Sprint intervals vs an hour of cardio: what an RCT involving biopsies revealed

A recent cross-over RCT in the American Journal of Physiology compared a single session of sprint intervals with an hour of moderate cardio in the same men. The sprinters performed six times less mechanical work – and achieved a greater and longer-lasting increase in insulin sensitivity. I'll break down what this means in practice, the limitations of this finding, and when an hour is still worthwhile.

What have [they/you] done

Design – a randomised cross-over study. 12 healthy recreationally active young men attended the laboratory on three occasions, with intervening periods between visits. Each completed three protocols:

SIE (sprint interval exercise) Seven all-out sprints of 30 seconds each at 130% intensity — a total of 35 minutes, including rest periods and a warm-up
MICE (moderate-intensity continuous exercise): 60 minutes on the exercise bike at 65% intensity
Against Control session without exercises

Cross-over means that each participant served as their own control – the comparison is not between groups of people, but between three states within the same body. This sharply reduces noise from individual differences.

Insulin sensitivity was measured using a hyperinsulinaemic euglycaemic clamp, which is the gold standard in metabolic research. In simple terms: insulin is administered intravenously at a fixed rate, and glucose is given as needed to maintain a stable blood glucose level. The amount of glucose that needs to be added indicates how much glucose the tissues «consume» under the action of insulin. This is not a surrogate like HOMA-IR from morning blood samples; it is a direct physiological measurement.

Separate biopsies of skeletal muscle were performed to examine glycogen stores and the phosphorylation of signalling proteins in the insulin pathway.

What did you find

The main figure from the abstract: The overall mechanical work at SIE was six times less than at MICE. Essentially, sprinters completed less distance on the exercise bike, even though the load during peak minutes was supramaximal.

Despite this, SIE called Greater and more sustained increase in insulin sensitivity at a whole-body level. Not «no worse» – but bigger. The duration of the effect was also longer.

At muscle level:

Glycogen was lower before the clamp after SIE (more effective «depot depletion»)
During clamping, glycogen was replenished more intensely – the muscle more actively drew in glucose
Reduced phosphorylation of glycogen synthase (GS) and GSK-3β are markers of glycogen synthesis activation.
Akt at Thr308 and Ser473 - no significant difference between protocols

The last point is interesting: the classic insulin cascade via Akt does not explain the difference. A parallel pathway is likely at work – more directly through the rearrangement of glycogen metabolism. That is, the SIE effect does not go through «enhancement of the insulin signal», but through a change in the state of the muscle, in which insulin acts more effectively.

What does this mean in practice

First and foremost — Intensity is more important than volume, However, at least for the acute metabolic response. This is not a new thesis, but this RCT gives it a point: 6 times less work – a greater effect. Previously, it was easy to attribute interest in HIIT to «less rigorous studies» or «short-term markers.» Here: clamp, biopsies, crossover design on the same people.

The second — Sprint intervals are time-efficient in the strongest sense of the word.. You invest 35 minutes (of which there are 3.5 minutes of actual work, in 30-second bursts), and you get more than an hour of steady pedalling. If the goal is metabolic health, rather than endurance training per se, the argument against «no time for an hour» disappears.

Thirdly, this doesn't mean an hour of moderate cardio needs to be thrown out. The study measures a single acute session and the response in the hours following it. Chronic adaptations from MICE – mitochondrial density, aerobic base, cardiovascular endurance – lie outside this window. If the goal is broader than acute insulin, MICE and SIE are complementary.

What are the limits of this conclusion

The list of restrictions is long and should be read carefully:

Twelve men. Not women, not elderly, not diabetics, not people with obesity. Precisely the group where metabolism is most flexible without any interventions.
Healthy and active. If someone hasn't moved for a year and has insulin resistance, transferring the numbers directly is risky. The reaction could be different in volume and duration.
Acute reaction, not a chronic effect. RCT measured one session. How many such sessions per week give an optimum, how the effect accumulates, whether the SIE advantage is maintained with regular training - separate questions
130% is the absolute maximum. «Doing intervals in the gym» with a heart rate of 80% below the maximum isn’t the same as SIE on paper. If the intensity is lower, the effect is likely to be lower too. Exactly by how much is unknown
30 seconds x 7 reps sounds tough. Switching to this protocol from scratch is a bad idea. Cardiovascular conditioning should be at a good level.

What to take

If time is limited, metabolic health is a priority, and basic cardio fitness allows for it, then short, intense intervals yield more per minute of time invested than steady-state cardio. This is now confirmed not only by surrogate markers but also by clamps and biopsies.

If time is limited, endurance is important, or your starting fitness level is low, it's not worth dropping steady-state cardio for SIT. They are different tools for different tasks.

And here’s another detail that people rarely pay attention to. In the study, the SIE protocol included not just seven sprints — the full session lasted 35 minutes. So «3 minutes of intervals» is just marketing. The warm-up, the intervals between sprints, and the cool-down are essential; otherwise, your heart and legs won’t thank you for the 130%.

Source: Murphy E, Laurens C, Frances L, et al. Acute metabolic and molecular responses to sprint interval versus moderate-intensity continuous exercise in healthy young men. American Journal of Physiology – Endocrinology and Metabolism. 2026 May 1;330(5):E562-E571. DOI: 10.1152/ajpendo.00548.2025