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

Abstract

High intensity interval training is frequently implemented using the 4 × 4 protocol where four 4-min bouts are performed at heart rate (HR) between 85 and 95% HR max. This study identified the HR and power output response to the 4 × 4 protocol in 39 active men and women (age and VO₂ max = 26.0 ± 6.1 years and 37.0 ± 5.4 mL/kg/min). Initially, participants completed incremental cycling to assess VO₂ max, HR max, and peak power output (PPO). They subsequently completed the 4 × 4 protocol, during which HR and power output were monitored. Data showed that 12.9 ± 0.4 min of 16 min were spent between 85 and 95% HR max, with time spent significantly lower in interval 1 (2.7 ± 0.6 min) versus intervals 2-4 (3.4 ± 0.4 min, 3.4 ± 0.3 min, and 3.5 ± 0.3 min, d = 2.4-2.7). Power output was highest in interval 1 (75% PPO) and significantly declined in intervals 2-4 (63 to 54% PPO, d = 0.7-1.0). To enhance time spent between 85 and 95% HR max for persons with higher fitness, we recommend immediate allocation of supramaximal intensities in interval one.

Keywords: exercise prescription; heart rate; high intensity interval training; intermittent exercise.

Figure 1

(a) Mean heart rate response to each of the four intervals during the 4 × 4 protocol (mean ± SD). * = p < 0.05 for interval 1 versus intervals 2–4; (b) overall HR response during the 4 × 4 protocol for all 39 participants. Note that minutes 4–8, 11–15, 18–22, and 25–29 represent the four intervals of the session. The 2 lines represent the HR zone from 157 to 176 b/min, representing 85–95% HR max.

Figure 2

(a) Mean power output response to each of the four intervals during the 4 × 4 protocol (mean ± SD). * = p < 0.05 versus intervals 2–4; (b) overall power output response during the 4 × 4 protocol for all 39 participants. Note that minutes 4–8, 11–15, 18–22, and 25–29 represent the four intervals of the session.