Preferred movement patterns during a simple bouncing task.

Elastic tissues in the human body can store and return mechanical energy passively, reducing the metabolic cost of cyclical movements. However, it is not clear whether humans prefer movement patterns that optimize this storage and return. We investigated the preferred movement pattern during a bouncing task for which non-invasive techniques can identify the resonant frequency, which is the least metabolically costly. We quantified the preferred and resonant bounce frequencies for three mechanical conditions. During 'normal' trials, subjects bounced while reclined on a sled that moves along a track. During 'added mass' trials, mass was added to the sled. During 'added stiffness' trials, a spring was attached between the sled and the supporting frame, parallel to the track. Subsequently, we quantified the preferred bounce frequencies during ischemia, a technique that disrupts the available sensory feedback. Mechanical condition had a significant effect on both the preferred and resonant frequencies. Changes in preferred frequency scaled with resonant frequency, but the preferred frequency was significantly lower than the resonant frequency. These results indicate that humans adapt their preferred bouncing pattern in response to changes in mechanical condition. Humans may prefer a lower than resonant frequency because of an inability to sense metabolic cost during our relatively short trials. In contrast, during ischemia the preferred bounce frequency remained constant even when mechanical condition was varied, indicating that feedback is necessary to adapt the preferred frequency to changes in mechanics. These findings suggest that disrupted sensory feedback may prevent humans from choosing the optimal movement pattern.