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

Abstract

New findings: What is the central question of this study? Can bilateral knee extensor force accuracy be improved following 4 weeks of unilateral force accuracy training and are there any subsequent alterations to central and/or peripheral motor unit features? What is the main finding and its importance? In the trained limb only, knee extensor force tracking accuracy improved with reduced motor unit firing rate variability in the vastus lateralis, and there was no change to neuromuscular junction transmission instability. Interventional strategies to improve force accuracy may be directed to older/clinical populations where such improvements may aid performance of daily living activities.

Abstract: Muscle force output during sustained submaximal isometric contractions fluctuates around an average value and is partly influenced by variation in motor unit (MU) firing rates. MU firing rate (FR) 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. Ten healthy participants (seven females, three males, 27 ± 6 years, 170 ± 8 cm, 69 ± 16 kg) underwent a 4-week supervised, unilateral knee extensor force accuracy training intervention. The coefficient of variation for force (FORCE^CoV ) and sinusoidal wave force tracking accuracy (FORCE^Sinu ) 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. Knee extensor muscle strength remained unchanged following training, with no improvements in unilateral leg-balance. FORCE^CoV and FORCE^Sinu significantly improved in only the trained knee extensors by ∼13% (P = 0.01) and ∼30% (P < 0.0001), respectively. MU FR variability significantly reduced in the trained VL by ∼16% (n = 8; P = 0.001), with no further alterations to MU FR or neuromuscular junction transmission instability. 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 FR variability which was apparent in the trained limb only.

Keywords: electromyography; firing rate variability; motor unit; muscle force accuracy; neuromuscular function.

FIGURE 1

(a) Example force and intramuscular electromyography (iEMG) data recorded during a sustained isometric muscle contraction at 25% maximal voluntary contraction (MVC) in the vastus lateralis muscle. (b) Example raw data from a sinusoidal force tracking task at 25% MVC pre‐ and post‐training in the trained limb. The red line represents the requested target force, whilst the blue line represents the observed force. Subsequent calculation of area under the curve (N s) allowed for quantification of force tracking accuracy following training. (c) Unilateral balance data allowing measurement of centre of pressure (CoP), and the displacement of this (travel distance, mm), during static one‐legged standing. (d) Example raster and shimmer plots of near fibre motor unit potentials (NF MUP) extracted from decomposed iEMG recordings from the vastus lateralis muscle during 25% MVC, allowing quantification of NF MUP jiggle, an indicator of neuromuscular junction transmission instability. Inter‐discharge interval timings (ms) are indicated for each NF MUP firing in the motor unit potential train

FIGURE 2

Knee extensor maximum voluntary contraction (MVC) force (N) (a) and displacement of centre of pressure during unilateral balance tasks (b) in n = 10 young individuals (n = 9 for unilateral balance due to one participant not achieving the minimum balance time required time to complete the test) pre‐ and post‐training in the trained and untrained limb. Group means shown as bars with individual data overlaid

FIGURE 3

Knee extensor coefficient of variation for force (FORCE^CoV, %) (a) and knee extensor sinusoidal wave force tracking accuracy (FORCE^Sinu; calculated as area under the curve; N s) (b) at 25% maximal voluntary contraction pre‐ and post‐training in the trained and untrained legs of n = 10 young individuals. Group means shown as bars with individual data overlaid. *P < 0.05, ****P < 0.0001

FIGURE 4

Vastus lateralis (VL) motor unit (MU) firing rate (FR) variability (%) (a), VL MU FR (Hz) (b), and VL neuromuscular junction transmission (NMJ) instability (%) (c) at 25% maximal voluntary contraction, pre‐ and post‐training (n = 8), in the trained and untrained legs. Group means shown as bars with individual data overlaid for data visualisation only. All analyses were based on multi‐level linear regression models, where MUs were clustered to each muscle/participant. **P = 0.001