Inducing postactivation potentiation alters human motor unit behavior of the elbow extensors during unconstrained velocity contractions

Abstract

The purpose was to test whether inducing post-activation potentiation (PAP) altered motor unit (MU) activity during dynamic isotonic contractions. From 12 participants (3 females), 39 MUs were recorded from the anconeus (n=31) and lateral triceps brachii (n=8) with fine-wire electrodes during elbow extensions at 50 and 75% of peak power with, and without PAP. To induce PAP participants produced a 2s ramp conditioning contraction (CC) up to maximal isometric elbow extension with a 3s hold. Following the CC (~2s), independent electrical stimulation to the triceps and anconeus showed twitch torques were potentiated by 84 and 66%, respectively, (both p<0.001). Compared to baseline (i.e., without PAP), at both intensities (50 and 75%) PAP increased MU recruitment thresholds (40% and 80%, p<0.001) with lowered mean MU rates (-20 and -26%), and instantaneous rates at recruitment threshold (-26 and -25%) (all p<0.001). Firing rates increased 20% (p<0.001) from 50 to 75% power, but rates during potentiated contractions targeting 75% were lower than baseline at 50% (-10%, p<0.001). Dynamic contractions provide a more functional paradigm to assess MU activity with PAP and showed larger effects across a wider range of contractile intensities compared to previously described isometric tasks. Findings indicate that peripheral feedback from the potentiated muscle is likely not the primary mechanism in modifying MU behaviors as changes occurred at recruitment which is relatively insensitive to afferent feedback. Therefore, MU activity during dynamic contractions is responsive to activation history force potentiation and can make compensatory adjustments to optimize contractile output.