Post-activation Potentiation : Effects on performance, tensile, and contractile properties of the plantar flexor muscles

Abstract: Aim The present thesis aimsto provide scientific based evidence that might expand the limited information available regarding the post-activation potentiation (PAP) effects on the rate of torque development (RTD) and muscle activity during repeated isokinetic concentric maximal voluntary contractions(MVC). Additionally, we aim to provide new insights related to the possible effects that such repeated maximal contractions might have on the muscle tendon unit (MTU) stiffness. Methods Nine active participants were tested in two separate sessions. Participants performed plantar flexor supra-maximal twitches evoked via electrical stimulation of the tibial nerve and concentric MVC sat 60°/s. These were evoked/performed before and from 5s to 15 minutes after a 6s plantar flexion maximal voluntary isometric contraction (MVIC). Twitch RTD and time to peak torque (TTP) were measured. Concentric MVC peak torque (PT), soleus, medial and lateral gastrocnemius muscle activation, as well as RTD during different portions of the rising torque curve (e.g.50-100ms) were assessed. Passive stiffness index of the MTU was calculated using the torque-angle relation attained during the 5°/spassive ankle angle rotation that followed each maximal concentric contraction. Results TwitchRTD significantly increased immediately after the conditioning contraction (CC)and remained enhanced for 5s-8min, with increases of 59.7%-6.0%. Twitch TTP significantly decreased from 5s-1min postCC by 9.9%-2.0%. TTP then increased by 4.6%-2.2% from 3-8min. In the maximal voluntary concentric contractions, there was a significant increase of 5.7, 6.0 and 5.9% at 1.5, 3, and 5min respectively in the PT. Voluntary RTD showed significant increases during the 100-200ms, 50-200ms and 0-200ms phases. These increases were seen at 3min (7.3%), 1.5-5min (8.0-6.9%), and 1.5-5min (8.6-9.5%) respectively. Stiffness showed no significant changes and any changes in EMG do not appear to be due to PAP effects. Conclusions The results from the current study show that PAP affects voluntary performance at a lower angular velocitythan previously reported and in a time frame where twitch contractile properties were also potentiated. The lack of stiffness changes suggests that the acute effects might be mainly related to mechanisms within the muscle and that similar testing protocols should expect low interference from factors related to passive MTU stiffness changes.