Skeletal muscle is made up of two main types of muscle fibers – slow-twitch and fast twitch muscle fibers. These fibers can be further broken down into subtypes that include type 1 (slow-twitch), type 2A, type 2B, and type 2X. In general, your fast-twitch muscle fibers are the ones you recruit when you need to generate lots of force, or example when you lift a heavy weight. You also call on these fibers when you generate force quickly, for power moves such as plyometric training kettlebell swings, and sprinting. Fast-twitch muscle fibers have a high firing rate, making them ideal for strength and power, but they have a drawback – they fatigue quickly.
In contrast, slow-twitch muscle fibers have a slower firing rate and don’t generate the same degree of force as the fast twitch muscle fibers but they have another advantage. They’re more resistant to fatigue. Long after the fast-twitch muscles are exhausted, the slow-twitch muscles keep firing. That’s an advantage if you’re doing an activity that doesn’t require that you produce a lot of force but one that you do for a longer period of time. For example, slow-twitch fibers are the main fibers you recruit when you jog or lift light weights for a high number of reps. Therefore, slow-twitch muscle fibers are referred to as endurance fibers.
Fast Twitch Muscle Fibers versus Slow-Twitch Muscle fibers
Most people have roughly 50% slow-twitch and 50% fast twitch muscle fibers. People who excel at strength sports or sprinting likely have a ratio of fast-twitch to slow-twitch fibers that’s higher than the average person whereas high-performing marathon runners would likely have a preponderance of slow-twitch fibers. So, genetics have a hand in what your fast-twitch to slow-twitch ratio is and, unfortunately, you can’t convert a slow-twitch fiber to a fast-twitch fiber through training – or can you?
Remember how we said there are sub-types of fast twitch muscle fibers? One sub-type called type 2A has more endurance capabilities than type 2B, which are the super-force generators. Type 2A has some of the characteristics of both slow-twitch and fast-twitch fibers, even though they’re classified as fast-twitch. Should you need more endurance capabilities, you can convert some of your type 2B fibers to type 2A through training to give you more muscle endurance. For example, if you became a marathon runner and started running long distances, some of your type 2B could convert to 2A to support the type of training you’re doing. What doesn’t seem likely is that you can convert a fast-twitch muscle fiber to a slow-twitch or vice versa.
You can also impact your fast and slow-twitch muscle fibers by how you train. Though you can’t change a slow-twitch fiber to a fast-twitch one, should you focus mainly on strength and power training, your fast twitch muscle fibers will hypertrophy while your slow-twitch fibers will atrophy to some degree. Slow-twitch fibers atrophy by losing some of the mitochondria that give them their endurance capabilities.
Why You Should Focus More Time on Fast-Twitch Training
As people age, they often devote more of their time to workouts that use slow-twitch muscle fibers, like walking, yoga, and lifting lighter weights. That’s not an ideal approach. As you know, you begin losing muscle mass and strength after the age of 30 and this age-related assault on your muscle tissue speeds up after the age of 50.
The best weapon against muscle loss is exercise but what TYPE of exercise? The reality is you don’t lose the two types of muscle fibers (fast-twitch and slow-twitch) that you have at the same rate. It’s your fast twitch muscle fibers that take the biggest hit as you grow older. That’s why it’s important to devote more time to retaining fast-twitch muscle fibers. Why you ask? Strength and power are what keeps you functioning at your highest level! It’s fast-twitch fibers that you use to boost yourself out of a chair. You don’t want those fibers to atrophy.
How does this impact your training? Rather than turning to more endurance-based exercise as you age, the type that activates mainly slow-twitch fibers, make sure you’re continuing to work the fast twitch muscle fibers, the ones that you lose at a higher rate. These fibers often get slighted as people age. To work those fibers, you need high-intensity weight lifting, kettlebell workouts, and plyometrics. If you’re not up for jumping around, as plyometric training dictates, you can still challenge yourself by lifting heavy and doing kettlebell training. Remember, kettlebell training and weightlifting that focuses on compound exercises also build functional strength, exactly what you need as you age.
Fast Twitch Muscle Fibers and Sarcopenia
You have probably heard the term “sarcopenia,” a word that refers to the age-related loss of muscle tissue. Research shows it’s fast twitch muscle fibers that atrophy the most with age and are the biggest contributors to sarcopenia. Sarcopenia is a national epidemic, due to aging and lack of physical activity. Unfortunately, as people become sarcopenic with age, they lose functionality AND are at higher risk for metabolic problems that can eventually lead to type 2 diabetes and heart disease.
These are all reasons why you need to challenge yourself with heavy weights when you lift and do a high proportion of compound exercises to build functional strength. Don’t neglect balance either. You also lose balance as you age and become more susceptible to falling and hip fractures. Try doing exercises like one leg squats or do exercises while standing on a Bosu ball for some sets.
Nutrition Matters Too
When you’re lifting heavy and doing strength and power training, make sure you’re giving your body enough protein for muscle repair. Research suggests that past the age of 60, you may need more protein than you need earlier in life. One reason is your muscles become more resistant to the anabolic stimuli that promote new muscle protein synthesis. It’s important to get enough leucine-rich protein sources to help overcome this resistance. Fortunately, physical activity itself helps conquer some of this anabolic resistance you experience with age.
The Bottom Line
Make sure your exercise program is well-rounded and you’re spending at least half of your workout time doing activities that target fast twitch muscle fibers. After all, those are the ones you lose more of as you age. If you plan on staying fit and function during your retirement years, strength and power training will help you do it.
Clin Interv Aging. 2010; 5: 217–228. Published online 2010 Sep 7.
Exerc Sport Sci Rev. 2014;42(2):45-52.
Am J Clin Nutr May 2005. Vol. 81 no. 5 953-963.
Today’s Dietitian Vol. 17 No. 4 P. 16. April 2015.
Medscape Multispecialty. “Anabolic Resistance of Muscle Protein Synthesis with Aging”
Related Articles By Cathe: