Physical activity is found to prompt a clean-up of muscles by increasing the activity of protein Ubiquitin, which tags onto worn-out proteins and causes them to degrade.

It is universally accepted that maintaining muscular function is essential and a big part of the human body's ability to do so depends on proteins – the building blocks of muscles.

The proteins do, however, degrade when worn-out and are eventually eliminated in a clean-up process that allows them to be replaced by freshly synthesised proteins.

While extensive knowledge has been accumulated about how muscles regulate the build-up of new proteins during physical training, much less is known about how muscle contractions and exercise serve to clean-up the worn-out proteins.

Now, researchers at the University of Copenhagen’s Department of Nutrition, Exercise and Sports, have demonstrated that a single, intense, 10-minute bicycle ride results in a significant increase in the activity of Ubiquitin, the 'death marker protein' and a subsequent intensification of the targeting and removal of worn-out proteins in muscles.

Ubiquitin itself is a small protein and attaches itself to the amino acid Lysine on worn-out proteins, after which the protein is transported to a Proteasome, which is a structure that, according to the research, "gobbles up proteins and spits them out as amino acids".

These amino acids can then be reused in the synthesis of new proteins. As such, Ubiquitin contributes to a very sustainable circulation of the body's proteins.

Professor Jørgen Wojtaszewski, one of the researchers, says the findings serve to strengthen the entire foundation for the effect of physical activity.

"Basically, it explains part of the reason why physical activity is healthy," he said.

"The beauty is that muscle use, in and of itself, is what initiates the processes that keep muscles 'up to date', healthy and functional."

Professor Bente Kiens, another member of the team, added: "The important role of Ubiquitin for 'cleaning-up' worn-out proteins in connection with muscular activity was not fully appreciated. Now we know that physical activity increases Ubiquitin tagging on these proteins."

For the study, a group of men, aged between 26 and 28 years-old, completed 8-11 minute training sessions an exercise bikes.

Blood tests and muscle biopsies were taken prior to and upon the completion of their training session.

The muscle biopsies were studied using mass spectrometry, which demonstrated how Ubiquitin was used on a large scale to clean up damaged proteins.

• The study, titled Quantification of exercise-regulated ubiquitin signaling in human skeletal muscle identifies protein modification cross talk via NEDDylation in the FASEB Journal. To access the article, click here.