Parasympathetic reactivation during muscle metaboreflex activation immediately following isometric exercise of different intensities in healthy young adults

Following exercise cessation, parasympathetic reactivation occurs, decreasing heart rate (HR) back to baseline levels. Heart rate variability (HRV) is the fluctuation in the time between RR intervals (RRI) and an indicator of autonomic activity. While HRV responses to exercise and muscle metaboreflex activation, a common component of the exercise response, have been studied, HRV during the transition from the end of exercise to isolated muscle metaboreflex activation of graded levels has not been investigated. Therefore, the objective of this study was to examine HRV immediately after exercise during isolated muscle metaboreflex activation following exercise of different intensities in healthy young adults. It was hypothesized that there would be lower HRV immediately after exercise during isolated muscle metaboreflex activation following exercise of higher intensities. Ten healthy young adults (mean age 22±3 years; 3 females/7 males) attended four lab visits in a randomized order. In each visit, participants rested for a 5-minute baseline period. Ten seconds before the end of baseline, a cuff placed around the right thigh was inflated to 200mmHg to cause limb circulatory occlusion. Participants then performed 1.5 minutes of 30%, 50%, or 70% maximal voluntary contraction isometric right calf exercise or continued to rest for 1.5 minutes (0% control trial). Participants then stopped exercising and rested or continued to rest for an additional 30 seconds with maintained limb circulatory occlusion. RRI (electrocardiography) was continuously recorded, from which HR was calculated. Time-domain HRV was assessed using the root mean square of successive RRI differences (RMSSD), standard deviation of normal-to-normal intervals (SDNN), and percentage of successive RRIs that differed by more than 50 milliseconds (ms; pNN50). One-way repeated measures analysis of variance with Šídák’s post-hoc tests were used to analyze the data for significant differences with P set at < 0.05. During the rest/exercise phase, RMSSD, SDNN, and pNN50 were significantly lower during the 30% (P=0.0021, P=0.0009, and P=0.0264, respectively), 50% (P=0.0019, P=0.0001, and P=0.0097, respectively), and 70% (P=0.0003, P=0.0001, and P=0.0107, respectively) trials compared to the 0% trial. During isolated muscle metaboreflex activation following exercise, RMSSD, SDNN, and pNN50 were similar across all trials (RMSSD: range of means 46.87±20.20ms - 52.90±28.85ms, P=0.6665; SDNN: range of means 63.38±19.75ms - 71.00±23.41ms, P=0.5416; pNN50: range of means 24.57±15.75% - 26.53±15.58%, P=0.9288). In conclusion, HRV is similar immediately after exercise during isolated muscle metaboreflex activation following exercise of different intensities. These findings suggest that there is a combination of rapid parasympathetic reactivation following the end of exercise and an additional parasympathetic activation due to muscle metaboreflex activation-induced increases in blood pressure mediated by the arterial baroreflex. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.