The Role of Skeletal Muscle in Healthspan and How Exercise Promotes Muscle Health

Skeletal muscle quality, which includes both muscle mass (the number and size of muscle fibers) and muscle strength (force production and contractility), is critical for maintaining mobility, balance, and motor coordination. Loss of muscle mass and strength can significantly impact healthspan, reducing the overall quality of life. Age-related loss of skeletal muscle mass, known as sarcopenia, along with increased frailty, is one of the leading causes of morbidity among the aging population, affecting over 10% of individuals over 60 years of age. However, regular exercise can significantly delay or even prevent the loss of muscle mass, thereby extending healthspan.

Clinical research has found that aging can result in a 20–25% reduction in muscle cross-sectional area compared to younger individuals. Importantly, resistance (anaerobic) exercise has been shown to significantly delay age-associated sarcopenia by preserving muscle mass and cross-sectional area. Evidence also suggests that endurance exercise can help improve sarcopenic conditions.

Mechanisms Behind Exercise-Mediated Muscle Preservation

Although the specific mechanisms by which exercise promotes and conserves muscle mass are not fully understood, several key targets have been identified. One of the main regulators of muscle growth is myostatin, a protein that inhibits muscle development and is often upregulated in individuals with sarcopenia. Both endurance and resistance exercise have been shown to decrease myostatin levels in muscle and blood, helping to reduce muscle wasting. Additionally, resistance exercise activates anabolic signaling pathways, such as the Akt-mTOR pathway, which are crucial for muscle protein synthesis. Exercise also reduces the activity of muscle-wasting proteins like MuRF-1 and Atrogin-1, further promoting muscle growth and maintenance.

Improving Muscle Strength

Muscle strength, the capacity for maximum force production per unit of muscle mass, is vital for overall quality of life and tends to decline with age. Resistance exercise is known to improve muscle strength by increasing both the amplitude and velocity of muscle contractions. Studies show that a 12-week resistance exercise program can increase muscle fiber peak power by 30–42% in healthy young men. Additionally, endurance exercise has been observed to improve the contractile properties of muscle fibers, even in older adults. For example, women in their 70s with a lifelong history of aerobic exercise exhibit increased strength in specific muscle fibers compared to sedentary counterparts.

Exercise Capacity and Its Benefits

The practical measure of skeletal muscle function is exercise capacity, which is often assessed through activities such as treadmill running, cycling, or a six-minute walk. Exercise capacity is inversely correlated with all-cause mortality, and numerous clinical trials show that exercise interventions can improve this capacity in a variety of populations. These improvements are attributed to several physiological and biochemical adaptations in skeletal muscle, including mitochondrial biogenesis, angiogenesis (growth of new blood vessels), and fiber type transformation.

• Mitochondrial Biogenesis: Endurance exercise promotes the formation of new mitochondria, enhancing energy production within the muscle. This process is regulated by the activation of key signaling molecules, such as PGC-1α, which helps coordinate the expression of genes involved in mitochondrial function.

• Angiogenesis: Exercise-induced angiogenesis expands the capillary network in skeletal muscles, improving gas and nutrient delivery. This adaptation is crucial for increasing exercise capacity and overall muscle function.

• Fiber Type Transformation: Regular endurance exercise promotes a shift in muscle fiber types, enhancing muscle endurance and overall performance. This transformation is regulated by pathways such as the calcineurin-NFAT (nuclear factor of activated T-cells) signaling pathway.

Exercise and Skeletal Muscle Metabolism

Metabolic diseases, like insulin resistance and obesity, are major barriers to extending healthspan. Since skeletal muscle accounts for about 40% of body weight and is responsible for the majority of postprandial glucose uptake, it plays a key role in metabolism. Fortunately, exercise intervention can make skeletal muscle more sensitive to insulin, helping to delay or prevent metabolic diseases such as type II diabetes. Muscle contractions during exercise lead to the activation of pathways that facilitate glucose uptake, improving overall metabolic health. Additionally, exercise promotes mitochondrial function and dynamics, contributing to efficient energy production in muscle cells.

Exercise-Induced Muscle Antioxidant Effects

Oxidative stress, caused by the overproduction of reactive oxygen species (ROS), is a major factor in aging, diabetes, and cardiometabolic diseases, making it a significant impediment to increasing healthspan. Exercise, however, stimulates the body’s natural antioxidant defenses. One of the most important antioxidant enzymes activated by exercise is extracellular superoxide dismutase (EcSOD), which neutralizes ROS on the cell surface and within the extracellular matrix. Research shows that EcSOD, which is primarily expressed in skeletal muscle, is upregulated by exercise and travels through the bloodstream to the heart and lungs, where it helps protect against oxidative damage.

Summary

A complex network of signaling and transcriptional changes mediates the adaptations in skeletal muscle in response to exercise, leading to improvements in muscle mass, strength, and endurance capacity. These changes not only enhance physical performance but also promote healthspan by reducing the risk of age-related decline, metabolic diseases, and oxidative stress. Further research will continue to uncover the sophisticated processes behind these exercise-induced benefits, shedding light on the importance of maintaining skeletal muscle health for a longer, healthier life.