Galloway JC^{1, 2, 3}, Bhat A², Heathcock JH², Lobo MA², Manal K.³; ¹Dept. of Physical Therapy; ² Biomechanics and Movement Science Program; ³Center for Biomedical Engineering Research, Univ. of Delaware, Newark, DE. Jacgallo@udel.edu.

PURPOSE: Recent work on reaching of healthy adults and adults with neurological injury suggests that shoulder dynamics are less complex than elbow dynamics. Specifically, the combinations of muscular and non-muscular torques are less complex at the shoulder vs. the elbow for most movement directions. It is unknown if this feature applies to vertical plane reaching, which involves the additional torque due to gravity. The purpose of this project was to determine if this feature generalizes to fast and slow vertical reaches across multiple directions in sitting and supine. SUBJECTS: Five healthy adults (3 females, 2 males aged 23-35 years) participated. METHODS: Subjects performed multiple fast and slow point-to-point arm movements to each of 12 targets in the parasagittal plane. We recorded reflective markers placed on the right shoulder, elbow, wrist, index finger and left shoulder with a 6-camera high-speed motion analysis system. We used joint kinematics and mass-segment parameters in equations of motions to calculate muscle, interaction and gravitational torques (MT, IT, GT) and the total or Net torque (NT) at the shoulder and elbow. Ratios of each individual torque and torque combination to NT were calculated as reflection of the joint dynamics. RESULTS: As in horizontal reaches, the shoulder displayed different dynamics as compared to elbow across both speeds and body positions. For the majority of directions within a speed or body position, the shoulder displayed less complex dynamics vs. the elbow. Specifically, shoulder NT was composed primarily of MT and GT, whereas elbow NT was composed of various combinations of MT, GT and IT for different directions. Similarly, shoulder dynamics varied less across speeds and body positions than the elbow. CONCLUSIONS: These results suggest that shoulder dynamics are less complex to control than elbow dynamics as a general feature of reaching movements. From these results, we predict that. 1) Elbow dynamics are a key impairment in pediatric and adult patients with dysfunctional reaching, 2) An important requirement for controlling the elbow is learning the differential effects that speed and body position have on GT and IT, as well as how GT changes throughout a reach. These results highlight the importance of training within and across important contexts such as speed and body. FUNDING: Funded in part by Foundation for Physical Therapy scholarships to J. H. and M. L. and by Univ. of Delaware Research Foundation.

Reprint Information
Requests for reprints should be directed to the corresponding author of the article. Students and other academic customers may receive permission to reprint copyrighted material from Physical Therapy by contacting the Copyright Clearance Center Inc, 222 Rosewood Dr, Danvers, MA 01923. Similar inquiries by all others should be made to the APTA Editorial Office, Attn: Physical Therapy.