Task constraints act at the level of synergies and at the level of end-effector kinematics in manual reaching and manual lateral interception.

  • Tuitert Inge
  • Golenia Laura
  • Otten Egbert
  • Bootsma Reinoud
  • Bongers Raoul

  • Emergent synergies
  • Reaching
  • Interception
  • UCM analysis
  • End-effector kinematics

ART

To coordinate the redundant degrees of freedom (DOF) in the action system, synergies are often proposed. Synergies organi e DOF in temporary task-specific units emerging from interactions amongst task, organism, and environmental constraints. We examined whether task constraints affect synergies, end-effector kinematics, or both. To this end, we compared synergies and end-effector kinematics when participants (N = 15) performed discrete movements of identical amplitude in manual reaching (stationary targets) and manual lateral interception (moving targets, with different angles of approach). We found that time-velocity profiles were roughly symmetric in reaching, whereas they had a longer decelerative tail and showed an angle-ofapproach effect in interception. Uncontrolled Manifold analyses showed that in all conditions joint angle variability was primarily co-variation, indicating a synergistic organi ation. The analysis on the clusters of joint angle configurations demonstrated differences between reaching and interception synergies, whereas more similar synergies were used within interception conditions. This implies that some task constraints operate at the level of synergies while other task constraints only affect end-effector kinematics. The results support a two-step process in the organi ation of DOF, consisting of synergy formation and further constraining of synergies to produce the actual movement, as proposed by Kay (1988).