Coactivity Effects upon Carpal Tunnel Contact Forces

Abstract

Contact force on the carpal tunnel structures due to flexor tendon forces have been identified as an important contributor to the compression of the median nerve. Therefore, a pilot study was conducted to assess the increase in carpal contact force due to the antagonistic coactivity of the finger extensor muscles. Surface EMG activities of the superficial finger flexor and extensor muscles of four subjects were measured during several isometric power grip exertions at seven different wrist angles. The results showed that a linear relation between EMG and muscle force holds under the prescribed isometric conditions. An EMG-assisted model was developed to predict tensile forces in an equivalent flexor tendon. For a given angle, the model predicts increased tensile force in the flexor tendon with increased extensor (antagonist) coactivity in response to isometric grip exertions. It was found that if one accounts for muscle coactivity, predicted force in the flexor tendons would be as much as 33% greater than force predicted by models which neglect coactivity. This increase would also be observed in carpal contact force since this force is linearly related to the flexor tendon force. Models that neglect coactivity severely underestimate flexor tendon forces and consequently contact forces in the carpal tunnel.