Is there sufficient evidence to explain the cause of sexually dimorphic behaviour in force steadiness?
Applied Physiology, Nutrition and Metabolism
Applied Physiology, Nutrition, and Metabolism 30 of sex-related physiology in tandem with results from FS studies indicate that differences in fibre 31 type, contractile properties and number of motor units (MUs) are unlikely contributors to 32 differences in FS between females and males. MU type, behaviour of the population (inclusive of 33 number of active MUs from the population), agonist-antagonist activity, maximal strength, and 34 tendon mechanics are probable contributors to sexually
... rphic behaviour in FS. To clearly 35 determine underlying causes of sex-related differences in FS, further study and reporting 36 between females and males is required. Females and males are included in many studies; 37 however, rich data on sexually dimorphic behaviour is lost when data are collapsed across sex or 38 identified as non-significant without supporting values. This poses a challenge to identifying the 39 underlying cause of females having higher CV of force than males. This review provides 40 evidence of sexually dimorphic behaviour in FS and suggests that physiological differences 41 between females and males effect neuromuscular noise, and in-turn contribute to sex-related 42 differences in FS. 43 44 Keywords: neuromuscular noise, females, women, motor unit, common synaptic input 45 Page 2 of 34 https://mc06.manuscriptcentral.com/apnm-pubs Applied Physiology, Nutrition, and Metabolism 63 defined as the magnitude of variability in the force oscillations around a pre-established target 64 force. In absolute terms, FS is quantified as the standard deviation (SD) of force output around 65 the target and, in relative terms as the coefficient of variation (CV) of force (ratio of standard 66 deviation in force fluctuations relative to the mean force). It is well established that with 67 increases in isometric force the absolute size (SD) of force fluctuations increase in a linear 68 fashion (Tracy and Enoka 2002). For relative force variability, there is a non-monotonic decrease Page 3 of 34 https://mc06.manuscriptcentral.com/apnm-pubs Applied Physiology, Nutrition, and Metabolism Page 4 of 34 https://mc06.manuscriptcentral.com/apnm-pubs Applied Physiology, Nutrition, and Metabolism 102 and/or identity classified as gender -woman or man. This distinction between sex and gender 103 will likely need to evolve with the FS literature as elements of physical activity, sport, and 104 musical training influence FS (Enoka et al. 2003) and are delineated through gender rather than 105 sex. 106 Force steadiness studies centering on age, muscle group, intensity and type of 107 contraction, as well as those inferred to gendered conditions (e.g., physical activity, training) are 108 extensive (Enoka et al. 2003, Oomen and van Dieën 2017). The study of sex-related differences 109 in FS is relatively diminutive, though shrewd evaluation of the literature revealed a number of 110 investigations undertaken in females and males. However, data is lost to reporting, as many 111 studies do not report female and male data or statistically collapse across sexes. This was 112 historically explicable given many scientists and labboratory personnel were males and 113 immediately accessible for recruitment. Moreover, when one considers how adding an 114 experimental group alters statistical power, and that menstrual cycle hormones potentially Page 5 of 34 https://mc06.manuscriptcentral.com/apnm-pubs Applied Physiology, Nutrition, and Metabolism 129 insight into potential sites of sex-related sources of neuromuscular noise contributing to 130 differential force output between females and males, sex-related physiology literature will be 131 applied in tandem with the knowledge from general FS studies to provide a framework of the 132 morphological and physiological factors that can likely be disregarded and those that should be 133 considered in the investigation of neuromuscular noise contributing to sex-related differences in 134 FS. 135 136 Likely Factors to Discount Page 6 of 34 https://mc06.manuscriptcentral.com/apnm-pubs Applied Physiology, Nutrition, and Metabolism 154 The human literature reporting whole muscle contractile properties in females and males 155 aligns with the fibre type data. The elbow flexors (Hicks and McCartney 1996), plantar flexors 156 (Simpson et al. 2018), knee extensors (Berger et al. 2012) and dorsiflexors (Hicks and 157 McCartney 1996) have significantly lower peak tension (~25%), and substantially slowed half 158 relaxation time (~30%) in females compared to males. While modelling studies suggest that an 159 increase in twitch contraction time improves FS (see Fig 9, Dideriksen et al. 2012), most Page 7 of 34 https://mc06.manuscriptcentral.com/apnm-pubs Applied Physiology, Nutrition, and Metabolism 178 have been applied in humans. Most use a general concept for MU number estimation (MUNE) 179 and two (Yerdelen et al. 2006, Gawel and Kostera-Pruszczyk 2014) have compared the estimated 180 MU numbers between females and males. These studies show no differences between the sexes, 181 but suggest that the age-related decline in the number of MUs was greater in females compared 182 with males for small muscles of the hand. The challenge with assessing sex-dependent 183 differences in MU number relative to age-related decline is that biological age is connected to Page 8 of 34 https://mc06.manuscriptcentral.com/apnm-pubs Applied Physiology, Nutrition, and Metabolism 499 2110-0. 500 Kukulka, C.G., and Clamann, H.P. 1981. Comparison of the recruitment and discharge properties 501 of motor units in human brachial biceps and adductor pollicis during isometric contractions. 502 Brain Res. 219(1): 45-55. 503 Mierzejewska-Krzyzowska, B., Bukowska, D., Taborowska, M., and Celichowski, J. 2014. Sex 504 differences in the number and size of motoneurons innervating rat medial gastrocnemius 505 muscle. Oomen, N.M.C.W., and van Dieën, J.H. 2017. Effects of age on force steadiness: A literature 524 review and meta-analysis. Ageing Res. Rev. 35: 312-321. Elsevier B.V. 525