*

• Background: The autonomic nervous system regulates involuntary functions through two divisions: the sympathetic division raises heart rate, contractility, and blood pressure, and the parasympathetic division lowers them. During exercise, sympathetic activation redistributes blood flow to working muscles; during recovery, parasympathetic reactivation restores cardiovascular balance. Heart rate variability (HRV), the variation in time between heartbeats, is a non-invasive marker of cardiac autonomic modulation. High HRV reflects ANS adaptability and health; low HRV is linked to cardiac disease, aging, poor inflammatory regulation, and mental health disorders
• Sex Differences: Evidence is mixed on whether HRV differs by sex at rest or after exercise. Other postexercise responses do differ: postexercise hypotension occurs through vasodilation in females but through reduced cardiac output in males
• Knowledge Gap: Sex-specific postexercise autonomic responses to HIIE vs. CE are not well understood
• Purpose: Compare the acute effects of HIIE and CE on postexercise cardiovascular and autonomic nervous system dynamics between sexes
• Hypotheses: HIIE will elicit greater sympathetic dominance and slower parasympathetic reactivation than CE. The HIIE vs. CE difference will vary between males and females

Study Design

3 visits to the Exercise Physiology Laboratory, Dept. of Kinesiology

Visit 1: maximal exercise test to determine aerobic capacity (VO₂ max)

Visits 2 & 3: HIIE or CE, in randomized order

Exercise Protocols

• HIIE (28 min total):
• Four 4-min high-intensity bouts (85-95% VO₂ reserve)
• Four 3-min moderate-intensity recovery phases (40-50% VO₂ reserve)
• CE (25 min total):
• Steady-state exercise at 65% VO₂ reserve

Postexercise Measurements

Participants seated for 30 min after each exercise bout

VO₂ and ECG recorded continuously, BP, VO₂ analyzed in 5 min windows

HIIE was followed by lower vagal-related HRV (ln RMSSD, ln HF) than CE throughout 30 min of recovery, with no difference at baseline, suggesting greater and more sustained parasympathetic withdrawal. Slower reactivation after HIIE was supported only as a trend (condition × time, p = 0.028-0.064). HRV recovery was similar in males and females, apart from a trend toward lower ln LF in males. Males had higher diastolic BP after CE only. Larger studies of sex-specific ANS recovery are needed.

• After both workouts, the heart beat faster and the time between beats was shorter, as expected.
• Heart rate variability (HRV), a sign of how well the body’s calming system is working, dropped after exercise. It tended to drop more after interval exercise, but that difference was only a trend, not a firm result.
• A measure of the body’s calming (“rest and digest”) system was lower after the interval workout than after the steady workout.
• Another HRV measure (low-frequency power) was also lower after the interval workout.
• The balance between the body’s “stress” and “calm” systems tipped toward stress after both workouts.
• Men and women responded similarly overall. One measure (low-frequency power) tended to be lower in men after exercise.

Influence Of High-Intensity Interval Exercise On Postexercise Autonomic Nervous System Dynamics Between Sexes

CESAR A. CARRERA JR., B.S., ACSM-EP, MATT LEE, PH.D, FACSM, KENT A. LORENZ, PH.D, JAMES R. BAGLEY, PH.D

Exercise & Muscle Physiology Laboratory; Department of Kinesiology; San Francisco State University; San Francisco, CA 

EXERCISE

PHYSIOLOGY LAB

Figure 1. Experimental protocol. After a 5-min seated baseline, participants completed either high-intensity interval exercise (HIIE; 28 min) or continuous exercise (CE; 25 min) on separate visits, followed by 30 min of postexercise recovery. ECG and VO₂ were recorded continuously. Heart rate variability (HRV), blood pressure (BP), and VO₂ were analyzed during the baseline and the 5-10, 15-20, and 25-30 min postexercise windows (blue). The intervention block is not drawn to scale.

• Main effect of time on RR interval, which decreased postexercise (p<0.001)
• Main effect of time on ln LF/ln HF showed it was higher postexercise (p<0.001)

ln HF was lower after HIIE (condition effect, p < 0.001); condition × time trend (p = 0.047)

ln RMSSD was lower after HIIE (condition effect, p < 0.001); �condition × time trend (p = 0.064)

ln LF tended to be lower in males postexercise�(sex × time trend, p = 0.043)

ln LF was lower after HIIE (condition effect, p < 0.001); condition × time trend (p = 0.028)

ln RMSSD was lower in males overall (sex effect, p < 0.001); sex × time, p = 0.517

LFnu was higher in males overall (sex effect, p = 0.003); sex × time, p = 0.628.

Panel 1. Postexercise (A) ln RMSSD and (B) ln HF following HIIE and CE. Values are estimated marginal means from the linear mixed model (sex × condition × time; diagonal covariance; VO₂ max covariate for ln RMSSD and ln HF). Effects were evaluated at a Bonferroni-adjusted α = 0.025. (n = 23; 22 per condition, 21 at the 15–20 min window). Error bars represent the 95% confidence interval. * CE significantly different from HIIE (p < 0.05, pairwise).

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Panel 2. Postexercise (C) ln LF power following HIIE and CE. Values are estimated marginal means; error bars represent the 95% CI. * CE significantly different from HIIE (p < 0.05, pairwise). Postexercise (D) ln LF power in males and females, collapsed across conditions. Values are estimated marginal means; error bars represent the 95% CI. † Females significantly different from males at 5-10 min (p < 0.05, pairwise).

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Panel 3. Postexercise (E) ln RMSSD and (F) LFnu in males and females, collapsed across conditions. Values are estimated marginal means; error bars represent the 95% CI. † Females significantly different from males (p < 0.05, pairwise).

A

B

Effect sizes: Partial η² for the CE vs HIIE comparison in ln RMSSD, ln HF, and ln LF at each time point. Dashed lines mark conventional thresholds for medium (.06) and large (.14) effects.

Diastolic blood pressure: DBP mean values following HIIE and CE in males and females. Values are estimated marginal means; error bars represent the 95% CI. † Males had higher diastolic BP than females after CE (p < 0.001), but not after HIIE (sex × condition, p = 0.021).

Table 1. Values are mean ± SD; VO₂ max range in parentheses. BMI = body mass index. Hedges’ g = standardized effect size (male vs female). P values from independent-samples t-tests.

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C

D

E

F

INTRO/BACKGROUND

METHODS

RESULTS

BOTTOM LINE

Bottom line: The interval workout turned the body’s calming system down more, and it stayed lower through the 30-minute recovery. Whether it switched back on more slowly was only a trend, not a firm result. Men and women recovered mostly the same.

CONCLUSION

References

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