Active Voice: Impact of a Highly Sedentary Lifestyle on Muscle Mass Regulation
By Brandon J. Shad, Ph.D., and Gareth A. Wallis, Ph.D.

Brandon J. Shad, Ph.D. Gareth A. Wallis, Ph.D.

Our environment has been engineered to encourage sedentary behavior at every opportunity. This is concerning, as sedentary behavior is becoming increasingly recognized as a significant risk factor for the development of type 2 diabetes, cardiovascular disease, some cancers and, ultimately, increased risk of premature death. It is no surprise that guidelines around the globe, such as those advocated in the current Physical Activity Guidelines for Americans, now highlight the importance of sitting less and moving more.

While we have become increasingly aware of the negative impact that sedentary behavior can have on many aspects of our health, we know comparatively less about the effects of a highly sedentary lifestyle on skeletal muscle. Recent observational evidence links high engagement in sedentary behavior with low muscle mass and strength. This has serious health consequences across the lifespan, given that sedentary behavior is highly prevalent. Furthermore, skeletal muscle mass and function are essential both for performing activities of daily living and functional independence into old age. Understanding whether a sedentary lifestyle negatively impacts skeletal muscle physiology is clearly of substantial importance.

We conducted our study in collaboration with several physical activity and skeletal muscle physiology experts affiliated with research institutions in the U.K., the Netherlands and Australia. Our findings are presented in the October 2019 issue of Medicine & Science in Sports & Exercise. Our subjects were 11 physically active young men, who first completed one week of their usual habitual physical activity, averaging about 13,000 steps per day. These participants then reduced their daily step count to a target level of roughly 1,500 steps per day); in this study condition, they completed no structured physical activity for one week in order to simulate a highly sedentary lifestyle. The use of stable-isotope techniques (i.e., heavy water ingestion), collection of skeletal muscle biopsies and blood samples allowed us to make multiple important observations.

The major new finding was that one week of a highly sedentary lifestyle significantly reduced the rate at which we build skeletal muscle; more specifically, muscle protein synthesis showed a 27% decline in the highly sedentary condition. Becoming highly sedentary also altered the expression of several genes within skeletal muscle that are associated with muscle mass regulation and oxidative metabolism. Using an oral glucose tolerance test, we also confirmed previous findings demonstrating that being highly sedentary significantly reduces insulin sensitivity.

What do our findings mean? They provide the first evidence in healthy young individuals that a highly sedentary lifestyle can have a direct deleterious effect on the dynamic physiological processes that regulate how much skeletal muscle we have. This is important, as it suggests that high engagement in sedentary behaviors may increase the risk of muscle loss and poor physical function. We suggest regular physical activity promotion and minimizing sedentariness throughout the lifespan to maintain skeletal muscle health.

About the authors:
Brandon J. Shad, Ph.D., is a researcher at the University of Birmingham in the United Kingdom. His research focuses on understanding the physiological processes that regulate skeletal muscle mass in response to physical activity and inactivity. He received an ACSM International Student Award for presenting these research findings at the 2018 ACSM Annual Meeting in Minneapolis.

Gareth A. Wallis., Ph.D., is a researcher and senior lecturer in Exercise Metabolism and Nutrition at the University of Birmingham in the United Kingdom. He conducts research that informs a broad spectrum of issues from health to performance, in which the interaction of nutrition with exercise metabolism is placed at the core.

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily represent positions or policies of ACSM. Active Voice authors who have received financial or other considerations from a commercial entity associated with their topic must disclose such relationships at the time they accept an invitation to write for SMB.