Neuromuscular dynamics during isometric knee contractions: effects of target force, knee angle, and tibial rotation on force steadiness.

This study investigated neuromuscular variations across low to moderate target force levels, three knee joint angles and tibial rotation positions, examining their correlations with muscle morphology and their collective impact on force steadiness (FS). Twelve young adult males performed knee extension tasks under three different tibial rotation conditions: internal rotation (IR), neutral rotation (NR), and external rotation (ER). All tasks involved submaximal isometric contractions at the knee joint. Participants performed submaximal isometric contractions at three knee joint angles (30°, 60°, and 90°) and three target force levels (10%, 40%, 70% of maximum voluntary isometric contraction (MVIC)). The electromyographic (EMG) signals from vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) were recorded. FS was quantified using standard deviation (SD) and coefficient of variation (CV) of force output, while EMG steadiness was assessed using SD and CV of the EMG signals. Muscle morphology features, including muscle thickness (MT) and echo intensity (EI), were assessed via ultrasound imaging. Submaximal isometric contractions revealed significant FS differences across target force levels and knee angles (angle × target force interaction: F(4,44) = 3.073, P = 0.026). With increasing target force from 10 to 70% MVIC, quadriceps EMG signals showed progressive amplitude increases in all three muscles (RF, VM and VL, p < 0.05).While tibial rotation showed no significant effect on FS measures (P > 0.05), it significantly influenced EMG characteristics at 60° knee angle (tibial rotation × target force × muscle interaction: F(8,88) = 3.357, P = 0.026). Strong correlations were found between force steadiness and EMG steadiness (r = 0.557-0.657, P < 0.001). Additionally, RF echo intensity positively correlated with EMG_SD in both ER and NR positions (r = 0.644 and 0.619 respectively, P < 0.05), while quadriceps muscle thickness negatively correlated with EMG_CV in ER position (r = -0.600, P < 0.05). FS revealed that absolute fluctuations (SD) increased with target force levels, while relative fluctuations (CV) decreased. Target force levels and knee joint angles significantly influence FS, whereas tibial rotation does not directly affect FS during isometric contractions. However, tibial rotation interestingly affects EMG steadiness, and the variability in EMG-derived muscle activation correlates with both FS and muscle morphology parameters. These findings provide valuable insights into neuromuscular control mechanisms during force production.