Motor primitives of pointing movements in a three-dimensional workspace

A central question of motor control is how the motor system deals with redundant degrees of freedom. Redundancy can be reduced by coupling multiple degrees of freedom into a single motor primitive. Previous studies measuring motor primitives in aimed limb movements were restricted to two-dimensional target planes. We asked whether a limited number of motor primitives would also be sufficient to capture the posture variance of aimed limb movements in a three-dimensional target volume. To this end, participants had to point towards virtual targets uniformly spaced in a three-dimensional workspace. Results showed that three motor primitives captured 89.7 ± 2.8 % of the data variance of unrestrained pointing movements. Each motor primitive corresponded to a valid movement of the arm. The findings imply that complex postures in a three-dimensional target volume can be reduced to three motor primitives. The reduction results in a unique mapping of target position and posture and, thus, solves the redundancy problem. The reduction further indicates that, in a pointing task, the motor system does not control hand rotation independent of hand translation.