Shortly after age 50, the rate at which a person loses muscle mass begins to accelerate. Figure 1 is a graphical representation of the average number of quadriceps muscle fibers present in adults aged 18 to 82. Looking at the center of the graph, it is clear that the number of muscle fibers remains stable until around age 50. After that, muscle mass declines rapidly, dropping from 600,000 muscle fibers at age 50 to 320,000 muscle fibers by age 80. The loss of muscle fibers is compounded by even greater losses of strength and power and an increase in fatigability. The resultant weakness produces a downward spiral of frailty that strongly correlates with the development of osteoarthritis, disability, fractures, and reduced lifespan.
Age-related muscle loss is referred to as sarcopenia, which is Greek for “poverty of flesh.” Unfortunately, the prevalence of sarcopenia is increasing with each generation; the number of elderly individuals who are frail is expected to double within the next 40 years. One theory for the growing prevalence of sarcopenia posits that the genes responsible for repairing damaged muscles were programmed during the late Paleolithic period, when constant activity led to continual repair and remodeling of muscles. Our current, more sedentary lifestyle does not match our evolutionarily programmed Paleolithic genes. Supporting the belief that human genes were designed for activity is the fact that modern hunter-gatherers develop sarcopenia at much slower rates than similar-aged sedentary populations.
While research confirms that regular exercise can reduce the speed of muscle wasting, there is a surprising amount of controversy regarding what type of exercise is best-suited for maintaining muscle mass. According to the American College of Sports Medicine, the only way to effectively build muscle is to perform multiple sets of 8 to 12 repetitions using heavy weights. The theory is that the muscle must be pushed to the limit of its capacity to stimulate muscle repair.
The problem with using heavy weights is that as a person ages and muscle fibers begin to disappear, the vanishing muscle fibers are replaced with fat and scar tissue, which significantly weakens muscles and tendons. As a result, heavy resistance exercise increases the likelihood of a muscle tear, which explains why so few seniors are consistent with strength-training programs.
Fortunately, recent research has proven this theory wrong: exercising with light weights, when done properly, is very effective at stimulating muscle repair. To understand how light weights can increase muscle remodeling, it is first necessary to understand how muscles work. Basically, all muscles are made of small cells, or myofibers, which are classified as either slow-twitch (type I) or fast-twitch (type IIB) fibers. Slow-twitch fibers are designed to produce low levels of force for long periods of time. Slow-twitch muscles use oxygen for energy and therefore require a lot of capillaries to supply them with oxygen-rich blood. Slow-twitch muscle fibers are the body’s preferred fibers when running marathons. Conversely, fast-twitch fibers function without oxygen and produce large amounts of force in very short periods of time. Fast-twitch muscle fibers are essential for success in sprinting and weightlifting. Because fast-twitch muscle fibers function without oxygen, they have limited need of a blood supply.
The central nervous system selectively recruits either slow- or fast-twitch muscle fibers depending on the activity being performed. When exercising with light weights, the body chooses the same small percentage of the available slow-twitch muscle fibers over and over again. These slow-twitch muscle fibers generate adequate amounts of force and rarely fatigue, making them the body’s “go to” muscles for almost all daily activities. When lifting a heavy weight, however, the slow-twitch fibers are initially activated, but when they are unable to generate adequate force, the body begins to recruit fast-twitch fibers. The heavier the weight, the more fast-twitch muscle fibers are recruited.
The forced recruitment of fast-twitch muscle fibers is the key to preventing age-related muscle loss because stimulation of fast-twitch muscle fibers causes specialized satellite cells to repair and rebuild muscle fibers. Satellite cells are considered the stem cells of muscles because they have the ability to differentiate into myoblasts, the precursor repair cells that are responsible for muscle growth, remodeling, and repair. Until recently, it was believed the recruitment of fast-twitch muscle fibers with subsequent satellite cell activation could only occur by lifting heavy weights. This is why the American College of Sports Medicine continues to recommend heavy-resistance exercise for strength training.
Stay tuned for Part 2 where we’ll touch on some resistance related research and we’ll provide a Simple Workout Routine to Maintain Muscle Mass.
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