The relationship of agonist muscle single motor unit firing rates and elbow extension limb movement kinematics

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 (∼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.NEW & NOTEWORTHY Inducing postactivation potentiation (PAP) increased motor unit (MU) recruitment thresholds, but decreased firing rates during dynamic isotonic contractions at 50% and 75% peak power compared with baseline (no PAP). Compared with previously described isometric tasks, modifications in MU activity with PAP were larger across a wider range of contractile intensities (75% maximum). Single MUs are responsive to activation history force potentiation and make compensatory adjustments in relation to the active state of the muscle.

Effects of the Proprioceptive Neuromuscular Facilitation Contraction Sequence on Motor Skill Learning-Related Increases in the Maximal Rate of Wrist Flexion Torque Development

Background: The proprioceptive neuromuscular facilitation (PNF) reciprocal contraction pattern has the potential to increase the maximum rate of torque development. However, it is a more complex resistive exercise task and may interfere with improvements in the maximum rate of torque development due to motor skill learning, as observed for unidirectional contractions. The purpose of this study was to examine the cost-benefit of using the PNF exercise technique to increase the maximum rate of torque development.

Methods: Twenty-six participants completed isometric maximal extension-to-flexion (experimental PNF group) or flexion-only (control group) contractions at the wrist. Ten of the assigned contractions were performed on each of three sessions separated by 48-h for skill acquisition. Retention was assessed with 5 contractions performed 2-weeks after acquisition. Torque and surface electromyographic (sEMG) activity were analyzed for evidence of facilitated contractions between groups, as well as alterations in muscle coordination assessed across test sessions. The criterion measures were: mean maximal isometric wrist flexion toque; the maximal rate of torque development (dτ/dt_max); root-mean-square error (RMSE) variability of the rate of torque versus torque phase-plane; the rate of wrist flexion muscle activation (Q₃₀); a coactivation ratio for wrist flexor and extensor sEMG activity; and wrist flexor electromechanical delay (EMD).

Results: There were no significant differences between groups with respect to maximal wrist flexion torque, dτ/dt_max or RMSE variability of torque trajectories. Both groups exhibited a progressive increase in maximal strength (+23.35% p < 0.01, η² = 0.655) and in dτ/dt_max (+19.84% p = 0.08, η² = 0.150) from the start of acquisition to retention. RMSE was lowest after a 2-week rest interval (−18.2% p = 0.04, η² = 0.198). There were no significant differences between groups in the rate of muscle activation or the coactivation ratio. There was a reduction in coactivation that was retained after a 2-week rest interval (−32.60%, p = 0.02, η² = 0.266). Alternatively, EMD was significantly greater in the experimental group (Δ 77.43%, p < 0.01, η² = 0.809) across all sessions. However, both groups had a similar pattern of improvement to the third consecutive day of testing (−16.82%, p = 0.049, η² = 0.189), but returned close to baseline value after the 2-week rest interval.

Discussion: The wrist extension-to-flexion contraction pattern did not result in a greater maximal rate of torque development than simple contractions of the wrist flexors. There was no difference between groups with respect to motor skill learning. The main adaptation in neuromotor control was a decrease in coactivation, not the maximal rate of muscle activation.

Post-activation potentiation induced by concentric contractions at three speeds in humans

NEW FINDINGS

What is the central question of this study? Is the degree of in human muscle affected by different shortening velocities, or contraction type? What are the main findings and their importance? The PAP response following maximal concentric contractions was independent of velocity. Slow and moderate velocity maximal contractions produced PAP responses like those from maximal isometric contractions when matched for contraction duration. Despite contraction type differences in cross-bridge and Ca²⁺ kinetics, maximal contractions, regardless of contraction modality, likely generate sufficient Ca²⁺ to induce maximal PAP.

ABSTRACT

Post-activation potentiation (PAP) is the acute enhancement of contractile properties following a brief (<10 s) high-intensity contraction. Compared with isometric contractions, little is known about the PAP response induced by concentric conditioning contractions (CCs) and the effect of velocity. In the dorsiflexors of 11 participants, twitch responses were measured following 5 s of maximal effort concentric CCs at each of 10, 20 and 50°/s. Concentric PAP responses were compared to a maximal isometric voluntary contraction (MVC) matched for contraction time. Additionally, concentric CCs were compared to isometric CCs matched for mean torque, contraction area and time. The PAP response following maximal concentric CCs was independent of velocity and there was no difference in the PAP response between concentric CCs and an isometric MVC. During maximal contractions, regardless of contraction modality, there is likely sufficient Ca²⁺ to induce a similar full PAP response, and thus there was no difference between speeds or contraction type. Following concentric CCs there was a significantly larger peak twitch torque than following their isometric torque matches (49-58%), and faster maximal rates of torque development at the three speeds (62-77%). However, these responses are likely related to greater EMG in concentric contractions, 125-129% of isometric maximum compared to 38-54%, and not to contraction modality per se. Thus, PAP responses following maximal concentric CCs are not affected by velocity and responses are not different from an isometric MVC. This indicates maximal CCs of 5 s produce a maximal PAP response independent of contraction type (isometric vs. concentric) or shortening velocity.