The sensory origin of the sense of effort is context-dependent

Florian Monjo, Jonathan Shemmell, Nicolas Forestier

The origin of the sense of effort has been debated for several decades and there is still no consensus among researchers regarding the underlying neural mechanisms. Some advocate that effort perception mainly arises from an efference copy originating within the brain while others believe that it is predominantly carried by muscle afferent signals. To move the debate forward, we here tested the hypothesis that there is not one but several senses of effort which depend on the way it is evaluated. For this purpose, we used two different psychophysical tests designed to test effort perception in elbow flexors. One was a bilateral isometric force-matching task in which subjects were asked to direct similar amounts of effort toward their two arms, while the other consisted of a unilateral voluntary isometric contraction in which subjects had to rate their perceived effort using a Borg scale. Throughout two distinct experiments, effort perception was evaluated before and following different tendon vibration protocols intended to differentially desensitize muscle spindles and Golgi tendon organs, and to affect the gain between the central effort and muscle contraction intensity. By putting all the results together, we found that spindle afferents played divergent roles across tasks. Namely, while they only acted as modulators of motor pathway excitability during the bilateral task, they clearly intervened as the predominant psychobiological signal of effort perception during the unilateral task. Therefore, the sensory origin of the sense of effort is not central or peripheral. Rather, it is context-dependent.