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

The purpose of this study was to test whether inducing postactivation 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 2-s ramp conditioning contraction (CC) up to maximal isometric elbow extension with a 3-s hold. Following the CC (similar to 2 s), independent electrical stimulation to the triceps and anconeus showed twitch torques that were potentiated by 84% and 66%, respectively (both P < 0.001). Compared with baseline (i.e., without PAP), at both intensities (50% and 75%) PAP increased MU recruitment thresholds by 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 with 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 that is relatively insensitive to afferent feedback. Therefore, MU activity during dynamic contractions is responsive to activation history force potentiation and adapts to optimize contractile output.