EMG-EMG coherence during three-digit grasping

40 Fingertip force control requires fine coordination of multiple hand muscles within 41 and across the digits. While the modulation of neural drive to hand muscles as a function 42 of force has been extensively studied, much less is known about the effects of fatigue on 43 the coordination of simultaneously active hand muscles. We asked eight subjects to 44 perform a fatiguing contraction by gripping a manipulandum with thumb, index, and 45 middle fingers while matching an isometric target force (40% maximal voluntary force) 46 for as long as possible. The coordination of twelve hand muscles was quantified as 47 electromyographic (EMG) muscle activation pattern (MAP) vector and EMG-EMG 48 coherence. We hypothesized that muscle fatigue would cause uniform changes in EMG 49 amplitude across all muscles and an increase in EMG-EMG coherence in the higher 50 frequency bands but with an invariant heterogeneous distribution across muscles. 51 Muscle fatigue caused a 12.5% drop in the maximum voluntary contraction force 52 (P < 0.05) at task failure and an increase in the standard deviation of force (P < 0.01). 53 Although EMG amplitude of all muscles increased during the fatiguing contraction (P < 54 0.001), the MAP vector orientation did not change, indicating that a similar muscle 55 coordination pattern was used throughout the fatiguing contraction. Lastly, EMG-EMG 56 coherence (0-35 Hz) was significantly greater at the end than at the beginning of the 57 fatiguing contraction (P < 0.01) but was heterogeneously distributed across hand 58 muscles. These findings suggest that similar mechanisms are involved for modulating and 59 sustaining digit forces in non-fatiguing and fatiguing contractions, respectively. 60