Applied Neuromuscular Physiology Lab, Department of Kinesiology, Applied Health, and Recreation, Oklahoma State University, Stillwater, Oklahoma, USA.
Neuromuscular Research Laboratory, Department of Sports Medicine and Nutrition, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
Acta Physiol (Oxf). 2023 Sep;239(1):e14024. doi: 10.1111/apha.14024. Epub 2023 Aug 8.
Motor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate- and high-intensity contraction intensities. Additionally, the influence of fiber composition on potential sex-related differences remains unquantified.
Eleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAP ), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMG ) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAP , IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies.
There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAP , whereas females displayed greater slopes (p = 0.001-0.007) for the IFR and MFR versus RT relationships. N-EMG at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018-0.031) with the slopes for the MUAP , IFR, and MFR versus RT relationships at 70% MVC (r = -0.599-0.585).
Both sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex-related differences in MU recruitment and firing rate patterns and N-EMG at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.
在中等和高强度收缩强度下，尚未比较男女之间的股外侧肌（VL）运动单位募集和放电率模式。此外，纤维成分对潜在性别相关差异的影响尚未量化。
11 名男性和 11 名女性进行了 40%和 70%最大自愿收缩（MVC）。从 VL 记录的表面肌电图（EMG）信号被分解。分析募集阈值（RT）、MU 动作电位幅度（MUAP）、初始放电率（IFR）、平均放电率（MFR）和稳定扭矩时的归一化 EMG 幅度（N-EMG）。计算 MUAP、IFR 和 MFR 与 RT 关系的 Y 截距和斜率。通过肌肉活检确定 I 型肌球蛋白重链同工型（MHC）。
在 40%MVC 时，男女之间 MU 特征没有性别差异。在 70%MVC 时，男性的 MUAP 斜率更大（p=0.002），而女性的 IFR 和 MFR 与 RT 关系的斜率更大（p=0.001-0.007）。女性在 70%MVC 时的 N-EMG 更大（p<0.001）。女性的 I 型%MHC 更高（p=0.006），并且与 70%MVC 时 MUAP、IFR 和 MFR 与 RT 关系的斜率相关（r=-0.599-0.585）（p=0.018-0.031）。
男女均表现出 MU 放电率与募集阈值之间的反比关系。然而，70%MVC 时男女之间 MU 募集和放电率模式以及 N-EMG 的性别差异可能归因于女性更高阈值 MU 的 I 型%MHC 更大和较小的抽搐力。有证据表明，肌肉纤维成分可能解释了男女之间不同的 MU 行为。
