What if muscle spindles were also involved in the sense of effort?

Effort perception is widely acknowledged to originate from central processes within the brain, mediated by the integration of an efference copy of motor commands in sensory areas. However, in this brief article, we aim to challenge this perspective by presenting evidence from neural mechanisms and empirical studies that suggest that reafferent signals from muscle spindles also play a significant role in effort perception. It is now imperative for future research (a) to investigate the precise mechanisms underlying the interactions between the efference copy and reafferent spindle signals in the generation of effort perception, and (b) to explore the potential for altering spindle sensitivity to affect perceived effort during ecological physical exercise, and subsequently, influence physical activity behaviors. Abstract figure legend The perception of effort is intimately tied to the neural signals related to motor command magnitude. The efference copy has long been recognized as the primary signal that the brain processes to generate a sense of effort. However, recent evidence suggests that reafferent muscle spindle signals may also play a crucial role in effort perception, particularly through interactions with the efference copy. During voluntary contractions, alpha-motoneurons (innervating force-producing extrafusal fibers) and gamma-motoneurons (innervating intrafusal fibers) are coactivated. The fusimotor commands transmitted by gamma-motoneurons to intrafusal fibers are conveyed through muscle spindles to reach afferent fibers. Reafferent spindle signals thus provide feedback to the brain regarding the amount of fusimotor commands received by intrafusal fibers. While the precise mechanisms underlying the interactions between the efference copy and reafferent muscle spindle signals remain speculative, we propose that future research should focus on identifying these mechanisms to further our understanding of the generation of effort perception. This article is protected by copyright. All rights reserved.