 2020 Mar 1; 105 (3).

ContextAs many sports are divided in male/female categories, governing bodies have formed regulations on the eligibility for transgender individuals to compete in these categories. Yet, the magnitude of change in muscle mass and strength with gender-affirming treatment remains insufficiently explored.ObjectiveThis study explored the effects of gender-affirming treatment on muscle function, size, and composition during 12 months of therapy.Design, Settings, ParticipantsIn this single-center observational cohort study, untrained transgender women (TW, n = 11) and transgender men (TM, n = 12), approved to start gender-affirming medical interventions, underwent assessments at baseline, 4 weeks after gonadal suppression of endogenous hormones but before hormone replacement, and 4 and 12 months after treatment initiation.Main Outcome MeasuresKnee extensor and flexor strength were assessed at all examination time points, and muscle size and radiological density (using magnetic resonance imaging and computed tomography) at baseline and 12 months after treatment initiation.ResultsThigh muscle volume increased (15%) in TM, which was paralleled by increased quadriceps cross-sectional area (CSA) (15%) and radiological density (6%). In TW, the corresponding parameters decreased by -5% (muscle volume) and -4% (CSA), while density remained unaltered. The TM increased strength over the assessment period, while the TW generally maintained their strength levels.ConclusionsOne year of gender-affirming treatment resulted in robust increases in muscle mass and strength in TM, but modest changes in TW. These findings add new knowledge on the magnitude of changes in muscle function, size, and composition with cross-hormone therapy, which could be relevant when evaluating the transgender eligibility rules for athletic competitions.© Endocrine Society 2019. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

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