The aging process affects everyone; however, but the rate at which it does so varies widely. These differences relate to genetic factors, environmental factors, health conditions, lifestyle choices and more. So, we all age at different rates and how we live influences the aging process.
One of the downsides of aging, besides developing those pesky wrinkles, is you lose muscle strength and size. With age, elderly individuals often experience physical decline that makes it difficult for them to do physical tasks on their own. For example, older, deconditioned adults may have trouble getting out of bed or climbing stairs without assistance. Fortunately, strength training helps preserve muscle strength and mass. It’s the best antidote for physical aging.
You might wonder what causes the loss of strength that comes with aging? While there are a variety of factors, scientists recently discovered another reason muscles generally lose strength with age. A meta-analysis that combined 29 studies looking at the effects of aging on muscle strength. It found that, with age, the rate at which motor units discharge slows and this slowing plays a role in the age-related loss of strength.
How Muscles Contract
To understand this, let’s briefly review how muscles contract. A motor unit is a group of muscle fibers innervated by a single neuron, or nerve cell, called a motor neuron. Each motor neuron branches out to different muscle fibers. So, when you tell your bicep to flex, your brain sends a command down to the appropriate motor neuron in your spinal cord, which then sends an electrical signal to your bicep muscle fibers. These fibers contract, causing your bicep to flex.
Your nervous system can activate individual motor units through a process called selective recruitment. This means that one or more motor units can activate at one time depending on the force you need to generate to complete a task. If you’re lifting a heavier weight and need to generate more force, your brain will activate more motor units, allowing your muscles to contract with more force. Likewise, if you’re lifting a lighter weight, fewer motor units will be activated.
How Aging Affects Muscle Contractions
Based on the results of this meta-analysis, motor units get “lazier” with age and their discharge rate slows. A younger person will have a high motor neuron discharge rate than an older individual and the younger person’s discharge rate will gradually slow with age and their ability to generate force will decline. They also noted that discharge rates were the slowest when there’s a need to generate greater amounts of force, such as lifting a heavy weight.
The take-home message is that motor neurons discharge at a slower rate as they age. Since the motor neuron plays a key role in muscle contraction, this contributes to decreased forced production and loss of strength in elderly individuals.
Motor Units Decline with Age Too
We also lose motor units with age, independent of the rate at which they fire. The loss of motor units and contraction velocity accelerates after the age of 80, meaning by the age of 80, you may have half the strength you had at age 20. That’s a problem when it comes to functionality. Your muscles not only generate less force, but they do so less quickly. So, power declines too. Nervous system aging also contributes to the loss of strength and power.
Muscle mass also declines with age. Interestingly, men lose more absolute and relative muscle mass than women with age, especially after the age of 70. The reason for this isn’t clear. In one study, men lost 4.7% of their muscle mass every decade while women lost only 3.7% every ten years. However, men have more muscle mass to begin with.
Strength Training Moderates This Effect
It’s no secret that some adults are strong and physically fit even at an advanced age. It’s possible to have functional capacity and muscle strength consistent with someone 20 years younger – but you had better move your body! Strength training offsets the loss of strength that goes with aging, but older adults need more intense stimulation of their muscles to make gains due to motor unit changes and a phenomenon called anabolic resistance, age-related resistance to muscle growth. To make meaningful gains, older adults need to train at an intensity 60 to 85% of their one-rep max, rather than using light weights, as many do.
The earlier you start strength training, the better, but you can increase strength and muscle size at any age. Strength loss begins after the age of 30 and speeds up during the sixth decade. It further accelerates after the age of 80. So, strength training needs to be a lifelong habit. And there is more than one reason to do it. High-intensity strength training helps prevent age-related bone loss too.
Another problem that older people encounter is a loss of stamina. Strength training improves stamina, or muscle endurance, too. This gives you greater ability to do the things you enjoy without tiring out.
The Bottom Line
One reason your muscles generate less force with age is because motor units fire at a slower rate and you also lose motor units. Strength training is the best way to counter the loss of muscle strength that occurs with aging. But you knew that, right?
References:
- Mayer F, Scharhag-Rosenberger F, Carlsohn A, Cassel M, Müller S, Scharhag J. The intensity and effects of strength training in the elderly. Dtsch Arztebl Int. 2011 May;108(21):359-64. doi: 10.3238/arztebl.2011.0359. Epub 2011 May 27. PMID: 21691559; PMCID: PMC3117172.
- Faulkner JA, Larkin LM, Claflin DR, Brooks SV. Age-related changes in the structure and function of skeletal muscles. Clin Exp Pharmacol Physiol. 2007;34:1091–1096.
- Kamen G, Knight CA. Training-Related Adaptations in Motor Unit Discharge Rate in Young and Older Adults. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences. 2004;59(12):1334-1338. doi:10.1093/gerona/59.12.1334.
- Siparsky PN, Kirkendall DT, Garrett WE Jr. Muscle changes in aging: understanding sarcopenia. Sports Health. 2014 Jan;6(1):36-40. doi: 10.1177/1941738113502296. PMID: 24427440; PMCID: PMC3874224.
- Orssatto LBR, Borg DN, Pendrith L, Blazevich AJ, Shield AJ, Trajano GS. Do motoneuron discharge rates slow with aging? A systematic review and meta-analysis. Mechanisms of Ageing and Development. 2022;203:111647. doi:10.1016/j.mad.2022.111647
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