Voluntary muscle activation and evoked volitional-wave responses as a function of torque

Introduction: This study employed a unique stimulation paradigm which allowed for the simultaneous assessment of voluntary activation levels (VA) via twitch-interpolation, and the evoked V-wave responses of the plantar flexors during submaximal and maximal contractions. Test-retest reliability was also examined. Methods: Fourteen participants repeated a stimulation protocol over four visits to assess VA and evoked V-wave amplitude across torque levels ranging from 20% to 100% MVC. MVC torque and EMG amplitude were also measured. Results: VA increased nonlinearly with torque production and plateaued by 80% MVC. V-wave amplitude increased linearly from 20% to 100% MVC. There were no differences in any dependent variable across visits (p > 0.05). VA demonstrated moderate to substantial reliability across all torque levels (ICC = 0.76-0.91) while V-wave amplitude exhibited fair to moderate reliability from 40% to 100% (ICC = 0.48-0.74). Discussion: We were able to reliably collect VA and the V-wave simultaneously in the plantar flexors. Collection of VA and V-wave during the same contraction provides distinct information regarding the contribution of motor-unit recruitment and descending cortico-spinal drive/excitability to force production.