Principal Investigators: Wendy M. Murray, PhD and P. Hunter Peckham, PhD Project Staff: Kevin Kilgore Project Category: Spinal Cord Injury - 2003 Objective: The primary objective of this research is to establish quantitative measures of hand impairment that predict the ability to perform a simple task. A second objective of this research is to establish a strong collaboration between the VA Functional Electrical Stimulation (FES) Center of Excellence and the VA Palo Alto Health Care System (PAHCS) Center of Excellence on Mobility. This project provides a mechanism for the two VA Centers to develop comparable tools and techniques for quantifying hand and upper extremity function after spinal cord injury. In addition, this work will lead to continual collaboration on quantitative evaluation and computer simulation of the tetraplegic upper extremity. Research Plan: This project proposes to use mechanical analysis to characterize the mechanical criteria to complete a simple task, develop hardware and software necessary to quantify hand impairment and the ability to complete the task and transfer the technology from the VA Palo Alto Health Care System to the Cleveland VA Center of Excellence, test subjects, and use computer simulations to link hand impairment and function. Work Accomplished: The main goal of this project was to quantify how basic elements of hand biomechanics influence an individual's ability to complete a simple task. To accomplish this task, a custom device designed at the VA Palo Alto Center was transferred to the VA Center of Excellence on FES. Transfer included:
After the device was transferred to Cleveland, one neuroprosthesis user and one able-bodied subject were tested to evaluate how the forces produced by the muscles of the thumb influenced the ability to pick up a paperweight. Neuroprosthesis user - The subject's typical functional grasp generated 14.8 N of pinch force by stimulation of the adductor pollicis and the flexor pollicis longus; and the subject was able to easily lift the paperweight with this grasp pattern. When the pinch force was reduced below 9 N, the subject was no longer able to accomplish the task under normal conditions. When the paperweight was covered with a rough surface, the subject was able to lift the weight with 8 N of pinch force. The subject could also lift the paperweight at 8 N if the abductor pollicis brevis (AbPB) was stimulated. Based on previous studies with this subject, the pinch force vector has an increased flexion magnitude (normal to the surface of the paperweight) when the AbPB is activated. Able-bodied subject - An experiment was performed to identify the contribution of the adductor pollicis and abductor pollicis brevis muscles to the 3-dimensional thumb force vector. The subject was seated with the hand grasping and holding the pinch meter. The custom device was utilized. Surface electrodes were placed on: 1) the ulnar nerve just proximal to the wrist in order to recruit the adductor pollicis muscle and 2) the median nerve just proximal to the wrist in order to recruit the abductor pollicis brevis muscle. Thumb force in all three directions was sampled at 200Hz and recorded for ten seconds. Stimulation was applied to each electrode individual at two stimulus levels, and then in combination to both electrodes simultaneously. The results demonstrated that the abductor pollicis brevis muscle produces a force vector with components of flexion, abduction and distal force (in order of decreasing magnitude). The adductor pollicis produced a force vector with components of flexion, distal and adduction force (in order of decreasing magnitude). At high levels of stimulation, activation of the abductor pollicis brevis alone caused the thumb to slip off of the load cell. The slip occurred at 7.8 N of flexion force, 2.7 N of abduction force and 0.74 N of distal force. Comparison of force levels with the adductor pollicis indicates that the abduction force was responsible for causing the slip to occur, and slip was shown to occur between 1.6 and 2.7 N. Distal forces as high as 5.7 N and flexion forces as high as 17.1 N did not produce slip. Stimulation of both abductor pollicis brevis and adductor pollicis simultaneously demonstrated that muscle forces could be combined to increase pinch force and reduce slip. With low level stimulation applied to both muscles, a pure flexion force of 12.5 N could be generated, with no abduction or adduction force. Discussion and Implications: These results illustrate the importance of considering all components of the forces generated on an object if functional outcome is to be accurately predicted. In the case of a simple disk-shaped object, we demonstrated that increasing the force normal to the object (flexion force in this case) was not sufficient to maintain grip on the object. Instead, once the abduction-adduction forces exceeded a threshold, the object slipped out of the grip and increasing flexion forces alone were insufficient to stop slip. The force on the object must be re-directed in order to maintain grip. In the case of the thumb intrinsic muscles, we demonstrated that the combination of abductor pollicis brevis and adductor pollicis could produce a pure flexion force with no slip component. We have developed instrumentation that allows us to measure all three forces in pinch, as well as object position in space. This instrumentation is currently being utilized as a part of ongoing research at the VA Palo Alto, and, as a result of this project, has now been successfully transferred and implemented to the VA Center of Excellence in FES in Cleveland. This project will allow the development of future studies, and continued collaboration between the two centers. We are currently planning to perform experiments on additional subjects in order to facilitate funding submissions and future publication of this work. Funding Source: VA RR&D Merit Review Funding Status: Completed |
