Peripheral contributions to the perception of effort.

The effort of any brief static or dynamic muscular contraction is probably sensed as force. Force sensation may operate according to one of the following three mechanisms: 1) feedforward, where a copy of central motor outflow is fed directly to the sensory cortex; 2) feedback, afferent input to the cortex from peripheral receptors activated by contracting muscle; and 3) feedforward + feedback, expected and actual results of contraction are compared and continuously adjusted. Using both static and dynamic contractions as models, data have been obtained that support either feedforward or feedback mechanisms. It is suggested that experimental circumstances have much to do with isolating one part of a system that probably contains feedforward and feedback elements.