Evidence for adaptive shoulder-elbow control in cyclical movements with different amplitudes, frequencies, and orientations.

The evolution of joint dynamics and muscle patterning in the shoulder and elbow was studied for cyclical line drawing tasks at different frequencies, amplitudes, and orientations in the horizontal plane. Three main modes of control were identified: elbow-centered, shoulder-centered, and elbow-shoulder, each referring to the principal joints or joint combinations that were used to achieve the behavioral goals. The contribution of the shoulder joint was most prominent across the majority of movement orientations and largely paralleled changes in the dynamic (inertial) forces in the end effector (shoulder-centered control). The two joints either exchanged roles during the performance of the right diagonal movement (elbow-centered control) or shifted from a single-joint strategy to a dual-joint strategy during the performance of large amplitudes with low or medium cycling frequencies (shoulder-elbow control). These behavioral results support the existence of a modular control mode that allows the central nervous system to effectively tune motor commands to meet a broad variety of orientations, amplitudes, and frequencies. This refers to the emergence of a context-dependent control mode for the shoulder and elbow that optimizes the implementation of the underlying motor goals under a rich combination of spatial and temporal manipulations.