Training induced improvements in knee extensor force accuracy are associated with reduced vastus lateralis motor unit firing variability

Background Muscle force output during sustained submaximal isometric contractions fluctuates around an average value and is known to be influenced by variation in motor unit (MU) firing rates. MU firing rate variability seemingly reduces following exercise training interventions, however, much less is known with respect to peripheral MU properties. We therefore investigated whether targeted force accuracy training could lead to improved muscle functional capacity and control, in addition to determining any alterations of individual MU features. Methods Ten healthy participants (7 females, 3 males, 27±6 years, 170±8 cm, 69±16kg) underwent a 4-week supervised, unilateral, force accuracy training intervention. The coefficient of variation for force (FORCECoV) and sinusoidal wave force tracking accuracy (FORCESinu) were determined at 25% maximal voluntary contraction (MVC) pre- and post-training. Intramuscular electromyography was utilised to record individual MU potentials from the vastus lateralis (VL) muscles at 25% MVC during sustained contractions, pre- and post-training. Results Knee extensor muscle strength remained unchanged following training, with no improvements in unilateral leg-balance. FORCECoV and FORCESinu significantly improved in only the trained knee extensors by ~13% ( p =0.01) and ~30% ( p <0.0001) respectively. MU firing rate variability significantly reduced in the trained VL by ~16% ( n =8; p =0.001), with no further alterations to MU firing rate or neuromuscular junction transmission instability. Conclusion Our results suggest muscle force control and tracking accuracy is a trainable characteristic in the knee extensors, which is likely explained by the reduction in MU firing rate variability apparent in the trained limb only. ### Competing Interest Statement The authors have declared no competing interest. * FORCECoV : coefficient of variation for force FORCESinu : sinusoidal wave force tracking accuracy iEMG : intramuscular electromyography MU : motor unit MUP : motor unit potential MUPT : motor unit potential train MVC : maximal voluntary contraction NF : near fibre NMJ : neuromuscular junction RET : resistance exercise training VL : vastus lateralis 1RM : 1-repetition maximum.