Effects of related sensory inputs on motor performances in man studied through changes in perceived heaviness.

1. The perception of the heaviness of lifted objects was studied using a weight-matching task when sensory inputs from parts related to the lifting task were altered.2. A weight lifted by flexing the index finger feels heavier when the thumb is anaesthetized and lighter when the thumb is electrically stimulated. A weight lifted by flexing the distal joint of the thumb feels heavier when the sensory input from the skin and joint of the thumb is abolished by anaesthesia. It also feels heavier when the index (but not the little) finger is anaesthetized, and lighter when the index (but not the little) finger is electrically stimulated.3. A weight lifted by extending the thumb feels lighter when the sensory input from the thumb is abolished.4. The perceived heaviness of a weight lifted by flexing the thumb is increased during anaesthesia of the thumb when the flexor of the thumb is the prime mover but not when it is acting simply as a postural fixator to support the weight.5. The sensation of the heaviness of lifted objects derives from the effort or centrally generated voluntary motor command employed in the lift. Our results indicate that the motor commands to either thumb or index finger flexors are facilitated by sensory inputs arising from a wide sensory field usually involved in co-operative motor performances carried out by both muscle groups together.