Characterizing the Heart Rate Response to the 4 × 4 Interval Exercise Protocol.

Area of Science:

• Exercise Physiology
• Sports Science
• Cardiovascular Training

Background:

• High-intensity interval training (HIIT) is a popular training method.
• The 4x4 protocol, involving four 4-minute bouts at 85-95% maximal heart rate (HR max), is commonly used.
• Understanding the physiological response to this protocol is crucial for optimizing training.

Purpose of the Study:

• To investigate the heart rate (HR) and power output response during the 4x4 HIIT protocol.
• To determine the time spent within the target HR zone (85-95% HR max) during each interval.
• To analyze the changes in power output across the four intervals.

Main Methods:

• 39 active men and women (VO2 max: 37.0 ± 5.4 mL/kg/min) participated.
• Participants underwent an incremental cycling test to determine VO2 max, HR max, and peak power output (PPO).
• The 4x4 HIIT protocol was performed, with HR and power output continuously monitored.

Main Results:

• Participants spent an average of 12.9 minutes within the 85-95% HR max zone out of 16 total interval minutes.
• Time spent in the target HR zone was significantly lower in the first interval (2.7 min) compared to subsequent intervals (3.4-3.5 min).
• Power output was highest in the first interval (75% PPO) and significantly decreased in intervals 2-4 (54-63% PPO).

Conclusions:

• The initial interval of the 4x4 protocol does not effectively achieve the target 85-95% HR max intensity.
• Power output significantly declines throughout the 4x4 protocol.
• To maximize time within the target HR zone, especially for fitter individuals, initiating the first interval at supramaximal intensity is recommended.