Effort-Based Pacing Matches Speed-Based Interval Training for Aerobic Gains With Less Strain
Running intervals by perceived effort gave the same time near VO2max as speed-based pacing, with lower heart rate, lactate and perceived exertion.
Summary
VO2max, one of the strongest predictors of lifespan, improves most when you spend time near your maximum oxygen uptake during hard intervals. Researchers compared two ways of setting interval intensity in 13 young men: running at a fixed 85% of maximal aerobic speed, or running at a self-rated effort of 5 out of 10 and adjusting speed as fatigue built. Both approaches kept runners at 90% or more of VO2max for about half of each session (48% vs 49%). But the effort-based approach felt easier, with lower peak heart rate, breathing rate, blood lactate and session effort ratings. Runners naturally started fast and slowed down, which prevented the steady creep in heart rate and breathing seen with fixed pacing. The takeaway: you can get the same aerobic stimulus without a lab test, using only how hard it feels.
Detailed Summary
Cardiorespiratory fitness, measured as VO2max, is among the strongest known predictors of all-cause mortality, and high-intensity interval training (HIIT) is considered one of the most effective ways to raise it. The key driver appears to be accumulating time at or above 90% of VO2max. Traditionally, coaches prescribe interval speed as a fixed percentage of maximal aerobic speed (MAS), which requires a maximal lab test. This study asked whether rating of perceived exertion (RPE) could do as well, or even better.
Thirteen physically active male physical education students (average age 23.6 years, VO2max 51.6 mL/kg/min) completed a maximal treadmill test, then two randomized HIIT sessions on a 400-meter track at least 48 hours apart. Each session had three 3-minute intervals with 2 minutes of passive recovery. One was run at RPE 5 on Borg's CR-10 scale, with runners starting fast and adjusting speed to hold effort constant. The other was run at a fixed 85% MAS, paced by beeps. Pilot work showed 85% MAS matched the average speed of the first RPE interval. Heart rate, gas exchange, GPS speed and lactate were recorded.
The primary outcome did not differ. Runners spent 48% of session time at or above 90% VO2max with RPE and 49% with MAS, and time at 90% of maximal heart rate was also similar. Average speed, VO2, ventilation and average heart rate were comparable. This contradicted the authors' hypothesis that RPE would produce more time near VO2max. Both sessions produced peak VO2 values above the incremental test maximum, and time near VO2max exceeded the 25-43% reported in earlier MAS-based studies.
The difference was in strain. With RPE, peak heart rate was lower (183.7 vs 187.9 bpm, p=0.028, d=0.41), as were peak respiratory frequency (56.7 vs 59.8 breaths/min, p=0.046, d=0.53), post-exercise lactate (9.6 vs 11.7 mmol/L, p=0.049, d=0.82) and session RPE (5.1 vs 6.0, p=0.021, d=1.2). Yet peak speed was higher with RPE (17.9 vs 16.4 km/h, p=0.008, d=1.1) because runners started fast. Lower lactate likely reflected lower mean speed and greater aerobic reliance.
Interval-by-interval data explained why. With RPE, speed declined across intervals while VO2, ventilation and mean heart rate stayed flat. With fixed pacing, heart rate, VO2 and ventilation crept upward, a cardiorespiratory drift that adds fatigue. By the third interval, peak heart rate (182.2 vs 187.6 bpm), peak VO2 (54.6 vs 57.4 mL/kg/min) and peak respiratory frequency were all lower with RPE. The authors suggest minute ventilation acted as the main regulator of speed, and speculate that RPE could allow extra intervals.
Effort-based prescription needs no maximal test and no detailed pacing plan, which suits gyms, clinics and recreational athletes. Caveats are substantial: 13 young, active men, one RPE level, one short session format, a single session per condition, and no verification test for VO2max. These are acute responses, not training adaptations over weeks, and may not transfer to women, older adults or clinical populations.
Key Findings
- Time at or above 90% VO2max was nearly identical: 48% of session time with RPE-based vs 49% with MAS-based HIIT (no significant difference)
- Peak heart rate was lower with RPE prescription: 183.7 vs 187.9 bpm (p=0.028, d=0.41)
- Post-exercise blood lactate was lower with RPE: 9.6 vs 11.7 mmol/L (p=0.049, d=0.82, large effect)
- Session RPE was lower with RPE prescription: 5.1 vs 6.0 au (p=0.021, d=1.2, large effect)
- Peak speed was higher with RPE (17.9 vs 16.4 km/h, p=0.008, d=1.1) because runners started fast then slowed, while average speed did not differ
- By the third interval, peak VO2 (54.6 vs 57.4 mL/kg/min, p=0.048) and peak respiratory frequency (54.3 vs 58 breaths/min, p=0.036) were lower with RPE, indicating less cardiorespiratory drift
- Both sessions exceeded the 25-43% of session time near VO2max reported in earlier MAS-based HIIT studies, with some peak VO2 values above the incremental test maximum
Methodology
This was a randomized, within-subject crossover study of 13 male physical education students (mostly recreationally active) who first completed a maximal incremental treadmill test with breath-by-breath gas analysis to determine VO2max and MAS. They then completed two HIIT sessions at least 48 hours apart on a 400-m track, each with three 3-minute intervals and 2-minute passive recovery, prescribed at RPE 5 (Borg CR-10) or 85% MAS (beep-paced). Heart rate, gas exchange, GPS speed, post-session lactate and session RPE were measured. Data were analyzed with repeated-measures ANOVA, Tukey post hoc tests and Cohen's d (significance p<0.05).
Study Limitations
The sample was small (13) and consisted only of young, mostly recreationally active men, limiting generalizability to women, older adults and clinical populations. Only one RPE level, one interval format and one session per condition were tested, and no verification bout confirmed true VO2max, with several participants showing supramaximal peak VO2. The study measured acute responses, not training adaptations, and the claim that RPE could allow extra intervals is speculative. No conflicts of interest were reported in the supplied text.
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