Perceptions of effort and heaviness during fatigue and during the size-weight illusion.

Previous work has shown that force perception and the sense of motor effort are different attributes of sensorimotor function. This study explores the hypothesis that one reason force and effort perceptions are distinct is to inform an individual of impaired motor function when muscular force lags effort. This hypothesis predicts that effort and force perceptions will dissociate when motor function is impaired by fatigue but not during the size-weight illusion. All subjects reported a distinct increase in effort when lifting a standard test weight as fatigue developed. When fatigue was sufficiently marked so that they could barely lift the test weight, they rated their effort as similar to that required to lift a maximal weight in the unfatigued state. The perceived heaviness of the test weight also increased as fatigue developed, but this fatigue-weight illusion was smaller than the increase in effort for all subjects and displayed greater variability. In contrast, both the perceived weight of a small object and the effort required to lift it increased in parallel when small and large objects were lifted sequentially. The size-weight and size-effort illusions appear to be examples of a common phenomenon in which perceptual experience is rescaled to maintain acuity under different working conditions. The fatigue-weight illusion also has the effect of increasing perceptual acuity as the subject's weight lifting range decreases due to fatigue.