Physical exercise stimulates brain function through its effect on the muscles

Exercise plays a major role in brain health by triggering the release of molecules linked to cognitive function from muscles, a new study has found.

A study published in the journal Proceedings of the National Academy of Sciences shows that when nerves that stimulate muscles are activated, they also send signals to the brain, releasing bioactive molecules and nanoparticles that improve brain function.

The findings highlight the importance of maintaining musculoskeletal health not only for cardiovascular health or mobility, but also to counteract neurodegeneration, said Hyunjun Kong, PhD, a professor in the department of chemical and biomolecular engineering at the University of Illinois at Urbana-Champaign and a co-author of the study.

He explained that: "Maintaining neuronal innervation is critical to allowing muscles to produce biological factors that are beneficial to the brain. With regular muscle contractions, the muscles not only release these beneficial factors, but they also help maintain the innervation needed to ensure that the nerves continue to send signals to the muscles. These signals are needed to regulate the release of neurotrophic factors to the brain."

The study stimulated muscles with glutamate to see how nerve function responded. The researchers used models of muscle tissue, one innervated and one uninnervated, and found that the innervated tissue sent more signals to the brain.

Because some neuron function in muscles can decline with age or injury, the researchers were interested in how this loss would affect brain health.

How does exercise affect brain function?

In this study, the researchers did not observe people exercising. Instead, they studied models of muscle tissue, which means it was impossible to draw direct conclusions about specific forms of exercise and their effects on brain health.

Kong said previous studies on exercise and brain health have shown a direct correlation between the size of the hippocampus and regular physical activity. But for this new study, the researchers looked at the nervous system and musculoskeletal function to better understand the interactions between the brain and body.

“Our study did not prove how exercise can directly improve cognitive function,” Kong cautioned. “Several studies have already shown that regular exercise can improve cognitive function in adults. In these studies, participants were asked to exercise regularly and changes in the size of their hippocampus were tracked.”

"The results show that people who regularly do aerobic exercise have larger hippocampi and show improved performance on spatial memory tests. We focused on how muscle-related neurons affect one of the pathways between muscles and the brain." — Hyunjun Kong, PhD

Ryan Glatt, CPT, NBC-HWC, a senior brain health coach and director of the FitBrain program at the Neuroscience Institute in Santa Monica, California, who was not involved in the study, said it will take time to fully understand the effects of exercise on the brain, and future studies in humans will need to take it into account.

"The duration needed to observe the effects of exercise on cognitive function may vary depending on the type, intensity, and frequency of exercise, as well as individual differences in age, baseline cognitive function, and health status," Glatt said.

"Studies typically show effects over a period of weeks to months. It is important that future studies take these variables into account to accurately assess the time frame," he said.

What types of exercise are best for brain health?

Regular exercise has significant benefits for brain health. For example, boxing has been used to help people with Parkinson's disease.

Glatt noted that some types of exercise may be more effective than others at improving brain health, particularly those that require cognitive function.

"Aerobic exercise such as running, swimming and cycling, which improve cardiovascular health, is widely known for its positive effects on brain function," he said.

"However, activities that combine physical and cognitive demands - such as dance and team sports - may offer additional benefits due to the demands on coordination, rhythm and executive functions," he added.

Can exercise reverse or slow cognitive decline?

Kong suggested that physical activity could potentially reverse or slow down the cognitive decline associated with aging.

"As people age, they gradually lose well-formed neuromuscular junctions between nerves and muscles, which impairs the ability of muscles to be regulated by nerve signals and, consequently, reduces their ability to secrete factors critical to brain function," Kong explained.

"With appropriate training or stimulation of muscle contractions, muscles can produce factors that help maintain these neuromuscular junctions, preventing denervation. As a result, older adults can still have functionally innervated muscles that can produce important factors that improve cognitive function in the brain," he noted.

Glatt pointed out that a sedentary lifestyle with relatively low levels of physical activity may be associated with a higher risk of cognitive decline.

However, he cautioned that there are various individual differences related to genetics, lifestyle and environmental factors.

"Exercise is increasingly recognized as a supportive intervention for people with impaired cognitive function, including those suffering from age-related declines or conditions such as Alzheimer's disease. Physical activity can improve blood flow to the brain, reduce inflammation and stimulate the release of growth factors, which can help maintain or improve cognitive function," Glatt said.

"While exercise is beneficial for maintaining cognitive health and potentially slowing the progression of decline, evidence for its ability to reverse existing cognitive impairment is still inconclusive. Most research suggests that exercise may promote a slower rate of decline and better overall brain health, but more extensive research is needed to reverse established cognitive deficits," he cautioned.