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Why Strength Training Might Come At The Expense Of Endurance Muscles

junction in fast-twitch muscle

The neurotransmitter brain-derived neurotrophic factor (BDNF) is produced by the muscle and acts on both muscles and synapses, researchers at the University of Basel’s Biozentrum have found.

In their recent work, Prof. Christoph Handschin’s research group at the Biozentrum closely studied strength muscles and the myokine BDNF, which plays an important role in the formation of strength muscle fibers.

Handschin’s team has demonstrated that this factor is produced by the muscle itself and remodels the neuromuscular synapses, the neuronal junctions between the motor neurons and muscle. BDNF not only causes the strength muscles to develop, but at the same time leads to endurance muscle fiber number decline.

Muscles And Synapses

Generally, there are two types of muscle, depending on the type of fibers they are made of: There are the slow-twitch fibers for endurance muscles, which are formed mainly during endurance sports. Marathon runners primarily exercise this type of muscle.

A great deal less well studied is the second form of muscle consisting of fast-twitch fibers. These strength muscles gain in volume during strength training and provide substantial muscular power.

“It is interesting that BDNF is produced by the muscle itself and not only exerts an influence on the muscle. At the same time, it affects the neuromuscular synapses, which are the junctions between the motor neurons and muscle,”

explains Handschin.

This remodeling of the neuromuscular synapses during strength training results in the body developing more strength muscle fibers.

“However, strength muscle growth occurs at the expense of the endurance fibers. More precisely, through the release of BDNF, the endurance muscles are transformed into strength muscles,”

clarifies Handschin. This makes BDNF a factor proven to be produced by the muscle itself and to influence the type of muscle fibers formed.

Relevance To Muscle Training

The new knowledge gained about the myokine BDNF also provides a possible explanation for the decrease in endurance musculature seen as a result of strength training.

Innervation of the acetycholine receptors (green) on the muscle fiber by the motor neuron
Innervation of the acetycholine receptors (green) on the muscle fiber by the motor neuron (red).
Credit: University of Basel, Biozentrum

This correlation is already being taken into account in the training plan for high performance sports. Particularly in sporting disciplines such as rowing, which are geared towards strength and endurance, the muscle remodeling must be considered.

Moreover, in a follow-up study, the research group showed that in muscle lacking BDNF the age-related decline in muscle mass and function is reduced.

“We didn’t expect this result. It also makes the findings interesting for treatment approaches for muscle atrophy in the elderly,”

says Handschin.

Julien Delezie, Martin Weihrauch, Geraldine Maier, Rocío Tejero, Daniel J. Ham, Jonathan F. Gill, Bettina Karrer-Cardel, Markus A. Rüegg, Lucía Tabares, Christoph Handschin
BDNF is a mediator of glycolytic fiber-type specification in mouse skeletal muscle
Proceedings of the National Academy of Sciences Jul 2019, 201900544; DOI: 10.1073/pnas.1900544116

Top Image: Neuromuscular junction in fast-twitch muscle. Credit: Dr Guy Bewick, Aberdeen Univ.. CC BY-NC