Effects of fatigue on intra-muscle force-stabilizing synergies

We applied the recently introduced concept of intra-muscle synergies in spaces of motor units (MUs) to quantify indices of such synergies in the tibialis anterior during ankle dorsiflexion force production tasks and their changes with fatigue. We hypothesized that MUs would be organized into robust groups (MU-modes), which would co-vary across trials to stabilize force magnitude, and the indices of such synergies would drop under fatigue. Healthy, young subjects (n = 14; 7 females) produced cyclical, isometric dorsiflexion forces while surface electromyography was used to identify action potentials of individual MUs. Principal component analysis was used to define MU-modes. The framework of the uncontrolled manifold (UCM) was used to analyze inter-cycle variance and compute the synergy index, ∆VZ. The tests were repeated after a non-fatiguing exercise (control) and fatiguing exercise. Across subjects, fatigue led, on average, to a 43% drop in maximal force and fewer identified MUs per subject (29.6±2.1 vs. 32.4±2.1). The first two MU-modes accounted for 81.2±0.08% of variance across conditions. Force-stabilizing synergies were present across all conditions and was diminished after fatiguing exercise (1.49±0.40), but not control exercise (1.76±0.75). Decreased stability after fatigue was caused by an increase in the amount of variance orthogonal to the UCM. These findings contrast with earlier studies of multi-effector synergies demonstrating increased synergy index under fatigue. We interpret the results as reflections of a drop in the gain of spinal reflex loops under fatigue. The findings corroborate an earlier hypothesis on the spinal nature of intra-muscle synergies.