Background: Muscle force output during sustained submaximal isometric contractions fluctuates around an average value and is influenced by variability of motor unit (MU) firing (1). Traditional exercise training interventions have been shown to reduce such fluctuations in muscle force (2, 3), therefore, improving muscle force control/accuracy, which is often attributable to reduced variability in MU firing rate (4). However, much less is known with respect to MU properties following low intensity training. 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, 27±6 years, 170±8 cm, 68.7±15.7 kg) underwent 2 bilateral assessment visits (pre- and post-training) separated by 4-weeks of fully supervised unilateral knee extensor force accuracy training. Force accuracy training occurred 3x/week and consisted of 6 sinusoidal force tracking contractions at 10, 25 and 40% MVC in each training session. Knee extensor strength was assessed via maximal voluntary contractions. Unilateral balance was assessed during static one-legged standing. The coefficient of variation for force (FORCECoV) and sinusoidal wave force tracking accuracy (FORCESinu) was quantified pre- and post-training at 25% MVC. Intramuscular electromyography (iEMG) was utilised to record individual MU potentials from the vastus lateralis (VL) muscles at 25% MVC during sustained contractions. MU firing rate variability was calculated as the coefficient of variation of the interdischarge interval. Data were analysed via two-way repeated measures ANOVA for muscle strength, FORCECoV, FORCESinu and unilateral balance, with bonferroni post hoc tests used to identify statistical differences as a result of training. Multi-level mixed effects linear regression models used to analyse MU data. Statistical significance was accepted at p0.99). FORCECoV significantly improved in the trained leg by ~13% (2.80±0.58% vs. 2.39±0.40%, p=0.01) but not the untrained leg (3.02±0.71% vs. 3.00±0.65%, p>0.99). Similarly, FORCESinu significantly improved in the trained leg by ~30% (34.59±7.26 N·s vs. 23.40±3.85 N·s, p<0.0001) but not the untrained leg (32.22±6.76 N·s vs. 28.57±5.93 N·s, p=0.19). MU firing rate variability, significantly reduced by ~16% in the trained VL (n=8; ß=-2.018, 95% CI=[-3.202]-[-0.835], p=0.001), with no changes in the untrained leg (ß=0.862, 95% CI=[-0.271]-[1.995], p=0.14).
Conclusion: A 4-week period of targeted force accuracy training leads to improved muscle force control and accuracy in young healthy participants, which is associated with reduced MU firing rate variability. Importantly, these adaptations and possible mechanisms were evident in the trained limb only. These findings may influence interventional strategies to improve force accuracy in older and clinical populations to potentially aid improved balance and subsequently reduce the risk of future falls, a future direction of this work.