Motor unit behavior of lumbar multifidus during a forward trunk bending task performed under different speeds and loads in asymptomatic participants.

Background The lumbar multifidus (LM) plays a key role in static and dynamic stability; however, studies of LM motor unit behavior have yet to be extensively investigated. This study aimed to assess the test-retest reliability of motor unit behavior measurements using electromyography decomposition (dEMG) and to investigate the motor unit behavior under different speeds and loads in asymptomatic participants. Methods In this experimental repeated-measures design, 29 male and female asymptomatic participants were recruited. Motor unit behavior was measured during two sets of 60-second active trunk flexion exercises using dEMG under two speeds (15 and 25 repetitions/minute) and two loads (5% and 10% body weight). The action potential amplitude and motor unit firing rate were derived. Intraclass correlation coefficients (ICC) were used to determine within-session test-retest reliability, and a two-factor repeated-measure ANOVA was used to determine the effects of load and speed. Results Findings demonstrated acceptable within-session test-retest reliability (ICC > 0.70) for most parameters. Significantly greater peak and average amplitudes and average firing rates were seen with an increase in speed, while greater average amplitudes and firing rates were seen with an increase in load. Conclusion These findings support the use of measures of LM motor unit behavior. Exercises at greater speeds and loads increase LM firing rates and amplitudes. A better understanding of LM motor unit behavior may aid our understanding of rehabilitation protocols for low back pain.