Sprint intervals: can clusters make them better?

Rec = recovery; SIE30 protocol: 4 × 30-s work bout + 240-s active recovery; SIE15 protocol: 4 × 15-s work bout + 15-s active recovery + 15-s work bout + 225-s active recovery; SIE10 protocol: 4 × 10-s work bout + 10-s active recovery + 10-s work bout + 10-s active recovery + 10-s work bout + 220-s active recovery.

Physiological measurements were taken during sprint interval protocols as well. Pulmonary gases were collected throughout the interval training sessions. The data was used to calculate VO2peak during the work bouts, time spent above the respiratory compensation point, and time spent above 80% VO2max. The latter is considered particularly important as time spent above 80% VO2max has been related to significant improvements in aerobic endurance adaptations(2). Blood lactate levels were measured at the end of each protocol, with measurements taken at 1, 3, and 5 minutes post training. The researchers also asked the subjects for a rating of perceived exertion within 10 seconds of completing the training session.

The findings

When the results were calculated, there were some fascinating findings in terms of how clustered sprint interval training affected the physiological stress generated and the power outputs achieved during training. For oxygen uptake, 86% of maximum (VO2max) was reached during the 30-second intervals, and 96% and 94% of maximum was reached during the 15- and 10-second intervals, respectively. The peak levels of oxygen uptake measured during the recovery periods was also higher for both the 15- and 10-second intervals as compared to the 30-second protocol. Likewise, the key metric of ‘time spent above 80% of VO2max’ and the respiratory compensation point was significantly higher in the clustered interval groups as compared to the transitional intervals.

From an oxygen uptake perspective, clustered intervals were superior at generating high oxygen uptakes despite exercising for the same total time and taking breaks within the intensive intervals. Despite these differences in oxygen uptakes, there were no difference in peak heart rates, peak lactates, or session rating of perceived exertion. These measurements remained the same regardless of whether a traditional or clustered sprint interval protocol was used.

When it came to power outputs and the amount of work done during each sprint protocol, the trend was clear. Taking breaks improved power outputs, and more breaks paired with shorter intervals increased this effect (see table 1). Peak power was higher during the 10-second intervals than it was during the 15-second intervals, which was higher than the traditional intervals. The same effect was seen for the average power output over the course of the repetitions. Similarly, the minimum power output was higher as the clusters became shorter.

Since the work done is directly related to the power output of the activity, and power output was higher during the clustered protocols, so too was the work done across the different intervals. Interestingly, the fatigue index did not different across the protocols, with the subjects losing approximately 55% of their power output across each 30-second block of work. While the clustering the intervals allows for higher performance levels, those performance levels appear to drop by the same degree regardless of repetition length.

Table 1: Average power, total work and fatigue in traditional vs. clustered sprint intervals

Implications for athletes

The results of this study are a big win for the concept of cluster training when applied to sprint interval training. The clustered protocols allowed the subjects to spend more time with higher oxygen uptakes, create larger power outputs throughout the training, and do so without increasing lactate levels, heart rates, or rating of perceived exertion. These are valuable outcomes for two key reasons:

1)     The time spent at high oxygen uptakes is a key driver of aerobic adaptation. By clustering the intervals, the researchers were able to generate higher oxygen uptakes, and those higher uptakes were maintained for longer periods of time. Importantly, this was without changing the duration of the training or asking the subjects to perform more work. In short, the intervals were more effective and more efficient.

2)     Performance is often driven by high power outputs. The more athletes can practice generating higher power outputs, the more they can potentially learn to create and sustain these power outputs. Coaches can take advantage of clustered sprint interval training to enhance power outputs during the same work.

When applying these concepts to your training, consider opportunities where interval repetitions could be clustered to allow for higher performance output, particularly when a large amount of fatigue is likely to be present. This can allow for more high-quality practice, which can stimulate additional improvement over time. Likewise, if you’re using sprint interval training to drive aerobic adaptation as efficiently as possible, clustering the repetitions can help drive higher oxygen uptakes which can lead to enhanced aerobic development.

That’s not to say there isn’t value from performing traditional sprint intervals. Although there is aerobic activation and lower power outputs during the training, there is still a challenge present during extended sprinting, which may be lost when the intervals are broken up. Exercising a high intensity for a longer continuous duration creates a certain stress in the muscular system that is not wholly described just by power output, oxygen uptake, and lactate values. This is particularly true for athletes who need to exert themselves maximally for longer durations – eg runners competing at 200m, swimmers at 50m, track cyclists performing a 1-lap sprint etc. However, this study demonstrates there is additional value in using cluster strategies during some interval training sessions. But it’s not a binary choice that coaches and athletes must make; it’s possible to use both stimuli to keep training fresh, create challenge in novel ways, and stimulate a broader set of adaptations.

References

1. Eur J Appl Physiol.2025 Dec;125(12):3565-3578. doi: 10.1007/s00421-025-05857-4. Epub 2025 Jun 29

2. Front Sports Act Living. 2025 Jan 6:6:1507957