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      Response to Letter to the Editor From Kirk & Stebbings: The Impact of Gender-Affirming Hormone Therapy on Physical Performance

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          Abstract

          We thank Kirk and Stebbings for their interest in our paper (1). Kirk and Stebbings raise the point of absolute vs relative measures (2). Elite physical or sporting performance should be judged on absolute performance (ie, race times), which is the most important factor when assessing the impact of gender-affirming hormone therapy. However, given a lack of data (only 3 studies), our review also discusses contributing factors to physical performance (such as muscle mass and VO2 peak, which are dependent on weight) (1). When comparing muscle mass among groups, such as women from different ethnicities, it is accepted that age, weight, and body mass index are covariates (3, 4), and body mass index is influenced by height. There is great diversity in heights and weights across cisgender populations, especially among elite athletes. Kirk and Stebbings raise the hypothetical situation of a 95 and 65 kg woman; if they are both cisgender women, outside of sports with defined weight categories, weight and height are not considered to be an unfair “competitive advantage.” Kirk and Stebbings claim “performance advantages [are] provided by greater stature”; however, we note that their referenced studies by Norton and Olds and Monson et al do not support this definitive assertion (5, 6). Male advantage in physical performance is driven by much more than just stature. The review by Norton and Olds found great diversity in morphology for athletes of different sports, some requiring larger players whereas others demanded smaller morphological characteristics (5). Similarly, Monson et al concluded that “athletic success are [sic] impacted by a myriad of factors, and some of the most successful professional athletes do not have particularly long arms relative to their height” (6). Kirk and Stebbings’ letter implies an equivalence between transgender women and cisgender men, assuming incorrectly that the taller height in transgender women leads to an advantage that is equivalent to the advantage that cisgender men have over cisgender women (2). This assumption overlooks the significant body composition changes (muscle mass reduces 3-5% but fat mass increases 20-30%) with feminizing hormone therapy, leading to distinct differences in muscle mass, strength, cardiorespiratory fitness, and physical performance (1). We do not advocate for sex categories to be removed, and relative measures are not equal between sexes. While the relative measures of muscle mass and strength in transgender women are no different from cisgender women, they are clearly lower than that of cisgender men (7). Kirk and Stebbings draw attention to the only published small cross-sectional study examining cardiorespiratory fitness (7). Several absolute parameters show that transgender women have an intermediate value between cisgender women and cisgender men, expected due to height differences, with relative percentage fat mass and muscle mass comparable to cisgender women (7). It is imperative that VO2 peak is corrected or adjusted for weight when making comparisons between groups of women (whether they be transgender or cisgender) (1). Kirk and Stebbings reference De Pauw et al to support their assertion that absolute VO2 peak is most important; however, De Pauw et al's systematic review of cycling parameters in males in fact concludes that relative VO2 max (and absolute peak power output) are the most important performance parameters for cycling (8). Ideally, prospective controlled studies before and after gender-affirming hormone therapy with physical performance outcomes relevant for individual sports should be performed. Until further research is conducted, our conclusions are based on existing performance data that suggest no significant difference in transgender women relative to cisgender women in terms of long-distance running, 1.5-mile running times, or sit-ups after feminizing hormone therapy (1).

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          Guidelines to classify subject groups in sport-science research.

          The aim of this systematic literature review was to outline the various preexperimental maximal cycle-test protocols, terminology, and performance indicators currently used to classify subject groups in sport-science research and to construct a classification system for cycling-related research.
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            Cardiopulmonary capacity and muscle strength in transgender women on long-term gender-affirming hormone therapy: a cross-sectional study

            For transgender women (TW) on oestrogen therapy, the effects of prior exposure to testosterone during puberty on their performance, mainly cardiopulmonary capacity (CPC), while exerting physical effort are unknown. Our objective was to evaluate CPC and muscle strength in TW undergoing long-term gender-affirming hormone therapy. A cross-sectional study was carried out with 15 TW (34.2±5.2 years old), 13 cisgender men (CM) and 14 cisgender women (CW). The TW received hormone therapy for 14.4±3.5 years. Bioimpedance, the hand grip test and cardiopulmonary exercise testing on a treadmill with an incremental effort were performed. The mean VO2peak (L/min) was 2606±416.9 in TW, 2167±408.8 in CW and 3358±436.3 in CM (TW vs CW, p<0.05; TW vs CM, p<0.0001; CW vs CM, p<0.0001). The O2 pulse in TW was between that in CW and CM (TW vs CW, p<0.05, TW vs CM, p<0.0001). There was a high correlation between VO2peak and fat-free mass/height 2 among TW (r=0.7388; p<0.01), which was not observed in the other groups. The mean strength (kg) was 35.3±5.4 in TW, 29.7±3.6 in CW and 48.4±6.7 in CM (TW vs CW, p<0.05; TW vs CM, p<0.0001). CPC in non-athlete TW showed an intermediate pattern between that in CW and CM. The mean strength and VO2 peak in non-athlete TW while performing physical exertion were higher than those in non-athlete CW and lower than those in CM.
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              Morphological evolution of athletes over the 20th century: causes and consequences.

              Over the course of the past century it has become increasingly difficult to find athletes of the size and shape required to compete successfully at the highest level. Sport is Darwinian in that only the 'fittest' reach the highest level of participation. Not every physical characteristic could be expected to play a role in this selection process, but two that are important and for which substantial data assemblies exist, are height and mass. Measurements of elite athlete sizes were obtained from a variety of sources as far back as records allowed. We charted the shift in these anthropometric characteristics of elite sportspeople over time, against a backdrop of secular changes in the general population. Athletes in many sports have been getting taller and more massive over time; the rates of rise outstripping those of the secular trend. In open-ended sports, more massive players have an advantage. Larger players average longer careers and obtain greater financial rewards. In some sports it is equally difficult to find athletes small enough to compete. In contrast, there are sports that demand a narrow range of morphological characteristics. In these sports the size of the most successful athletes over the century has remained constant, despite the drift in the population characteristics from which they are drawn. A number of social factors both drive and are driven by the search for athletes of increasingly rare morphology. These include globalisation and international recruitment, greater financial and social incentives, and the use of special training methods and artificial growth stimuli. In many sports the demand for a specific range in body size reinforces the need to adopt questionable and illegal behaviours to reach the required size and shape to compete at the top level. Future scenarios also include 'gene-farming' through assortative mating and athlete gamete banks.
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                Author and article information

                Contributors
                Journal
                J Clin Endocrinol Metab
                J Clin Endocrinol Metab
                jcem
                The Journal of Clinical Endocrinology and Metabolism
                Oxford University Press (US )
                0021-972X
                1945-7197
                August 2024
                19 March 2024
                19 March 2024
                : 109
                : 8
                : e1682-e1683
                Affiliations
                Trans Health Research Group, Department of Medicine, The University of Melbourne , Parkville, 3010, Australia
                Department of Endocrinology, Austin Health , Heidelberg, 3084, Australia
                Trans Health Research Group, Department of Medicine, The University of Melbourne , Parkville, 3010, Australia
                Trans Health Research Group, Department of Medicine, The University of Melbourne , Parkville, 3010, Australia
                Department of Endocrinology, Austin Health , Heidelberg, 3084, Australia
                Trans Health Research Group, Department of Medicine, The University of Melbourne , Parkville, 3010, Australia
                Institute for Health and Sport, Victoria University , Footscray, 3011, Australia
                Institute for Health and Sport, Victoria University , Footscray, 3011, Australia
                Australian Regenerative Medicine Institute, Monash University , Clayton, 3800, Australia
                Author notes
                Correspondence: Ada S. Cheung, PhD, MBBS (Hons), Austin Health, 145 Studley Rd, Heidelberg, Victoria 3084, Australia. Email: adac@ 123456unimelb.edu.au .
                Author information
                https://orcid.org/0000-0001-5257-5525
                https://orcid.org/0000-0003-2959-5928
                https://orcid.org/0000-0001-8836-165X
                https://orcid.org/0000-0002-3666-5261
                https://orcid.org/0000-0002-4948-1445
                https://orcid.org/0000-0003-4046-8276
                Article
                dgae131
                10.1210/clinem/dgae131
                11244266
                38501536
                a4739176-2aba-4300-9ae4-1b407380c775
                © The Author(s) 2024. Published by Oxford University Press on behalf of the Endocrine Society.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs licence ( https://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reproduction and distribution of the work, in any medium, provided the original work is not altered or transformed in any way, and that the work is properly cited. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.

                History
                : 15 December 2023
                : 29 February 2024
                : 20 March 2024
                Page count
                Pages: 2
                Funding
                Funded by: Australian Government National Health and Medical Research Council;
                Award ID: #2008956
                Funded by: NHMRC Postgraduate Research Scholarship;
                Award ID: #2003939
                Award ID: #1194159
                Categories
                Letter to the Editor Response
                AcademicSubjects/MED00250

                Endocrinology & Diabetes
                Endocrinology & Diabetes

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