The neural basis of the senses of effort, force and heaviness

Effort, force and heaviness are related terms, having in common that they are all sensations associated with the generation of voluntary muscle contractions. Traditionally they have been thought to originate in the brain, as a result of copies of motor commands relayed to sensory areas. A stumbling block for the central hypothesis has been the lack of proportionality between the fall in muscle force from fatigue or paralysis and the increase in sensation generated while trying to achieve the required force. In recent times growing evidence has accumulated supporting a role for peripheral sensory receptors, in particular the muscle spindles, as contributing to these sensations. The review discusses the evidence for participation of sensory receptors and what this means for proprioception. In particular, it is not straightforward to envisage how muscle spindles might provide a reliable force signal in a contracting muscle, with or without support from the fusimotor system. An important additional consideration is the method of measurement. It has emerged that there is evidence of a task-dependency in the composition of the afferent signals contributing to the sense of force. The evidence suggests that the signal used in a two-arm force matching task is not the same as in a one-arm task. It will be important, in the future, to try and obtain more direct evidence about the afferent origins of the senses of effort, force and heaviness, how they might change from one task to another and what implications this has for motor control.