Muscle tissue deteriorates in both size and strength due to a lack of exercise, according to Vicci Hill-Lombardi, associate professor at Seton Hill University’s School of Health and Medical Sciences. This deterioration is known as muscle atrophy. Some muscle atrophy results from disease; however, disuse atrophy – caused from lack of muscle use – is much more common in today's society. Decreased activity, sedentary jobs and injuries leading to casts or slings all contribute to muscle deterioration.
Muscle at Rest
Muscle deterioration is seen most dramatically with bed rest and space travel, where muscles are almost completely at rest, according to H. James Phillips, associate professor in the physical therapy department at Seton Hall University. He says just one week of bed rest reduces muscle strength by over 30 percent. Anyone old enough to remember the Apollo and Gemini space flights will recall how hard it was for astronauts to walk on Earth after spending a week or more in a weightless environment. Phillips says this effect has been mostly prevented in the International Space Station by having astronauts exercise with resistance devices every day.
Muscle size and strength are very much dependent on the demands placed on them. Challenge them to do more – such as bouts of weight training – and they grow. Ask them to do less, and they shrink to a lower level. This is known as the SAID, or specific adaptation to imposed demands, principle, according to Hill-Lombardi. Fortunately, muscle tissue is resilient. Though muscles get smaller and weaker if you stop exercising for a period of time, they regain their size and strength when you resume your fitness routine. It takes less time to regain the size and strength than it originally took to attain that level of fitness, excluding extensive sedentary periods and injury.
Basal Metabolic Rate
Basal metabolic rate, or BMR – the number of calories you burn at rest – is largely tied to the amount of muscle mass you carry, according to Illinois certified personal trainer and Fitness Fusion CEO, Kerrie Kuntz. If a 30-year old stops moderate physical activity, the first consequence she experiences is a reduction in BMR and a subsequent weight gain. The weight gain results in increased fat tissue and decreased lean tissue, changing the proportion in her lean-to-fat body mass. Kuntz says this change affects internal and external physical health and can lead to the emotional distress that often accompanies a change in body image.
Another type of muscle deterioration is called "sarcopenia.” It is less dependent on exercise but an inevitable effect of aging. Beginning around age 50, everyone’s muscles get smaller. Though exercise slows the process to a certain extent, each year muscles shrink and get weaker. That's why 60-, 70- and 80-year-old athletes, regardless of how fit they are, simply do not look like younger athletes. Kuntz says this diminishment of function – although a normal part of the aging process – can be marginalized, if an individual maintains physical activity as he ages.