It’s a bummer, but if you’re sedentary, your muscles will gradually weaken with age and you’ll become less functional. Plus, the more sedentary we are, the more muscle function we lose. Lifestyle matters! Loss of muscle size and strength also leads to health issues, most notably, sarcopenia. Unfortunately, sarcopenia, the age-related loss of muscle tissue, leads to other problems, including an increased risk of falls, fractures, and poor metabolic health. A healthy body composition helps ward off chronic health problems, like type 2 diabetes. So, it’s important that we keep our muscles strong and preserve the muscle tissue we have through strength training.
By now, you might be wondering WHY we lose muscle strength with age. Why are we able to generate less muscle force at age 70 than we were at age 20? The most obvious reason is the loss of muscle size. As a muscle becomes smaller, the cross-sectional area of the muscle declines. Smaller muscles can’t generate as much force. However, this isn’t the full story. With age, the force a muscle generates per unit of area goes down too. So, factors other than a decrease in muscle size are at play. The actual quality of the muscle is reduced, independently of muscle loss. In fact, studies in the elderly show the decline in strength with size is up to three times greater than can be explained by the loss of muscle size. Let’s look at some of the factors that contribute to aging muscle
The Muscle Becomes Fattier
As we age, fat accumulates inside muscles. This makes the muscles fattier in composition. It’s not clear whether this phenomenon is primarily due to aging or whether it’s lack of physical activity that causes it. Nevertheless, this accumulation of fat in the muscle has health and fitness consequences. Research show fattier muscles generate less force and the additional fat impacts metabolic health. As fat builds up in the muscle, insulin sensitivity declines and the risk of metabolic syndrome and type 2 diabetes goes up.
The fact that healthy, young people develop fatty muscles after only a month of the muscle being immobilized suggests that lack of activity and contraction of the muscle is a major factor. So, we can reduce fat accumulation in the muscle by staying physically active. In fact, a 2010 study found that 12 weeks of three-times-weekly resistance training in older individuals reduced the age-related accumulation of fat inside muscle tissue.
The Selective Loss of Fast-Twitch Muscle Fibers
We lose muscle fibers with age, but the loss of fast-twitch muscle fibers is greater than the loss of slow-twitch ones. In fact, some fast-twitch muscle fibers are converted into slow-twitch ones. Why is this important? Fast-twitch fibers are fibers best equipped to generate strength and power. They can generate substantial force but fatigue quickly. In contrast, slow-twitch muscle fibers are optimized for muscle endurance. They can sustain muscle contractions for long periods of time but can’t generate as much force. So, if you lose more fast-twitch than slow-twitch, you lose strength and power more than you do endurance. On the plus side, strength training can help reduce some of this loss. Unfortunately, many older people focus on endurance exercise, like walking, rather than training with weights.
Changes in Motor Unit Recruitment
It’s ultimately our brain that tells our muscles to contract. Upper motor neurons in the brain connect with lower motor neurons in the brain stem and spinal cord. The lower motor neuron connects to the muscle fibers that it innervates. One motor neuron connects with many muscle fibers, making it possible for one signal to activate more than one muscle fiber. However, a single muscle may be innervated by many motor neurons. A motor neuron and the muscle fibers it connects with is called a motor unit.
As we age, this process breaks down to some degree. Motor unit recruitment becomes less efficient. Ideally, you want motor neurons to activate muscle fibers in an orderly synchronous manner. When recruitment is synchronized because the motor units are working together, the muscle can generate more force. But, recruitment of muscles becomes less well synchronized and more disorderly with age. This further reduces a muscle’s ability to generate force.
Leaky Calcium Channels?
At least in mice, leaky calcium channels play a role in the loss of muscle strength. As muscle cells age, calcium begins to leak from the muscle cell through channels called ryanodine receptor channels. As the calcium leaks, it reduces the ability of muscle cells to contract. This is similar to what happens to the muscle cells of people who have muscular dystrophy.
Why do the channels start leaking? Scientists believe that increased production of free radicals due to oxidative stress may play a role. The free radicals are released inside muscle cells where they damage energy-producing organelles called mitochondria. In turn, this stimulates the release of even more free radicals and further calcium leakage. Ultimately, this interferes with the ability of muscle cells to contract.
Scientists hope they can eventually develop a drug that stops this process. Can eating a diet that reduces oxidative stress and free radical formation help? No one knows at this point. However, there are other benefits to eating antioxidant-rich fruits, vegetables, and spices.
The Bottom Line
We can’t completely stop the aging process, but staying physically active helps reduce the loss of muscle tissue. It also helps you stay strong and functional as the years pass. One of the best things you can do for your health is to move your body and work your muscles against resistance. So, take advantage of this lifestyle activity that can truly have an impact on your health as you age.
References:
· ScienceDaily.com. “Study explains why muscles weaken with age and points to possible therapy”
· PLOS One. “Influence of muscle fiber type composition on early fat accumulation under high-fat diet challenge” August 1, 2017.
· J Nutr Health Aging. 2010 May; 14(5): 362–366.
· Science Daily. “Study explains why muscles weaken with age and points to possible therapy”
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