Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, 1301 Sunnyside Ave, Room 101BE, Lawrence, KS, 66045, USA.
Exp Brain Res. 2019 Oct;237(10):2653-2664. doi: 10.1007/s00221-019-05612-0. Epub 2019 Aug 8.
It is common practice to examine motor unit (MU) activity according to mean firing rate (MFR) and action potential amplitude (MUAP) vs. recruitment threshold (RT) relationships during isometric trapezoidal contractions. However, it is unknown whether the rate of torque development during the linearly increasing torque phase affects the activity of MUs during such contractions. Sixteen healthy males and females performed two isometric trapezoidal muscle actions at 40% of maximum voluntary contraction (MVC), one at a rate of torque development of 5% MVC/s (SLOW) and one at 20% MVC/s (FAST) during the linearly increasing torque phase. Surface electromyography (EMG) was recorded from the vastus lateralis (VL) via a 5-pin surface array sensor and decomposed into action potential trains of individual MUs, yielding MFRs and MUAP which were regressed against RT separately for each contraction. Surface EMG amplitude recorded from leg extensors and flexors was used to quantify muscle activation and coactivation. MFR vs. RT relationship slopes were more negative (P = 0.003) for the SLOW (- 0.491 ± 0.101 pps/%MVC) than FAST (- 0.322 ± 0.109 pps/%MVC) and the slopes of the MUAP vs. RT relationships (P = 0.022, SLOW = 0.0057 ± 0.0021 mV/%MVC, FAST = 0.0041 ± 0.0023 mV/%MVC) and muscle activation of the extensors (P < 0.001, SLOW = 36.3 ± 7.82%, FAST = 34.0 ± 6.26%) were greater for SLOW than FAST. MU firing rates were lower and action potential amplitudes were larger in relation to recruitment thresholds for a contraction performed at a slower rate compared to a faster rate of isometric torque development. Differences in MU activity can exist as a function of rate of torque development during commonly performed isometric trapezoidal contractions.
在等长梯形收缩期间，根据平均放电率（MFR）和动作电位幅度（MUAP）与募集阈值（RT）的关系来检查运动单位（MU）活动是常见做法。然而，尚不清楚在线性增加扭矩阶段期间扭矩发展速度是否会影响此类收缩期间 MU 的活动。16 名健康男性和女性以最大自愿收缩（MVC）的 40%进行了两次等长梯形肌肉动作，一次在扭矩发展速度为 5% MVC/s（SLOW），一次在 20% MVC/s（FAST）在线性增加扭矩阶段。通过 5 针表面阵列传感器从股外侧肌（VL）记录表面肌电图（EMG），并将其分解为单个 MU 的动作电位列车，分别针对每个收缩，将 MFR 和 MUAP 回归到 RT。从腿部伸肌和屈肌记录的表面 EMG 幅度用于量化肌肉激活和共激活。与 FAST（-0.322±0.109 pps/%MVC）相比，SLOW（-0.491±0.101 pps/%MVC）的 MFR 与 RT 关系斜率更负（P=0.003），MUAP 与 RT 关系斜率（P=0.022，SLOW=0.0057±0.0021 mV/%MVC，FAST=0.0041±0.0023 mV/%MVC）和伸肌的肌肉激活（P<0.001，SLOW=36.3±7.82%，FAST=34.0±6.26%）也更大。与更快的等速扭矩发展相比，较慢的收缩速度会导致 MU 放电率降低，动作电位幅度相对于募集阈值增大。在常见的等长梯形收缩期间，作为扭矩发展速度的函数，MU 活动可能存在差异。
