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

Abstract

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/d t _{m a x} ); root-mean-square error (RMSE) variability of the rate of torque versus torque phase-plane; the rate of wrist flexion muscle activation (Q ₃₀); 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/d t _{m a x} or RMSE variability of torque trajectories. Both groups exhibited a progressive increase in maximal strength (+23.35% p < 0.01, η ² = 0.655) and in dτ/d t _{m a x} (+19.84% p = 0.08, η ² = 0.150) from the start of acquisition to retention. RMSE was lowest after a 2-week rest interval (-18.2% p = 0.04, η ² = 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, η ² = 0.266). Alternatively, EMD was significantly greater in the experimental group (Δ 77.43%, p < 0.01, η ² = 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, η ² = 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.

Keywords: PNF; electromyography; flexor carpi radialis; motor learning; muscle mechanics.

FIGURE 1

The ideal torque-time curves shown to participants in the control group (top panel) and the experimental group (bottom panel).

FIGURE 2

Representative wrist torque (red), rate of torque (blue), and surface electromyographic (sEMG) activity (gray) for the control group (top panel) and experimental group (bottom panel). The graphs illustrate the onset and termination of the torque (τ) development phase of the contraction, where the maximum rate of torque development (dτ/dt_max) was the peak of the curve between these two points. The double threshold algorithm used to detect sEMG onset was the same for both the control and experimental groups. The figure inset associated with the bottom panel illustrates how the algorithm was applied to the PNF contraction pattern.

FIGURE 3

Representative phase plane trajectories for the control (left) and experimental (right) groups. Phase plane trajectories (thick dark lines) were constructed by plotting the rate of torque (y) versus torque (x), during the torque development phase of the contraction. The thin lines mapped onto the phase plane trajectories are variability ellipses with radii equal to one standard deviation in the rate of torque (y) and torque (x) directions. Maximal isometric flexion was initiated while the wrist was generating an extension torque and negative. The transition point between the negative torque in extension (blue) and a positive toque in flexion (red) in the experimental group is denoted by a transition in the color of the variability ellipses.