Autonomic control of cardiac rhythm at the onset of muscle metaboreflex activation following exercise in healthy young and old adults

Sympathetic and parasympathetic nervous system (SNS, PNS) control of cardiac rhythm during muscle metaboreflex activation (MMA) are important components of the physiological response to exercise but poorly understood. Therefore, SNS and PNS control of cardiac rhythm at the onset of MMA following exercise was examined in healthy young and old adults. Fourteen young (26 ± 2 years) and 10 old (62 ± 3 years) healthy adults performed 90-s, one-legged, isometric, calf exercise at 70% maximal voluntary contraction (MVC) and underwent a 0% MVC control trial, with unilateral circulatory occlusion beginning 5 s before exercise cessation and maintained for 30 s following exercise to isolate MMA. Absolute and normalized low-frequency (aLF, nLF) and high-frequency (aHF, nHF) bands and LF/HF were analyzed during a 30 s baseline period and the first 30 s of MMA following exercise (70% MVC) or continued rest (0% MVC). A time-trial interaction in LF/HF was observed in young adults (p = 0.043), with LF/HF elevated during 0–10 s of MMA compared to 10–30 s during the 70% MVC trial. This interaction was not observed in old adults (p = 0.490). These results indicate that healthy young adults exhibit restoration of PNS control of cardiac rhythm at the onset of MMA following exercise, whereas this restoration is absent in healthy old adults.