We read with interest and some concern the article titled “Intensity Matters: Effect of Different Work-Matched Efforts on Subsequent Performance in Cyclists,” pointing out that high-intensity interval exercise (HIIT) may induce greater fatigue than work-matched continuous exercise of moderate intensity (MICT). Using mechanical work to match efforts raises issues.^1,2

In this study, MICT and HIIT were designed to induce a work of 15 kJ/kg. However, a single HIIT session does not allow you to reach that amount, so the protocol added a 60-minute warm-up before the HIIT. The HIIT session was longer than the MICT, which is surprising since HIIT has been widely supported as a time-efficient way of training.

The critical power (CP) model was used to prescribe the power output (PO) of the exercises throughout the study. The 3-minute intervals of the HIIT session were performed at 346 W for a cumulative exercise duration of 24 minutes, whereas the CP model would estimate a maximal PO of 315 W for a continuous exercise of that duration. The W′ Balance model was developed from the CP to describe fatigue and recovery evolutions during intermittent exercises.³ Despite limits of the calculations (eg, no individualized recovery data), the W′ Balance model would suggest that fatigue might have occurred after ≈15 minutes for that HIIT session.³ Conversely, MICT was performed for 93 minutes at 199 W, whereas the CP model would suggest a maximal PO of 306 W for this duration. The accuracy of the CP model for exercises of long duration presents some limits; a power-law model would rather estimate a maximal PO of about 230 W.^4,5 Therefore, regardless of the calculations made, it is very likely that HIIT would have led much closer to exhaustion than MICT. The work–duration relationship for continuous as well as intermittent exercises is widely accepted and forms the basis of the CP model considered in the study; maximal work that can be accumulated at moderate intensities is significantly higher than for high intensities. Balancing the efforts made in HIIT and MICT using raw values of mechanical work suggests that this relationship is completely ignored.⁶ This results in a longer and much harder exercise in an HIIT session than in MICT.

Because of the intrinsic differences in the efforts compared, we are not convinced that the study provides strong evidence that an HIIT session may induce greater fatigue than MICT.