Change of a motor synergy for dampening hand vibration depending on a task difficulty

The present study investigated the relationship between the number of usable degrees of freedom (DOFs) and joint coordination during a human-dampening hand vibration task. Participants stood on a platform generating an anterior-posterior directional oscillation and held a water-filled cup. Their usable DOFs were changed under the following conditions of limb constraint: (1) no constraint; (2) ankle constrained; and (3) ankle-knee constrained. Kinematic whole-body data were recorded using a three-dimensional position measurement system. The jerk of each body part was evaluated as an index of oscillation intensity. To quantify joint coordination, an uncontrolled manifold (UCM) analysis was applied and the variance of joints related to hand jerk divided into two components: a UCM component that did not affect hand jerk and an orthogonal (ORT) component that directly affected hand jerk. The results showed that hand jerk when the task used a cup filled with water was significantly smaller than when a cup containing stones was used, regardless of limb constraint condition. Thus, participants dampened their hand vibration utilizing usable joint DOFs. According to UCM analysis, increasing the oscillation velocity and the decrease in usable DOFs by the limb constraints led to an increase of total variance of the joints and the UCM component, indicating that a synergy-dampening hand vibration was enhanced. These results show that the variance of usable joint DOFs is more fitted to the UCM subspace when the joints are varied by increasing the velocity and limb constraints and suggest that humans adopt enhanced synergies to achieve more difficult tasks.