Author(s): MAZZOLARI, R., PAUL, S., LIEBEL, P., HECKSTEDEN, A., Institution: MEDICAL UNIVERSITY OF INNSBRUCK, Country: AUSTRIA, Abstract-ID: 1710

INTRODUCTION:
During prolonged vigorous exercise, the relationship between external load and heart rate (HR) is not constant. Specifically, for a given running speed or power output, HR increases over time, or – conversely - running speed or power output declines when exercise is paced to maintain a constant HR [1,2]. Previous studies in untrained individuals show that, even when initial intensity is equivalent, the choice of exercise intensity reference (speed vs. HR) produces different cardiorespiratory and perceptual responses by the end of exercise, with these differences persisting even after 8 weeks of training [2]. Interestingly, recent evidence suggests that speed-HR decoupling may be inversely related to performance level [3]. This could make these two intensity reference methods more interchangeable for prescribing exercise to endurance athletes. However, it is important to bear in mind that for practical application, interchangeability must also be established at the individual level, a factor that has not yet been systematically studied.
METHODS:
We used a replicated crossover design in a cohort of distance runners. After an incremental treadmill test, participants completed three cycles of paired sessions in randomized order: a constant-HR session (HR-C) and a constant-speed session (SPEED-C). HR-C consisted of 30 minutes of exercise at a HR corresponding to the midpoint between the first and second lactate thresholds; SPEED-C consisted of 30 minutes at the running speed corresponding to the same midpoint. During sessions, we recorded HR, speed, oxygen consumption, and rating of perceived exertion. Data were analyzed using linear mixed models to assess both the average effect and the subject-by-prescription method interaction. Shrinkage-adjusted individual estimates were obtained, and the consistency of individual responses across outcomes was assessed via correlation analysis.
RESULTS:
Twenty-one runners (13 men, 8 women) completed at least two cycles and were included in the analysis. SPEED-C elicited a significantly larger physiological and perceptual response than HR-C for all measured markers (all p<0.001). A significant subject-by-prescription interaction was observed for all outcomes (all p<0.001). Moderate to large correlations were observed across all outcomes.
CONCLUSION:
The choice of exercise-intensity reference method significantly affects acute physiological and perceptual responses during continuous endurance exercise in distance runners. Importantly, the significant subject-by-prescription interaction and the consistency of responses across outcomes indicate that the difference between intensity prescription methods is highly individual. The relevance of this result for individualized exercise programs warrants further study.

[1] Zuccarelli 2018/ [2] Mazzolari 2025/ [3] Smyth 2022