The influence of maximal isometric activity on twitch and H-reflex potentiation, and quadriceps femoris performance

The possibility of post-activation potentiation (PAP), enhanced neuromuscular performance following prior contractile activity, has received considerable attention but with little assessment of physiological changes. Maximum twitch force and H-reflex amplitude can be used to quantify the effect of prior activity, and may account for any PAP. Quadriceps femoris maximum twitch force (twitch potentiation, TP), the EMG Hmax/Mmax ratio and associated twitch force ratio (reflex potentiation, RP), were measured for 18 min after a period of rest (CON condition) and a 10 s isometric maximal voluntary contraction (iMVC, EXP condition) in eight recreationally active males. In comparison to CON, the Hmax/Mmax ratio (electrical RP) was potentiated for 5–11 min after iMVC (P < 0.05), with the highest values recorded 5 min post (+42 ± 27% compared to mean of CON). Relative twitch force at Hmax (mechanical RP) was potentiated from 5 to 9 min post (P < 0.05). TP was greatest 10 s after iMVC (+67 compared to CON), and remained elevated for 18 min (P < 0.05). Optimal conditions for PAP were arbitrarily decided to occur 5 min after iMVC (TP, ~+16%; electrical RP, +42%), and performance (isometric rate of force development and isokinetic torque at 4.19 rad s⁻¹) was measured at this point after CON and EXP treatments. No measures of performance were improved with EXP (P = 0.26–0.91). Electrical and mechanical RP of the quadriceps femoris was demonstrated, but the clear RP and TP found to occur after a 10 s iMVC did not produce any performance benefit.