The reliability of surface EMG derived motor unit variables

Motor unit (MU) recordings obtained from surface electromyography (sEMG) decomposition are used to investigate the neural control of muscle in response to interventions, but our understanding of the longer-term reliability of MU variables is limited. This study examined the reliability of MU variables in the flexor carpi radialis (FCR) and tibialis anterior (TA) over a three-month period. Forty college-aged participants completed isometric wrist flexion (n = 20) and dorsiflexion (n = 20). There were 3 maximal isometric voluntary contractions (MVC) and 3 ramp contractions to 60% of MVC on four separate sessions separated by a total of 13 weeks. Intraclass correlation coefficients (ICC) were calculated from a fully nested ANOVA model. Maximal force was highly reliable (ICC = 0.94-0.99). The ICC values ranged from 0.49 to 0.92 for the FCR MU variables and from 0.58 to 0.96 for the TA MU variables. All MU variables exhibited a high degree of stability of means across test session and consistency within subjects, with the exception of the number of MUs detected in the TA. Poor ICC values did not reflect poor reliability but rather, convergence towards a narrow range of physiologically normal values. Surface EMG decomposition of a large population of MUs showed no differences in common drive between FCR (0.273) and for the TA (0.267) across test sessions. Forty percent of the sampled MUs in both muscles had a common drive of 0.30 or greater, which provides indirect support for the validity of the decompositions. MU variables may be used to monitor adaptations to a longer-term intervention study.