Activation patterns of the thumb muscles during stable and unstable pinch tasks

The ability to direct forces between the thumb and fingers is important to secure objects in the hand. We compared the coordinalion of thumb musculatu re in key and opposition pinch postures between stable and unstable tasks. The unstable task (producing thumb-tip force wearing a beaded thimble) requ ired well-directed forces; the stable task (producing thumb-tip force again st a pinch meter) did not. Fine-wire electromyography of thumb muscles and thumb-tip force magnitudes were recorded. We found no statistical differenc es in thumb-tip force between postures or stable versus unstable tasks, ind icating that the highest magnitudes of force can be accurately directed. Ab ductor pollicis brevis and extensor pollicis longus were significantly more activated in the unstable tasks, suggesting their importance in directing thumb-tip force. Understanding how pinch forces are directed might influenc e the choice of muscle-tendon transfers performed to restore function to th e severely paralyzed thumb. We introduce a device to quantify the ability t o control pinch force magnitude and direction simultaneously. Copyright (C) 2001 by the American Society for Surgery of the Hand.