Introduction: Vastus medialis (VM) and lateralis (VL) act as synergists during knee extension. During low-effort isometric contractions, common neural drive [1] and differences in firing rate between VM and VL [2] are thought to represent an attempt to balance mediolateral forces acting on the patellofemoral joint. In rats, the central nervous system is able to alter these motor strategies when the relative force contribution of VM and VL is altered experimentally [3]. It is unclear whether a decreased VM force production capability due to localized muscle soreness results in similar motor adaptations in humans.

Objectives: To investigate: 1) whether electrical stimulation can be used to induce muscle soreness in a specific muscle region; and 2) whether localized distal VM soreness alters single motor unit firing properties of VM and VL.

Methods: After approval of the University Ethics committee, 23 participants (24±5 years old, 10 females) performed knee extension contractions at 20% of their maximal effort, before and 2 days after a stimulation protocol designed to induce localised muscle soreness. The protocol consisted of 100, 5s-long electrical stimulations of the distal VM at intensities as high as tolerable. Pain pressure thresholds were collected to assess muscle soreness. Motor unit firing rates were extracted from high-density surface EMG collected using 64-channel grids. In participants where at least 3 VM and 3 VL motor units could be decomposed, the between-muscle coherence was calculated between the two cumulative spike trains. Low-frequency (1-5 Hz) coherence was extracted and compared between sessions. Wilcoxon tests were used to compare pain pressure thresholds, motor unit firing rate and between-muscle coherence between sessions or muscles.

Results: Participants reported localized pain over the distal VM (intensity: 2.6±1.6 out of 10) when climbing down the stairs. Pain pressure thresholds decreased over the stimulated VM region (-40 [-51, -20]%, N=23, p<0.001), but not over VL (-2 [-24, 11]% N=23, p=0.566). During submaximal contractions at a similar effort between sessions, motor unit firing rates did not differ between sessions (VM: N=172, p=0.875; VL: N=210, p=0.125). Motor unit firing rate was larger in VM than VL at baseline (8.9 [7.8, 10.5] vs 8.6 [7.4, 9.7] Hz; N=194, p=0.017), but this difference was not observed during muscle soreness (9.0 [7.7, 10.4] vs 9.0 [7.8, 10.1] Hz, N=188, p=0.502). Between-muscle motor unit coherence was lower during muscle soreness compared to baseline (-3.3 [-0.6, -4.6] Hz; N=12, p=0.015).

Conclusions: Localized electrical stimulation successfully induced localized muscle soreness. Similar to what has been observed in individuals with patellofemoral pain [2], localized muscle soreness resulted in a loss of muscle-specific patterns of activation, possibly by means of a reduced low-frequency neural drive between the VM and VL. Although the motor adaptation observed is small, these results support the notion that the central nervous system can alter the activation strategy between synergists in the presence of muscle soreness, possibly to compensate for differences in force production between synergists.