Muscle Fiber Composition & How It Changes with Age


Muscle Fiber Composition & How It Changes with Age

It’s no secret that you lose muscle strength mass with age. In fact, the loss of muscle tissue begins as early as age 30 and continues throughout life, speeding up after menopause. How much muscle can you lose? If you’re sedentary, about a third of a pound a year after the age of 30.  This adds up to significant muscle loss over time, about 40 to 50% of your muscle tissue by the time you’re 80. As you might expect, muscle loss is greater in people who don’t resistance train. What you might not realize is that your muscle fiber composition changes as well.

Muscle Fiber Composition – You Have Two Main Types of Fibers 

Muscle fibers are of two main types. These two types are called slow-twitch fibers and fast-twitch fibers. Slow-twitch muscle fibers are sometimes called “endurance” fibers because they’re resistant to fatigue. If you looked at them under the microscope, you’d see these fibers are loaded with mitochondria, giving them the ability to generate lots of ATP. Slow-twitch fibers have a great deal of “staying power,” making them ideal for running a marathon. When you walk around, do a light jog, or lift light weights, you’re using slow-twitch fibers.

In contrast, fast-twitch muscle fibers fatigue quickly. Unlike slow-twitch fibers where the inside of each muscle fiber is chock full of mitochondria, fast-twitch fibers are lacking in aerobic capacity. Yet, fast-twitch fibers can generate lots of force anaerobically and do it quickly. This gives them power capabilities.  When you need to generate force quickly or produce lots of force, you count on your fast-twitch muscle fibers to do the job. If you break into a sprint, lift a heavy weight, or swing a heavy kettlebell, your fast-twitch muscles do most of the work. As you can see, these fibers don’t get stimulated nearly as often as the slow-twitch fibers. If you don’t lift weights, these fibers may get almost no stimulation at all.

Muscle Fiber Composition and Aging

What does all of this have to do with the aging process? You don’t lose fast-twitch and slow-twitch fibers to the same degree. You lose more fast-twitch fibers as you age relative to slow-twitch. What happens is the nerve supply to fast-twitch muscle fibers dies off and a fiber that once had fast-twitch capabilities is converted to a slow-twitch fiber. While you still have a functioning muscle fiber, the fiber is now an endurance fiber. Unfortunately, it’s no longer designed for strength and power generation. As more and more fast-twitch fibers are converted to slow-twitch ones, you lose strength AND the ability to generate power.

As you know, power is the capacity to generate force quickly. It’s strength with a time component. Losing strength is bad enough but the reduction of power capabilities is just as damaging as the loss of strength, if not more so, for functionality. So many things we do each day has a power component to it. When you rise from a chair, you push upwards with force into a standing position. Unless you have that power component, getting out of a chair is challenging. Suppose you trip while walking on the sidewalk, you need enough power to shift your body to catch yourself. Fast-twitch fibers help you generate the power you need to avoid injury.

How much power capacity do we lose with age? When researchers measured power in a group of subjects using a device called an isokinetic dynamometer, they found that older participants generated less power at slow and fast walking speeds and it was most pronounced when they walked faster. So, you may not have a problem strolling at a slow or moderate pace due to the fact that your slow-twitch fibers are better preserved. However, the ability to generate power is more limited as fast-twitch fibers turn into slow-twitch ones.

The Importance of Power Training

What is the form of exercise most older people engage in if they exercise at all? They take a walk, usually not at a very fast one either. While any form of exercise is better than sitting in a chair, it’s not enough to preserve strength and power as we age. To reduce the shift of fast-twitch fibers to slow-twitch ones you have to stimulate the fast-twitch fibers. Walking won’t do that. What does target the fast-twitch fibers is heavy resistance training and training using lighter weights (around 60% of one-rep) max at a fast or explosive tempo.

Using heavy weights and a slow tempo builds strength but it’s not effective for building power. That’s why some form of power training is important for healthy aging. Research shows that it’s power capabilities that correspond most closely with better functionality later in life, not strength. Plus, research shows we lose power at a greater rate than we do strength with age

What does this mean? Older adults can benefit most from a combination of strength and power training. This means including sets that use lighter weight (around 60% to 70% of one-rep max) and a higher speed. Even better, add high speed/lower weight sets to your training now to make your muscles as powerful as possible now. Power capabilities are important at any age but they become even more vital as we age. Having more power can mean the difference between falling and breaking a hip and staying function.

The Bottom Line

As you age, you have to worry about losing muscle mass AND functionality, especially the ability to generate power. Keep lifting heavy weights at a slow to moderate speed to build strength and size but also include sets where you lift lighter and at a fast speed. Other ways to enhance power capabilities are kettlebell swings, plyometrics, and sprinting. Adding a power component to your workouts works your muscles in a different way and can help you avoid plateaus as well.



J Gerontol A Biol Sci Med Sci. 1995 Nov;50 Spec No:11-6.

Exerc Sport Sci Rev. 2014;42(2):45-52.

Journal of Aging and Physical Activity, 2007, 15, 349-359

Tufts Now. “Power Play”

Exercise Biology. “Why should older people do power training?”

Age Ageing. 2011 Sep;40(5):549-56. doi: 10.1093/ageing/afr005. Epub 2011 Mar 7

IDEA Health and Fitness Association. “Power Training For Older Adults”


Related Articles By Cathe:

4 Reasons You Lose Muscle Size & Strength with Age

3 Characteristics of Healthy, Youthful Muscle That Change as You Age

Why You Should Focus On Your Fast Twitch Muscle Fibers

3 Ways to Power Up Your Training and Get Better Results

The Best Ways to Build Muscle Endurance & Why You Should

Men May Be Stronger but Women Have Greater Muscle Endurance


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