Tracking of Individual Motor Units Following Concentric and Eccentric Exercise‐induced Fatigue Reveals Contraction‐type Specific Changes in Discharge Properties

Abstract

Muscle fatigue is characterised by a reduction in force output, with increases in force production partly mediated by adjustments in motor unit (MU) firing properties. Conflicting research has found MU firing rate (FR) to both increase and decrease following fatiguing exercise and this disparity may be influenced by contraction type. Using high density surface electromyography (HD-EMG) we have previously reported a decrease in FR following concentric (CON), and an increase following eccentric (ECC) contractions (1). However, HD-EMG also enables the tracking of individual MUs across tasks (2), potentially revealing more detailed MU-specific adaptations to fatigue. The aims of this study were to track the FR of individual MUs following CON and ECC contractions of the vastus lateralis (VL) muscle. HD-EMG was used to identify individual MU potentials (MUPs) from the VLs of 8 young volunteers (4 females; 21 ± 0.5 years). All MUPs were recorded during 4 sustained isometric contractions held at a target line of 25 and 40% of the participants maximum voluntary contraction (MVC) before and after completing CON and ECC fatiguing exercise (CON: ‘stepping up’ with one leg, ECC: ‘stepping down’ on the other leg, wearing a weighted vest +25-40% body weight). The coefficient of variation of force was calculated to assess force steadiness (FS). A total of 43 MUs were tracked pre and post fatigue. Paired t-tests were used for comparisons of tracked MUs. Significance was assumed when p<0.05. MVC decreased in both CON and ECC legs post exercise (-15.3% and -21.1% respectively; p<0.01) and EMG root-mean square amplitude (RMS) significantly decreased in both legs after fatigue (both p≤0.001). FS deteriorated in ECC only at both 25% and 40% MVC (p=0.004 and p=0.04 respectively), with no change in CON. Data from individual tracked MUs supported our previous finding with a decreased FR following CON (-4.7%; p=0.037) but no difference in FR with ECC (+6.9%; p=0.168). These data demonstrate that motor control and MU FR following exercise-induced fatigue differ according to exercise modality. HD-EMG enables specific changes in single motor units to be studied across sessions in addition to the overall MU population effects, and here the FR did not change following ECC exercise even with a notably reduced absolute force (21%). The possibility of differential MU recruitment strategies compensating for discharge characteristics and impacting motor control requires further investigation as this may have translational relevance in ageing, neuromuscular conditions and for post-exercise recovery from fatigue. 1. Jones EJ, Martinez-Valdes E, Negro F, McCormick D, Atherton PJ, Phillips BE, et al. Motor unit discharge properties following concentric and eccentric exercise-induced fatigue are dependent upon contraction type. American Physiological Society: Integrative Physiology of Exercise. 2020. 2. Martinez-Valdes E, Negro F, Falla D, Dideriksen JL, Heckman C, Farina D. Inability to increase the neural drive to muscle is associated with task failure during submaximal contractions. J Neurophysiol. 2020.