High-intensity interval training boosts muscle mitochondria, study shows

Schematic overview of participants and the 8-week HIIT protocol. Credit: Diabetologia (2026). DOI: 10.1007/s00125-026-06694-6

Researchers from the University of Southern Denmark investigated how eight weeks of high-intensity interval training affect the structure of mitochondria—the parts of muscle cells that produce energy. The study shows that training not only increases the number of mitochondria but also expands the active membrane—the cristae—inside them.

“We analyzed muscle samples from our study participants. We were able to measure that the number of mitochondria had increased after eight weeks of training and, importantly, our measurements showed that the active membrane inside the mitochondria had also expanded. So training does not just create more power plants but more efficient ones,” explains Martin Eisemann de Almeida, postdoc at the Department of Clinical Research, the Department of Sports Science and Clinical Biomechanics and Steno Diabetes Center Odense.

The findings suggest that high-intensity training can improve the muscles’ ability to produce energy. In the long term, this may influence how training can strengthen the muscles’ capacity to deliver energy, contributing to better endurance and physical function in individuals.

The effect is also observed in people with type 2 diabetes

The effect was observed in healthy participants, people with overweight and those with type 2 diabetes.

The results show that the muscle’s ability to adapt to training is preserved in people with type 2 diabetes—despite previous assumptions that it might be impaired. The findings therefore challenge the common perception that diabetes may impair the muscle’s adaptation to training.

Changes our understanding of how muscles produce energy

Previous studies were not able to demonstrate changes in the internal structure and quality of mitochondria, but this new study shows that the active membrane—the cristae—can be altered through training. What makes this study unique is its exceptionally high level of precision.

“We spent a year manually analyzing around 11,000 individual mitochondria. This made it possible to detect a change of around 7% in the active membrane—a difference that previous training studies were not able to demonstrate,” explains Eisemann de Almeida.

The results show that the density of mitochondrial cristae increased after the training period. In other words, the inner structure of the mitochondria changed in a way that makes them better able to supply energy.

The results need to be confirmed in larger studies

“This is a small study including only men, so if we are to draw firmer conclusions about the effect, it needs to be repeated in larger and more diverse groups,” explains Eisemann de Almeida.

“The study also does not tell us whether the changes are maintained in the long term, but it provides new evidence that muscle mitochondria are more adaptable than previously thought. The changes can most likely be maintained or further improved with continued training, while stopping training will probably cause the mitochondria to return to their baseline level.”

In the study, the researchers examined 44 men. The participants were divided into three groups: 15 with type 2 diabetes, matched with 15 people with overweight but without diabetes, and 18 people of normal weight. All participants underwent muscle biopsies—small tissue samples taken from the muscle—both before and after the eight-week training program. This allowed the researchers to investigate what eight weeks of high-intensity interval training did to muscle mitochondria.