Abstract：
Females typically live longer than males but, paradoxically, spend a greater number of later years in poorer health. The neuromuscular system is a critical component of the progression to frailty, and motor unit (MU) characteristics differ by sex in healthy young individuals and may adapt to ageing in a sex-specific manner due to divergent hormonal profiles. The purpose of this study was to investigate sex differences in vastus lateralis (VL) MU structure and function in early to late elderly humans. Intramuscular electromyography signals from 50 healthy older adults (M/F: 26/24) were collected from VL during standardized submaximal contractions and decomposed to quantify MU characteristics. Muscle size and neuromuscular performance were also measured. Females had higher MU firing rate (FR) than males (P = 0.025), with no difference in MU structure or neuromuscular junction transmission (NMJ) instability. All MU characteristics increased from low- to mid-level contractions (P < 0.05) without sex × level interactions. Females had smaller cross-sectional area of VL, lower strength and poorer force steadiness (P < 0.05). From early to late elderly, both sexes showed decreased neuromuscular function (P < 0.05) without sex-specific patterns. Higher VL MUFRs at normalized contraction levels previously observed in young are also apparent in old individuals, with no sex-based difference of estimates of MU structure or NMJ transmission instability. From early to late elderly, the deterioration of neuromuscular function and MU characteristics did not differ between sexes, yet function was consistently greater in males. These parallel trajectories underscore the lower initial level for older females and may offer insights into identifying critical intervention periods. KEY POINTS: Females generally exhibit an extended lifespan when compared to males, yet this is accompanied by a poorer healthspan and higher rates of frailty. In healthy young people, motor unit firing rate (MUFR) at normalized contraction intensities is widely reported to be higher in females than in age-matched males. Here we show in 50 people that older females have higher MUFR than older males with little difference in other MU parameters. The trajectory of decline from early to late elderly does not differ between sexes, yet function is consistently lower in females. These findings highlight distinguishable sex disparities in some MU characteristics and neuromuscular function, and suggest early interventions are needed for females to prevent functional deterioration to reduce the ageing health-sex paradox.
摘要：
女性的寿命通常比男性长，但是，矛盾的是,在更糟糕的健康环境中度过更多的晚年。神经肌肉系统是衰弱进展的关键组成部分，在健康的年轻人中，运动单位（MU）特征因性别而异，并且由于荷尔蒙分布不同，可能会以性别特定的方式适应衰老。这项研究的目的是调查早期至晚期老年人中股外侧肌（VL）MU结构和功能的性别差异。在标准化的亚最大收缩过程中，从VL收集了来自50名健康老年人（M/F：26/24）的肌内肌电图信号，并将其分解以量化MU特征。还测量了肌肉大小和神经肌肉性能。女性的MU射血率（FR）高于男性（P=0.025），MU结构或神经肌肉接头传递（NMJ）不稳定无差异。所有MU特征从低水平收缩增加到中等水平收缩（P<0.05），没有性别×水平相互作用。雌性有较小的VL横截面积，强度较低,受力稳定性较差(P<0.05)。从早期到晚期，两种性别均显示神经肌肉功能下降（P<0.05），无性别特异性模式。先前在年轻人中观察到的归一化收缩水平较高的VLMUFR在老年人中也很明显，MU结构或NMJ传播不稳定性的估计没有基于性别的差异。从早期到晚期，性别之间神经肌肉功能和MU特征的恶化没有差异，然而，男性的功能始终更大。这些平行的轨迹强调了老年女性的较低初始水平，并可能为识别关键干预期提供见解。关键点：与男性相比，女性通常表现出延长的寿命，然而，这伴随着较差的健康状况和较高的虚弱率。在健康的年轻人中，据广泛报道，在正常收缩强度下，女性的运动单位放电率（MUFR）高于年龄匹配的男性。在这里，我们在50人中显示，老年女性的MUFR高于老年男性，其他MU参数差异不大。从早期到晚期老年人的下降轨迹在性别之间没有差异，然而，女性的功能一直较低。这些发现突出了一些MU特征和神经肌肉功能的明显性别差异，并建议女性需要早期干预以防止功能恶化，以减少衰老的健康-性别悖论。
