Active Voice: What Contributes to the Age-Related Loss in Rapid Strength?
By Gena R. Gerstner, M.P.H., M.S., and Eric D. Ryan, Ph.D.
Gena R. Gerstner, M.P.H., M.S., is a Ph.D. student at the University of North Carolina at Chapel Hill and training under the supervision of Eric D. Ryan, Ph.D. She has been an ACSM member since 2016 and has presented her research at both the 2016 and 2017 ACSM Annual Meetings.
Eric D. Ryan, Ph.D., is a Stallings Fellow Associate Professor in the Department of Exercise and Sport Science at the University of North Carolina at Chapel Hill. He codirects the Neuromuscular Research Laboratory within the department and has been an ACSM member since 2003. His current research examines the influence of acute (i.e., passive stretching, vibration, fatigue and eccentric exercise) and chronic (i.e., aging, occupational demands, training and nutritional supplementation) stressors on neuromuscular function.
This commentary presents the authors’ views on the topic of their research article, which they and their colleagues had published in the July 2017 issue of Medicine & Science in Sports & Exercise® (MSSE).
A large percentage of older adults experience functional limitations with everyday tasks and suffer a high incidence of injuries which result in a significant economic burden. For example, one-third of older adults experience a fall each year, an event that often results in accelerated deteriorations in health. With the number of adults aged 65 years and older steadily increasing, understanding what impacts function and performance in older adults is critical.
Aging is associated with reductions in maximal and rapid strength. However, rapid strength has been reported to decrease to a greater magnitude as aging advances. These changes are related to mobility limitations and the increased risk of falling. Multiple studies have reported age-related reductions in maximal voluntary isometric strength for the muscles of the lower limb, specifically in the plantar flexors. The plantar flexors are important for carrying out functional activities and contribute to mobility and independence during activities of daily living in older adults. To identify key strategies to combat these changes, future studies are needed to help identify specific factors contributing to the age-related reductions in rapid strength.
In this study, as reported in the July 2017 issue of MSSE, we investigated differences in rapid strength performance between younger and older adults and sought to examine what neuromuscular factors may contribute to these differences. We originally hypothesized that there would be a reduction in rapid strength at early (0-50 milliseconds) and late (100-200 milliseconds) time intervals during a rapid isometric contraction. Additionally, we hypothesized that the early interval of rapid strength would be influenced by muscle activation and muscle quality (a proxy of muscle tissue composition), whereas the late interval of rapid strength additionally would be influenced by muscle size and architecture. Instead, we found that the older men had lower strength at the late interval, but not the early interval of contraction. The older men also had poorer muscle quality, smaller pennation angles, and lower muscle activation in the late interval of contraction. However, we did not observe differences in rapid strength between groups at the early time interval of the muscle contraction. Interestingly, lower late interval rapid strength values were related to poorer muscle quality, smaller pennation angles and lower muscle activation values. Beyond these findings, we observed that late interval rapid strength values were no longer related to pennation angle after taking maximum strength into account.
The lack of differences we found in the early phase of a rapid contraction may be explained by the similar early muscle activation values between groups. In contrast, the older men may be unable to sustain the same muscle activation during the late phase of contraction. Furthermore, additional factors such as poor muscle quality and smaller pennation angles also appear to play a role in the age-related changes in rapid strength. These findings add to our understanding of the importance of the various factors that influence the age-related reductions in rapid strength, which has been shown to significantly influence function and performance in older adults.