Fatigue-induced changes in motor unit behavior and intermuscular coherence across knee joint positions

Neuromuscular fatigue impairs force steadiness and alters motor unit (MU) behavior. However, it remains unclear how joint angle modulates the neuromuscular response to fatigue, especially in terms of MU characteristics and intermuscular coherence. This study examined the effects of fatigue and knee joint angle on MU behavior, force steadiness, and intermuscular coherence between the soleus (SOL) and gastrocnemius medialis (GM) during isometric plantarflexion.

Sixteen healthy males performed isometric plantarflexion at 30, 50, and 70 % of maximal voluntary isometric contraction (MVIC) in pre-and post-fatigue, under two knee joint conditions: straight leg (0°, STR) and bent leg (90°, BENT) positions, in a randomized order. Fatigue was induced via repeated sustained contractions at 60 % MVIC in each position. High-density surface electromyography from SOL and GM was decomposed to quantify MU parameters, including MU action potential amplitude (MUAP), firing rate (FR), recruitment threshold (ReTHR), and decruitment threshold (DeTHR). Intermuscular coherence between MU spike trains from SOL and GM was analyzed across frequency bands.

Post-fatigue, MIVC decreased significantly in both the STR and BENT conditions (p < 0.01). Force steadiness significantly worsened post-fatigue in the BENT condition at 50 % (p < 0.01) and 70 % (p < 0.05). In SOL, MUAP and FR increased post-fatigue across all conditions, while GM showed more selective increases at higher intensities. SOL exhibited higher MUAP in the BENT condition, whereas GM showed greater activity in the STR condition. ReTHR in SOL and DeTHR in both muscles showed significant changes related to posture and fatigue. Notably, beta-band intermuscular coherence showed significant interaction effects at 50 % and 70 % (p < 0.05), with increased coherence in the BENT condition post-fatigue (p < 0.05).

Fatigue induces muscle-specific and posture-dependent neuromuscular adaptations, characterized by changes in MU properties and intermuscular coherence, particularly in the bent knee position.