Learning a single limb multijoint coordination pattern: the impact of a mechanical constraint on the coordination dynamics of learning and transfer

The coordination dynamics of learning and transfer were studied in a single limb multijoint task requiring rhythmic elbow and wrist motions. Participants were required to learn a continuous 90° relative phase pattern between the elbow and wrist such that an angle-angle plot of elbow and wrist motion produced a circle with a diameter of 80°. Joint motion was restricted to elbow and wrist flexion-extension on the sagittal plane and the to-be-learned 90° relative phase pattern was always practiced with the learning arm supine. Cycling frequency was controlled by a pacing metronome set at 0.75 Hz. Issues regarding effector-independent and effector-specific transfer were addressed with three transfer conditions: (1) learning arm prone (LP), (2) non-learning arm supine (NS), and (3) non-learning arm prone (NP). Four subjects learned the required relative phase (90°) and amplitude (80°) pattern with their dominant arm and four with their non-dominant arm. The experiment produced three main findings with regard to elbow-wrist control processes: First, seven of eight participants spontaneously produced a wrist-lagging coordination pattern (wrist motion lagged elbow motion) in learning to produce a continuous relative phase pattern of 90° between the elbow and wrist. The wrist-lagging pattern may emerge as a result of the central nervous system exploiting the transfer of angular momentum from the elbow to the wrist as the elbow rotates up and down. The influence of interactive torque on elbow-wrist coordination represents an important mechanical constraint on the selection of intralimb coordination strategies during learning. The transfer conditions revealed that this mechanical constraint was effector-independent with regard to ipsilateral limb transfer (LP) and contralateral limb transfer (NS and NP). Second, consistent transfer of the learned relative phase pattern across ipsilateral and contralateral conditions demonstrates an effector-independent representation for this control variable. The effector-independent and effector-specific nature of joint amplitude transfer was dependent to some degree on learning arm, dominant or non-dominant, and the amount of practice, 1 day versus 5 days. Third, learning of the required 90° relative phase pattern may be characterized as a phase transition leading to the formation of a stable attractor in the elbow-wrist coordination landscape. The above findings are discussed with respect to motor programming and coordination dynamic viewpoints on effector-independent and effector-specific aspects of motor equivalence.