Changes in heart rate variability with respect to exercise intensity and time during treadmill running

Area of Science:

• Exercise Physiology
• Autonomic Nervous System Regulation
• Cardiovascular Dynamics

Background:

• Heart rate variability (HRV) reflects autonomic control of heart rate.
• Ultra-low frequency (ULF) and very-low frequency (VLF) HRV components are critical for heart rate regulation but understudied.
• This study investigates ULF and VLF HRV changes during graded treadmill exercise.

Purpose of the Study:

• To examine the impact of exercise intensity on ULF and VLF HRV.
• To assess the time-dependent changes in ULF and VLF HRV during running.
• To understand the role of these HRV components in exercise physiology.

Main Methods:

• ECG-derived RR intervals were recorded from 21 healthy males at rest and during moderate and vigorous treadmill running (45 min each).
• Time-dependent HRV analysis involved dividing exercise stages into three 14-min windows.
• ULF and VLF power spectral densities were calculated using Lomb-Scargle estimates.

Main Results:

• Mean ULF and VLF power significantly decreased with increasing exercise intensity (rest > moderate > vigorous).
• During moderate intensity, ULF and VLF power decreased over time (window 1 > window 2 > window 3).
• During vigorous intensity, VLF power decreased over time, while ULF power showed no significant time-dependent changes.

Conclusions:

• Exercise intensity significantly impacts ULF and VLF heart rate variability, with higher intensities reducing HRV.
• HRV, particularly VLF components, declines over the duration of vigorous exercise.
• Findings suggest prioritizing low-intensity exercise and early exercise stages for effective feedback control systems.