A grab reflex in the human hand.

The effect of stretching or unloading flexor pollicis longus indirectly has been investigated by altering the force on the wrist while the subject tracked a moving spot on a cathode ray oscilloscope by flexing the top joint of the thumb against a fixed stiff lever. A radialward pull on the wrist, which lifted the thumb off the lever, thereby indirectly unloading flexor pollicis longus, caused a burst of EMG activity about 50 to 60 ms later, rather than the silent period seen at the same latency when the muscle was unloaded directly. A similar burst of activity occurred in the long finger flexors. Letting the wrist go, which caused the thumb to hit against the lever thereby indirectly stretching flexor pollicis longus, produced an initial burst of EMG activity about 50 ms later, as occurred when the thumb was stretched directly; but this was terminated prematurely after about 10 ms by a paradoxical silent period. The response of flexor pollicis longus to indirect unloading was not altered by anaesthetizing the skin of the wrist under the chain pulling it, nor by anaesthesia of the thumb, so was not due to cutaneous stimuli. Nor was it affected by motor point anaesthesia of flexor pollicis longus, so it was unlikely to be due to changes in spindle input from that muscle. However, it was reduced considerably by bracing the arm, suggesting that it arose as a result of movement proximally at the shoulder. The response to indirect unloading of flexor pollicis longus by a pull on the wrist did not occur if the subject held the lever free in his hand, indicating that it was dependent on and adapted to the task. A similar response to a pull on the wrist occurred if the subject was asked merely to hold his thumb and fingers adjacent to a full sherry glass. In these circumstances wrist release also caused a thumb flexion. Such an automatic flexion response or 'grab reflex' helps the subject to maintain contact of his digits with an object when his hand is displaced from it, provided that is his intent. Although the response occurs too early to be voluntary, it depends on the nature of the task and the subject's prior instruction.