School of Kinesiology, Western University, London, Ontario, Canada.
Faculty of Health Sciences, School of Physical Therapy, Western University, London, Ontario, Canada.
J Neurophysiol. 2023 Nov 1;130(5):1321-1333. doi: 10.1152/jn.00068.2023. Epub 2023 Oct 25.
Aging is associated with neuromuscular system changes that may have implications for the recruitment and firing behaviors of motor units (MUs). In previous studies, we observed that young adults recruit subpopulations of triceps surae MUs during tasks that involved leaning in five directions: common units that were active during different leaning directions and unique units that were active in only one leaning direction. Furthermore, the MU subpopulation firing behaviors [average firing rate (AFR), coefficient of variation (CoV), and intermittent firing] modulated with leaning direction. The purpose of this study was to examine whether older adults exhibited this regional recruitment of MUs and firing behaviors. Seventeen older adults (aged 74.8 ± 5.3 yr) stood on a force platform and maintained their center of pressure leaning in five directions. High-density surface electromyography recordings from the triceps surae were decomposed into single MU action potentials. A MU tracking analysis identified groups of MUs as being common or unique across the leaning directions. Although leaning in different directions did not affect the AFR and CoV of common units ( > 0.05), the unique units responded to the leaning directions by increasing AFR and CoV albeit modestly ( = 18.51, < 0.001). The unique units increased their intermittency with forward leaning ( = 9.22, = 0.003). The mediolateral barycenter positions of MU activity in both subpopulations were found in similar locations for all leaning directions ( > 0.05). These neuromuscular changes may contribute to the reduced balance performance seen in older adults. In this study, we observed differences in motor unit recruitment and firing behaviors of distinct subpopulations of motor units in the older adult triceps surae muscle from those observed in the young adult. Our results suggest that the older adult central nervous system may partially lose the ability to regionally recruit and differentially control motor units. This finding may be an underlying cause of balance difficulties in older adults during directionally challenging leaning tasks.
衰老与神经肌肉系统的变化有关，这些变化可能对运动单位 (MU) 的募集和放电行为产生影响。在之前的研究中，我们观察到，年轻人在涉及向五个方向倾斜的任务中募集小腿三头肌 MU 的亚群：在不同倾斜方向活跃的共同单位和仅在一个倾斜方向活跃的独特单位。此外，MU 亚群的放电行为（平均放电率 (AFR)、变异系数 (CoV) 和间歇性放电）随倾斜方向而变化。本研究的目的是检验老年人是否表现出这种 MU 的区域性募集和放电行为。17 名老年人（年龄 74.8±5.3 岁）站在力台上，保持他们的重心在五个方向上倾斜。从小腿三头肌采集的高密度表面肌电图记录被分解为单个 MU 动作电位。MU 跟踪分析确定了在所有倾斜方向上共同或独特的 MU 组。尽管向不同方向倾斜不会影响共同单位的 AFR 和 CoV（>0.05），但独特单位通过增加 AFR 和 CoV 对倾斜方向做出反应，尽管幅度较小（=18.51，<0.001）。独特单位随向前倾斜增加其间歇性（=9.22，=0.003）。两种亚群 MU 活动的中侧重心位置在所有倾斜方向上都位于相似位置（>0.05）。这些神经肌肉变化可能导致老年人平衡能力下降。在这项研究中，我们观察到老年人小腿三头肌中不同 MU 亚群的募集和放电行为与年轻人不同。我们的结果表明，老年人的中枢神经系统可能部分丧失了区域性募集和差异控制 MU 的能力。这一发现可能是老年人在定向挑战性倾斜任务中平衡困难的潜在原因。
