Feedback from group III/IV afferents in skeletal muscle slows heart rate acceleration when transitioning from rest to low-intensity exercise in males.

Group III/IV skeletal muscle afferents respond to intramuscular mechanical and metabolic stimuli and are more active in fatigued muscle. These afferents project onto the brainstem and motor cortex and modulate cardiovascular and motor responses during exercise. The maximal rate of heart rate (HR) increase (rHRI), a marker of HR acceleration during the rest-to-exercise transition, is linearly related to exercise performance across various training and fatigue states, being faster with better exercise performance and vice versa. This study evaluated the effect of feedback from group III/IV afferents on rHRI in non-fatigued muscle. HR data were collected in eight recreationally active male participants (24 ± 1 years) during the transition from rest to 3 min of cycling at 100 W under control (CON) conditions and following lumbar intrathecal fentanyl (FENT) infusion to impair feedback from group III/IV afferents of the leg. rHRI was defined as the maximal first-derivative value of a logistic curve fitted to the HR data. Pre-exercise HR was similar in CON and FENT (CON 91.7 ± 16.7 bpm, FENT 99.8 ± 15.5 bpm, P = 0.250). HR increased more with exercise during CON than FENT (B = 10.01, 95% CI 5.33 to 14.70, P < 0.001) but rHRI was faster during FENT (B = 1.52, 95% CI 0.24 to 2.80, P = 0.020). In non-fatigued muscle, feedback from group III/IV afferents slowed rHRI. Increased feedback when muscle is fatigued may slow rHRI further which may explain the relationship between rHRI and exercise performance seen in previous studies, but further studies on fatigued muscle are required to confirm this.