Sex related differences in neuromuscular fatiguability between prepubertal children and young adults

Introduction Prepubertal children compared to adults manifest neuromuscular differences in a lower voluntary activation and fatigability, as well as a potentially lesser use of larger motor units innervating fast-twitch-fibers (Dotan et al., 2012). While children tend to fatigue more on a central than peripheral level, these trends vary based on exercise modality and muscle group (Ratel et al., 2015; Souron et al., 2022). Furthermore, females incline to fatigue less than males, but research on sex differences in children remains limited (Hunter, 2014). Therefore, the aim of this study was to investigate neuromuscular age and sex differences in a sustained isometric contraction until failure. Methods 16 prepubertal children (9.2 ± 0.9y) and 16 adults (23.4 ± 2.4y; m = f) performed a maximal voluntary isometric contraction (MVCpre), a failure-task at 60%MVCpre, followed by another MVCpost of knee extensors. Surface electromyography for rectus femoris (RF), vastus medialis (VM), and vastus lateralis muscles (VL) was analysed at 0, 25, 50, 75, and 100% of the individual time to failure (TTF). Linear models and Cohen’s d effect sizes were used to quantify age and sex differences of TTF, MVCpre-post, as well as root mean square (RMS) and median frequency (MDF) of each muscle throughout the fatiguing trial. Results Trivial differences were observed in TTF across age, sex, and age*sex-interaction (d ≤ 0.78). Children displayed a larger decline in MVCpre-post (d = 1.24), whereas sex and age*sex interactions were minimal (d ≤ 0.63). Initially (0% TTF), children had a lower activation in RF, VL and VL (d > 1.32), whereas age-differences across the failure-task were found for RMS VL, MDF RF, VL and VM (d ≥ 0.15), with no prominent sex-differences (d ≤ 0.14). Sex*age-interaction yielded detectable results only in RMS VM (d = -0.36). Conclusion The lower ability to generate an MVCpost combined with a smaller change in EMG parameters in children could be due to more central rather than peripheral fatigue. This is also supported by the lower initial activation in adults, suggesting that larger, more fatigable motor units are initially preserved, which may not be available in children. The small to large effects in neuromuscular fatigability between age and sex groups indicate the need for more in-depth research (tracking of motor units, change of voluntary activation and twitch properties) particularly with regard to sex-related disparities. References Dotan, R., Mitchell, C., Cohen, R., Klentrou, P., Gabriel, D., & Falk, B. (2012). Child-adult differences in muscle activation—A review. Pediatric Exercise Science, 24(1), 2–21. https://doi.org/10.1123/pes.24.1.2 Hunter, S. K. (2014). Sex differences in human fatigability: Mechanisms and insight to physiological responses. Acta Physiologica, 210(4), 768–789. https://doi.org/10.1111/apha.12234 Ratel, S., Kluka, V., Vicencio, S. G., Jegu, A.-G., Cardenoux, C., Morio, C., Coudeyre, E., & Martin, V. (2015). Insights into the mechanisms of neuromuscular fatigue in boys and men. Medicine & Science in Sports & Exercise, 47(11), 2319–2328. https://doi.org/10.1249/MSS.0000000000000697 Souron, R., Carayol, M., Martin, V., Enzo, P., Duché, P., & Gruet, M. (2022). Differences in time to task failure and fatigability between children and young adults: A systematic review and meta-analysis. Frontiers in Physiology, 13, Article 1026012. https://doi.org/10.3389/fphys.2022.1026012