Measurement of Multidirectional Arm Strength for People in a Seated Position
Authors: Stilwell George , Symons Digby , Gooch Shayne , Dunn Jennifer 
Published: November 2019

People with tetraplegia have significantly reduced upper body strength capabilities. The completion of activities of daily living require people living with tetraplegia to work at or near their physical limits. Improvements to the design of ‘workspaces’ and assistive devices can allow people with tetraplegia to live with increased independence. In order to make effective improvements to devices used by people with tetraplegia their strength capabilities must first be well understood. Each year there are approximately 10,400 new cases of people with tetraplegia in the US alone [1].

The purpose of this paper is to present an improved methodology for establishing multidirectional arm strength data for people with tetraplegia while in the seated position. A test rig has been designed that is a step forward in acquiring isometric upper body strength data. The test rig incorporates two three-axis load cells that allow isometric force to be measured rapidly in all directions. Using this test rig force can be measured for a full 360° rotation. Isometric strength can be measured for each arm over a 1.3m by 1.5m grid of points parallel to the sagittal plane using 100mm increments. Before the test rig used in this study was developed there was no practical way to obtain multidirectional upper body strength data in a reasonable amount of time.

A preliminary study has been completed using two able-bodied subjects and one subject with C6 tetraplegia. Results of this study confirm that this methodology can be used to characterize multidirectional upper body strength. A better understanding of the strength characteristics of all people working in a seated position can enable improvements to the design of such spaces. The results highlight the dramatic reduction in the range of motion and strength capabilities for a person with tetraplegia when compared to able-bodied subjects. In particular, the preliminary results show how a cervical spinal cord injury effects what directions voluntary force can be applied over a person’s range of motion. For a number of positions force can only be effectively applied in one direction. Analysis of retested points showed no indication of fatigue effecting results.

Future testing using the methodology established in this paper will be useful to provide greater insights into the strength capabilities of people with tetraplegia. The aim is to produce characteristic strength profiles for spinal cord injuries between C5 and C7. Such strength profiles will be of interest to designers of assistive devices and medical professionals. For designers, the strength information will enable changes to be made to the designs of ‘workspaces’ and assistive devices to help improve the independence of people with tetraplegia. For medical professionals this information will enable the benefits of surgical procedures such as tendon and nerve transfers to be evaluated quantitatively. This will enable people with tetraplegia to make more informed decisions when investigating possible rehabilitative surgeries.