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 analysed 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 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.
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 analysing around 11,000 individual mitochondria. This made it possible to detect a change of around 7 per cent in the active membrane – a difference that previous training studies were not able to demonstrate, explains Martin 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.
– 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 Martin 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 programme. This allowed the researchers to investigate what eight weeks of high-intensity interval training did to muscle mitochondria.
What are you investigating in this study? We are investigating how eight weeks of high-intensity interval training affects the structure of the muscle’s energy-producing units – the mitochondria – in both healthy individuals and people with type 2 diabetes.
What is the most important finding? In addition to increasing the number of mitochondria, training also increases the amount of active membrane – the cristae – inside them, which likely enhances their ability to deliver energy.
What can the results be used for? The results show that muscle mitochondria are more adaptable than previously thought. This new knowledge contributes to our understanding of how training improves the muscle’s energy supply and may have implications for future training and treatment strategies – particularly in relation to metabolic health and type 2 diabetes.
In muscle cells, small power plants – mitochondria – produce energy that allows the cell, and therefore the muscles, to work. Here, energy from food is converted into the form of energy that muscles use: ATP.
The inner structure of the mitochondria – the cristae – is folded, rather like a crumpled paper bag. Cristae form an active membrane – this is where energy production takes place. The more folds there are, and the more closely packed they are, the greater the working surface they have for producing energy.
Why is this important?
More numerous and more densely packed folds mean the mitochondria can produce more energy within the same space. This results in a more efficient energy system, without the mitochondrial network necessarily having to expand. This is important because it shows that energy production can be improved by altering the internal structure of the mitochondria – not only by creating more of them.
Method: The study included 44 men – 15 with overweight and type 2 diabetes, 15 with overweight but without diabetes, and 18 of normal weight. All participants were men aged 40–65. The training programme lasted eight weeks.
Participants completed high-intensity interval training three times a week using rowing and cycling. The training consisted of short, very intense intervals interspersed with breaks.
Before and after the training programme, the researchers took small muscle biopsies from the thigh, which were analysed by electron microscopy. This method makes it possible to visualise mitochondria and cristae in extremely fine detail.
Funding: The study was funded by Steno Diabetes Center Odense, which is supported by the Novo Nordisk Foundation (NNF17SA0030962-1). The project also received funding from the Region of Southern Denmark (19/37137), Odense University Hospital, the Novo Nordisk Foundation (NNF150C0015986), the University of Southern Denmark, the Christenson-Ceson Family Foundation and the Jeppe Juhl and wife Ovita Juhl Memorial Grant.
Read the study: The study Mitochondrial cristae density is increased following high-intensity interval training in men with type 2 diabetes is published in Diabetologia , a journal published by the European Association for the Study of Diabetes (EASD). Read the study: https://doi.org/10.1007/s00125-026-06694-6
Diabetologia
Experimental study
Human tissue samples
Mitochondrial cristae density is increased following high-intensity interval training in men with type 2 diabetes
8-Mar-2026