Self-Regulatory Depletion Effects, Physical Endurance and Muscle Activity: An Examination of Depletion Effects and Trait Self Control as an Effect Modifier

Abstract

<P> The limited strength model of self-regulation describes self-regulation as a limited, consumable, and renewable internal resource that is depleted when people attempt to control their emotions, thoughts or behaviours (Baumeister & colleagues 1994; 1996). Evidence indicates a consistent relationship across emotional, mental, and physical domains that task performance in all of these areas draws on the same limited resource and is governed by processes occurring within the central nervous system (Galliot, et al., 2007). The main purpose of this study was to examine the effects of self-regulation depletion on muscle activity (EMG) and physical stamina via an isometric task (ankle dorsiflexion). A secondary objective was to investigate trait self-control as an effect modifier of cognitive self-regulation depletion effects on physical stamina. It was hypothesized that individuals would show a greater decline in isometric endurance performance after undergoing a self-regulatory depletion manipulation compared to when they were exposed to· a non-depletion task of similar duration. It was also expected that participants would exhibit greater increases in EMG amplitude after being depleted compared to when not depleted. Additionally, it was hypothesized that people who scored lower on a measure of trait self-control would demonstrate greater depletion effects (i.e., greater pre-to-post performance differences) than those who scored higher on trait self-control. The study was a within-subjects cross-over design involving 31 informed and consenting sedentary university students (M^age= 21.72 ± 2.57 years). Participants were stratified by gender and randomized to experience either cognitive depletion (modified Stroop task) or non-depletion (colour word reading task) for their first trial. In each trial, they completed two isometric ankle dorsiflexion endurance trials at 50% of their MVC (predetermined by initial MVC) separated by the cognitive task. Due to an unexpected differential carryover effect of exposure order, analysis of the data was carried out for each testing session, with primary analysis focused on Time 1 as suggested by Grizzle (1965). Time 1 data indicated a trend towards significance (p = .13) for performance declines being greater in the depletion group compared to the non-depletion group, and a small effect size of .27 was detected, which is comparable to findings in other related studies (Bray et al., 2008; Muraven & Shmueli, 2006). No statistically significant differences emerged for muscle activity in the tibialis anterior for the depletion group compared to the non-depletion group. Those individuals who scored lower on trait self-regulation showed a non-significant trend towards greater depletion effects on muscular endurance performance than those who scored higher on trait self-regulation (p = .13; Cohen's d = .32). Results support the limited strength model of self-regulation and the trait self-control as an·individual difference factor affecting self-regulation, but raise questions regarding the role of central fatigue effects on muscular activation following self-regulatory depletion. </p>