Declining muscle strength is common as people age, which can lead to falls and affect a person's ability to move and function independently. It's often believed that a muscle's strength is directly tied to its mass. However, researchers at the Ohio University Heritage College of Osteopathic Medicine have discovered that muscle weakness in aging populations occurs in part due to a breakdown in communication between the motor cortex, the part of the brain that controls voluntary movement, and muscles. Historically, the motor cortex was thought to affect movement and coordination but not muscle strength.
"Several previous studies compared young adults with seniors and found no differences in the ability of the nervous system to activate muscles," said Brian Clark, Ph.D., professor of physiology and neuroscience at the Heritage College and executive director of the Ohio Musculoskeletal and Neurological Institute (OMNI). "When we ran the same comparison, we didn't see differences either. However, we went a step further and grouped older adults by their level of muscle strength. When we did this, the differences between our stronger and weaker seniors became clear."
For the study, research participants were asked to flex their wrist before, during and after receiving electrical pulses to stimulate the nerves in their forearm, which flexes the hand. Additionally, Clark's team used transcranial magnetic stimulation, a non-invasive and painless way to explore brain physiology, to examine the relationship between weakness and levels of brain excitability. Significant differences were found when comparing senior groups, with weaker seniors having higher levels of inhibition in the brain and an impaired ability to voluntarily activate their muscles. While several age-related factors contribute to dynapenia (age-related loss of muscle strength), this study found that degradation of the motor cortex is also partially responsible.
Although the study did not identify what happens in other muscle groups or how aging alters the way the motor nerves fire, Clark is currently conducting a four-year research project funded by the National Institutes of Health to better identify the specific neurological mechanisms that play a role in weakness in older adults.
"Our results have major implications for clinicians and scientists who are developing interventions to enhance muscle strength in older adults. The most effective approach will most likely employ multiple strategies, including ones that work with the nervous system as well as the muscular system," said Clark.
The results from the study have been published in the Journals of Gerontology Series A: Biological Sciences and Medical Sciences. Along with Clark, the paper is co-authored by S. Lee Hong, Ph.D., Timothy D. Law, D.O., and David W. Russ, P.T., Ph.D. of OHIO as well as Janet L. Taylor, M.D. from Australia.
"Many people are adversely affected by age-related muscle weakness," said Heritage College Executive Dean Kenneth H. Johnson, D.O. "Through innovative research projects like this, we build a better understanding of the body, which may lead to interventions that improve strength and mobility in older adults."
The study was supported in part through several grants from the National Institutes of Health.