Shoulder and elbow joint power differ as a general feature of vertical arm movements

Consistent patterns of joint power underlie coordinated lower extremity behaviors such as running and walking. Recent work found that shoulder and elbow power consistently differed during reaching movements in the horizontal plane. Moreover, joint power during horizontal reaching appears correlated with motor cortical activity. It is not known if the feature of differential joint power extends to vertical plane reaches or to reaches of different movement conditions. The purpose of this study was to test for differential shoulder and elbow power during the acceleratory and deceleratory phases of fast and normal speed vertical reaches in sitting and supine positions. Our results suggest that shoulder and elbow power typically differed both within and across conditions. First, shoulder power values were positive or negative dependent largely on movement direction and movement phase. That is, for each direction and phase, the shoulder either generated or absorbed energy independent of speed or body position. Second, and unexpectedly, reaches of certain condition combinations had similar shoulder power magnitudes across directions. In contrast, elbow power values for each direction varied between positive and negative values depending on phase, speed and position, and no two condition combinations overlapped across directions. Third, as target direction, movement phase and body position varied, shoulder power at fast and normal speeds were linearly correlated, as was shoulder power in sitting and in supine. In contrast, elbow power was linearly correlated only between speeds. These results join other studies to suggest that the neuromotor control of the shoulder may be less complex as compared to the elbow as a general feature of reaching movements. This differential control has important implications for the study of reaching impairments in neurorehabilitation populations, and provides a potentially important variable in the study of cortical firing patterns.