A study looking at how chronic inactivity affects muscles at the molecular level has helped to identify a phenotype that may predispose older adults to disability. The results were recently published by University of Vermont Associate Professor of Medicine Michael Toth, Ph.D., and colleagues in The Journal of Physiology.

Toth and his team examined 16 patients with advanced stage knee osteoarthritis (OA) – a long-term, debilitating condition – to determine whether or not their muscle tissue was functionally different from the tissue of a group of 15 subjects matched for sex, health status and body size. The group with OA reported very low levels of physical activity, while the control group had “recreationally active” lifestyles and no evidence of OA.

One goal of the study was to understand how the molecular structures that comprise a muscle fiber change with disuse. Although it’s been well-established that disease impairs muscle performance, the reasons behind this truth are less clear, in part because of the complex processes involved in muscle contraction.

“With age, we tend to develop disability at a higher rate,” Toth says. “No one is really sure why that happens.”

The capacity of the heart and lungs, the circulation system, the signal from the brain to the muscle, all play a role when studying the human as a whole. Toth and his team’s focus on the most fundamental components of a single muscle fiber allowed them to isolate and measure the “end result” of a muscle’s contraction – its speed and force – independent from these other variables.

The researchers found that muscle disuse did, indeed, affect the performance of muscle fibers on several key measures: force generation, power output and contractile velocity. Particularly surprising was the difference between the sexes, with women showing cellular and molecular adaptations to muscle disuse that explain their predisposition to develop physical disability.

“Although both men and women with knee OA showed reduced force-generating capacity in single muscle fiber primarily as a result of atrophy, only men exhibited an increase in velocity that might serve to offset losses in skeletal muscle power output,” state the study’s authors.

While men’s muscle fibers tend to compensate for overall less force with more velocity, muscle fiber in women doesn’t seem to work the same way, leading to an important finding.

“We may have identified a cellular and molecular phenotype that may predispose older adults to disability,” Toth says, noting that this conclusion could help hone in on exercises or physical therapy interventions specific to patients’ with this phenotype.

The study may lead to even wider implications down the road. With an aging population, 3.5 million adults per year are expected to need knee replacements by 2030. Targeted interventions that help patients both prepare for and recover from surgery could help improve quality of life and reduce health care costs for this population.

PUBLISHED

02-17-2015
Erin E Post