The senses of force and heaviness at the human elbow joint

Abstract

The present-day view of the neural basis for the senses of muscle force and heaviness is that they are generated centrally, within the brain, from copies of motor commands. A corollary of the motor discharge generates a sense of effort which underlies these sensations. In recent experiments on force and heaviness sensations using thumb flexor muscles, a rather different explanation has been invoked: Subjects were proposed to rely predominantly on inputs of a peripheral origin, in particular, the signals of muscle spindles. The present experiments have been carried out at the elbow joint to determine whether these new ideas apply more widely. The effects of fatigue of elbow flexor muscles have been studied in force and heaviness matching tasks using three exercise regimes, a sustained maximum voluntary contraction (MVC), a maintained contraction of 35 % MVC, and a maintained contraction of 35 % MVC combined with muscle vibration at 80 Hz. In force-matching experiments, subjects were required to contract both arms and while the reference arm generated the target force under visual control, it was matched by the indicator arm without visual feedback. During the 100 % MVC exercise, force in the exercising reference arm fell rapidly to almost a half of its original value over 90 s while force in the indicator did not fall, leading to a significant overestimation of the reference force. During the 35 % MVC exercise, subjects also overestimated the reference force and this persisted at 5 and 10 min after the exercise. When 35 % MVC was combined with vibration, the amount by which the indicator arm overestimated the reference force was significantly reduced. In heaviness matching experiments, subjects could move their arms through a small range. The reference arm was loaded with a weight, and weights were added or removed from the indicator until heaviness felt the same in the two arms. There was a small, but significant fall in the matching weight used after 100 % MVC exercise, that is, the weight held by the fatigued arm felt lighter. The 35 % exercise did not alter heaviness sensation while 35 % MVC exercise with vibration led to a significant reduction in perceived heaviness. To conclude, while the results of these experiments on elbow flexors are not as clear cut as for thumb flexors, the central effort hypothesis falls short, in a number of respects in explaining the data which are able to be interpreted in terms of a peripheral afferent contribution to the senses of force and heaviness.

Towards a three-dimensional framework of centrally regulated and goal-directed exercise behaviour: a narrative review

“…Similar results were provided by Brooks et al 125, who also concluded that the central command hypothesis alone cannot explain the perceptions of force and heaviness in their entirety and that a number of aspects are better interpreted in terms of a peripheral afferent contribution to the senses of force and heaviness. Collectively, these results suggest that afferent signals from primary sensory intrafusal fibres of muscle spindles and Golgi tendon organs significantly contribute to the senses of force and heaviness.…”

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

“…Here the authors saw the peripheral contribution, not as a pure exafference, rather as a reafference generated in response to the motor command. The experiments, carried out on thumb flexor muscles, have recently been extended to a study of elbow flexors where, however, the case for a peripheral contribution to the sense of force is not as strong as at the thumb (Brooks et al 2013).…”

“…The lengths have been so adjusted that active forces in the two legs match. The hypothesis is that if muscle stretch receptors contribute to the sense of force (Luu et al 2011;Brooks et al 2013), they should be more stretched at the longer muscle lengths and therefore generate more activity. It leads to the prediction that in a force matching task, the force generated in the leg with the longer muscle should be overestimated by the leg with the shorter muscle.…”

Influence de la fatigue mentale sur les performances physiques

“…Seule la perception de l'effort pendant l'exercice de pédalage subséquent était supérieure suite à la fatigue mentale en comparaison de la situation contrôle. La perception de l'effort, définie comme l'expérience consciente de la sensation d'épuisement, est connue pour être le résultat de l'interprétation par le cerveau des retours sensoriels (Gardner & Martin, 2000) et/ou de la décharge corollaire (Brooks, Allen, & Proske, 2013). Avec la fatigue mentale, les sujets avaient donc la sensation de produire un effort plus important comparé à la condition contrôle, bien que l'intensité de l'exercice soit identique.…”