The senses of active and passive forces at the human ankle joint.

The traditional view of the neural basis for the sense of muscle force is that it is generated at least in part within the brain. Recently it has been proposed that force sensations do not arise entirely centrally and that there is a contribution from peripheral receptors within the contracting muscle. Evidence comes from experiments on thumb flexor and elbow flexor muscles. Here we have studied the sense of force in plantar flexor muscles of the human ankle, looking for further evidence for such a mechanism. The active angle-torque curve was measured for muscles of both legs, and for each muscle, ankle angles were identified on the ascending and descending limbs of the curve where active forces were similar. In a plantar flexion force matching task, subjects were asked to match the force in one foot, generated on the ascending limb of the curve, with force in the other foot, generated on the descending limb. It was hypothesised that despite active forces being similar, the sensation generated in the more stretched muscle should be greater because of the contribution from its peripheral stretch receptors, leading to an overestimation of the force in the stretched muscle. It was found that provided that the comparison was between active forces, there was no difference in the forces generated by the two legs, supporting the central hypothesis for the sense of force. When total forces were matched, including a component of passive force due to muscle stretch, subjects seemed to ignore the passive component. Yet subjects had an acute sense of passive force, provided that the muscles remained relaxed. It was concluded that subjects had two senses, a sense of active force, generated centrally, and a sense of passive force, or perhaps muscle stretch, generated within the muscle itself.