Motivational intensity theory predicts that energy investment in goal pursuit is governed by the motivation to conserve resources and that it should consequently be a function of task demand: Trying to avoid wasting resources, individuals should invest only the energy that is required for task success and should disengage if success is impossible. Three experiments tested this hypothesis assessing the force exerted in an isometric handgrip task as indicator of energy investment. The results provided mixed evidence for motivational intensity theory.Supporting the theory, exerted force increased as a function of task demand if task success was possible and was low if success was impossible. However, exerted force exceeded required force in all possible conditions and participants did not disengage if task success was impossible. A meta-analysis of published studies involving disengagement conditions revealed that preceding research on motivational intensity theory also failed to provide support for the disengagement hypothesis. Our findings demonstrate the importance of task demand for energy investment but also challenge the assumption that energy investment is primarily driven by energy conservation concerns.

According to motivational intensity theory, individuals are motivated to conserve energy when pursuing goals. They should invest only the energy required for success and disengage if success is not important enough to justify the required energy. We tested this hypothesis in five experiments assessing exerted muscle force in isometric hand grip tasks as indicator of energy investment. Our results provided mixed evidence for motivational intensity theory. Corroborating its predictions, energy investment was a function of task demand. However, we did not find evidence for the predicted disengagement, and we observed that participants exerted in most conditions more force than required. Furthermore, the data could be better explained by a model that predicted an additive effect of task demand and success importance than by models drawing on motivational intensity theory’s predictions. These results illustrate the strong link between energy investment and task demand but challenge motivational intensity theory’s primacy of energy conservation.

In the last two decades, the strength model of self-regulation or self-control, respectively, (e.g., Baumeister 2002;Baumeister et al. 2000; see also Lopez et al. in press) has exerted a considerable impact on self-regulation research and theorizing. The model conceptualizes selfregulation as the capacity to alter one's own behavior (e.g., Baumeister 2002; Baumeister and Vohs 2007) and postulates that controlling behavior (for instance, overriding a habitual response) requires resources or energy, respectively. Resources that are mobilized to regulate behavior are supposed to be consumed in the regulation process and need to be restored. According to the strength model of self-regulation, all kinds of self-regulatory activity draw on the same resources. Performing a self-regulatory action should thus reduce the amount of resources that are available for subsequent self-regulation.Given that the model predicts that self-regulation efficiency is a direct function of the amount of available resources, performing a task that requires self-regulation should lead to decreased performance in subsequent tasks that rely on self-regulation. This effect is called ego-depletion effect and most of the empirical work on the strength model of self-regulation has focused on it (see Hagger et al. 2010, for a recent review of the empirical research on ego depletion).When presenting the strength model of selfregulation, authors have repeatedly likened self-regulation to muscle activity arguing that self-regulation resembles a muscle-the muscle metaphor. The ego-depletion effect should resemble muscle fatigue-the decrease in muscle performance after sustained physical exercise that is restored after rest. Self-regulation and muscle activity should both require energy resources, and the depletion of these resources should underlie both the ego-depletion effect and muscle fatigue. The strength model of self-regulation also claims that self-regulation can be trained and strengthened like a muscle. Repeatedly performing self-regulatory tasks should lead to higher self-regulation capacities and better performance in tasks that require self-regulation.In this chapter, we will take a closer look at the physiological foundation of the muscle metaphor. After an introduction to muscle functioning, we will discuss the two key elements of the muscle metaphor. We will elaborate on resource depletion as the cause of muscle fatigue, and we will discuss training effects on muscle strength.

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