Post-activation potentiation after isometric contractions is strongly related to contraction intensity despite the similar torque-time integral

Abstract

Post-activation potentiation (PAP) is defined as an enhanced contractile response of a muscle following its own contractile activity and is influenced by the intensity and duration of the conditioning contraction. The aim of this study was to determine if the combination of intensity and duration, that is, torque-time integral (TTI) is a determinant of PAP amplitude. We compared PAP amplitude following low-to-maximal voluntary conditioning contraction intensities with and without similar TTI in the knee extensors. Twelve healthy males completed two experimental sessions. Femoral nerve stimulation was applied to evoke single twitches on the relaxed quadriceps before and after isometric conditioning contractions of knee extensors. In one session, participants performed conditioning contractions without similar TTI (6 s at 100, 80, 60, 40 and 20% maximal voluntary contraction (MVC)), while they performed conditioning contractions with similar TTI in the other session (6 s at 100%, 7.5 s at 80%, 10 s at 60%, 15 s at 40%, and 30 s at 20% MVC). In both sessions, PAP amplitude was related to conditioning contraction intensity. The higher the conditioning contraction intensity with or without similar TTI, the higher PAP. Significant correlations were found (i) between PAP and conditioning contraction intensity with (r² = 0.70; P < 0.001) or without similar TTI (r² = 0.64; P < 0.001), and (ii) between PAP with and without similar TTI (r² = 0.82; P < 0.001). The results provide evidence that TTI has a minor influence on PAP in the knee extensors. This suggests that to optimize the effect of PAP, it is more relevant to control the intensity of the contraction rather than the TTI.

Keywords: electrical stimulation; knee extensors; muscle strength; neuromuscular performance; quadriceps; twitch.

FIGURE 1

Overview of the experimental protocol for the first (a) and the second (b) experimental session. Experimental sessions consisted of evoking single twitches before (PRE, three twitches) and after (POST, 11 twitches) each conditioning contraction. In the first experimental session (session with different torque–time integral), one conditioning contraction of 6‐s duration was performed at 100, 80, 60, 40 and 20% of maximal voluntary contraction (MVC). In the second experimental session (session with similar torque–time integral), one conditioning contraction was also performed at 100, 80, 60, 40 and 20% of MVC. However, the duration of each conditioning contraction was adjusted so that the same torque–time integral was observed among the conditioning contractions. Therefore, the duration of each conditioning contraction was set up with respect to its intensity, that is, 6 s at 100%, 7.5 s at 80%, 10 s at 60%, 15 s at 40%, and 30 s at 20% MVC.

FIGURE 2

Change in post‐activation potentiation (PAP) at different time points after the conditioning contractions of the knee extensors with and without similar torque–time integral (TTI). (a, b, c, d, e) represent data for conditionning contractions of 100%, 80%, 60%, 40%, and 20% MVC, respectively. Session with different TTI (), session with similar TTI (). Mean values ± SD. *Significantly different from PRE (P < 0.05). ^†Significantly different between sessions with and without similar TTI (P < 0.05).

FIGURE 3

Amplitude of post‐activation potentiation (PAP) following conditioning contractions. (a) Change in PAP at different time points after the conditioning contractions of the knee extensors (3, 6, 10, 20, 30, 60, 120, 180, 240, 300, 600 s) in the sessions with similar torque–time integral (TTI) and with different TTI. 100% (), 80% (), 60% (), 40% (), and 20% () MVC. Mean values ± SD. A significant interaction session × intensity × time was found for PAP (P = 0.035; F = 1.45; DF = 44). For the sake of clarity, symbols for statistics significance are not presented on the figure. (b) Typical recordings of twitch torque from one subject before and after the conditioning 6‐s maximal voluntary contraction (MVC) at each time points in the session with similar and different TTI. The black dashed line represents the twitch torque recorded before the conditioning contraction. CC, conditioning contraction.

FIGURE 4

Correlations between post‐activation potentiation (PAP) at 3 s after the conditioning contractions and torque of the conditioning contractions in sessions without (a) and with (b) similar torque–time integral (TTI). 100% MVC (), 80% MVC (), 60% MVC (), 40% MVC (), 20% MVC (). Each symbol (mean) represents one subject. CC, conditioning contraction.

FIGURE 5

Correlation between post‐activation potentiation (PAP) at 3 s after conditioning contractions without and with similar torque–time integral (TTI). 100% MVC (), 80% MVC (), 60% MVC (), 40% MVC (), 20% MVC (). Each symbol (mean) represents one subject.