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The effect of residual force enhancement on motor unit activity and torque steadiness

University of British Columbia

Activation reduction (AR), characterized by a decrease in electromyographic (EMG) activity for a given torque, is a phenomenon of residual force enhancement (RFE) arising from a greater relative contribution of passive force to overall torque in isometric submaximal contractions following active-lengthening, allowing for the same torque with less muscle activation. Although alterations in motor unit (MU) activity are thought to contribute to AR, there is a gap in this understanding. This study evaluated the underlying contribution of MU properties in the tibialis anterior (TA) to AR and tested for an influence on torque steadiness (TS). It was hypothesized that MU discharge rates (MUDRs) would be lower during AR than an isometric contraction, and this would contribute to reduced TS. Ten males (26 ± 4years) performed isometric contractions at 10 and 20% maximum voluntary contraction (MVC) torque before and after an active-lengthening contraction which occurred over a 30° ankle excursion, ending at the same ankle angle (40º of plantarflexion) as a reference isometric contraction. Tibialis anterior MUs were sampled with indwelling fine wire electrodes to determine MUDRs and discharge rate variability (MUDRV). Triceps surae and TA surface EMG were sampled to evaluate antagonist co-activation and AR, respectively. Steadiness was quantified as the coefficient of variation (CV) of torque. There was a ~44% AR at 10% MVC and ~24% AR at 20% MVC (p<0.05) following active-lengthening compared with the isometric, with no difference in antagonist co-activation (p>0.05). The CV of torque was ~22% greater during AR compared to isometric at 10% (p<0.05), with no difference observed at 20% (p>0.05). Overall, the number of MUs detected was ~42% greater in the isometric condition than AR. In the isometric conditions at 10% and 20% MVC, there were 51 and 47 MUs recorded with 27 and 30 of these units quantified in both isometric and AR conditions for these torques, respectively. Motor unit DRs decreased by ~23% (p<0.05) during AR, while MUDRV did not differ between isometric and AR (p>0.05). These findings indicate that AR can be attributed to lower MUDR and fewer detectable MUs, which likely contributes to a reduction in TS.

Text

2018-07-16

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/66509

Interdisciplinary Studies

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/