Differing isometric contractions can affect neural excitability

Abstract

Postactivation potentiation (PAP) of human skeletal muscle has been credited for improved performance in complex training situation, were a conditioning exercise is performed prior to a performance task. The mechanisms behind PAP have not been fully elucidated; it has been proposed that an increase in phosphorylation of the myosin regulatory light chains and/or increased spinal excitability could explain performance changes. However, which mechanism induces PAP has not been investigated as yet, while the optimum intensity of the conditioning exercise or the best rest period to induce optimal PAP levels has not been established. The purpose of the current investigation was to examine a range of different intensity conditioning contractions over a 60 minutes recovery period, on PAP response. Method: to gain reliability of the patellar tendon tap test a rubber tipped hammer was embedded into a machine. Twelve participants (8 males age 23.7 ± 4.9 yr, mass 78.3 ± 12.9 kg, height 1.75 ± 0.1 m and 4 females age 22 ± 3.2 yr, mass 60.6 ± 4.1 kg, height 1.66 ± 0.1 m) had three differing hammer tap forces (270 ± 0.79 N, CV 0.2 %, middle 252 ± 0.74 N, CV 0.2 % and lowest 121 ± 0.64 N, CV 0.5 %) delivered in a randomised order over 3 x 10 minute periods with sporadic intervals between each tap. The hammer tap force that reported the highest reliability was then used in a follow up study. The second study investigated skeletal muscles response to differing intensities of conditioning exercise, over a 60 minute rest period. The conditioning exercises consisted of 3 sets of isometric leg extensions of the dominant leg at 100%, 90%, 80% and 70% of a 1 RM, each set consisted of a 5 s contraction followed by 55 s passive rest. Subsequent neural excitability and intra-muscular activation were assessed in response to the patellar tendon tap test. Neural excitability was assessed via integrated EMG responses and goniometer movement of the lower leg. Results: The highest force (270 ± 0.79 N) gave the best reliability for hammer tap contact to the start of lower limb movement (17.3 %) compared to the other two hammer forces, this was used in the second study. In the second study no significant differences were observed between the hammer tapping the tendon and the start of muscle activation (F = 1.843, p>0.05) or muscle activation to the start of lower limb movement (F=1.587, p>0.05) between any of the conditioning contraction intensities or between any recovery periods. After 2 minutes recovery there was a trend for the maximal intensity conditioning exercise to reduce neural excitability. After 4 minutes rest all intensities of conditioning exercise caused a trend for an increase in neural excitability and intra-muscle activation. Conclusion: The poor reliability of the patellar tendon tap test maybe due to individual variations in tendon biology. Although this study improved the reliability compared to previous work, the technique was still too varied to ascertain conclusive results, beyond trends in data. Therefore, reliability of this technique needs to be addressed further to allow it to be recognised as a method to measure spinal excitability. At this moment in time no definitive rest period guidelines can be given to coaches for complex training or even if it enhances performances over traditional training methods. It is therefore recommended that coaches should view this training modality with caution, and not rely solely on this method, when more established valid methods to improve athletes’ explosive power performance, are readily available.