We are all born with a mix of slow and fast twitch muscle fibers, also called type 1 and type 2 muscle fibers.

Your slow twitch fibers are your endurance muscle fibers. They are involved in many of your daily activities, like walking, jogging and other light to moderate-intensity activities. They rely on blood and oxygen to produce a type of fuel called adenosine triphosphate (ATP) for energy to function. 

Your fast twitch muscle fibers are necessary for speed and power. They are the muscles you use for high-intensity activities like sprinting, jumping, powerlifting and soccer. They provide more muscle mass and definition.

Fast twitch muscles need fuel quickly, so they rely on carbohydrates stored in the muscles rather than oxygen for ATP energy production. This type of fuel can be produced quickly but in limited amounts, which is why your fast-twitch muscles tire out more quickly. 

The proportion of your muscle fiber type depends primarily on the amount you have when you are born and through genetics. Other factors affecting your body’s muscle fibers include age and fitness. If you regularly engage in activities that require quick movements, you may have a larger inventory of fast-twitch muscle fibers, while endurance runners, for instance, may have a larger proportion of slow-twitch muscle fibers. 

Age also plays a role in our muscle composition. For most people, muscle mass peaks around age 30. Then, by your late 30s or early 40s, muscle mass begins to decline by 3% to 8% per year unless you actively work to stop that decline by increasing your activity level and protein intake, says physical therapist Mike Wellsandt, DPT. 

Fast twitch muscles degenerate more rapidly than the slow twitch muscles as we age. The neuro input to these muscles also declines. “Maintaining your fast twitch muscle fibers is especially important as you get older as these are the muscles that help us react quickly for things like preventing a fall or avoiding a car,” says Wellsandt. “Fast twitch muscles also have a greater potential to grow with exercise than slow twitch muscle fibers and are generally able to produce more force than slow twitch.” 

These muscles may be harder to train as you age. If you neglect to maintain them, you may eventually lose the ability to recruit these fibers since the neuro connection to these muscles also degenerates, says Wellsandt. However, you can still enhance the efficiency of what you have. This requires engaging in higher-intensity movements that are quicker and more forceful, as well as doing resistance training with heavier weights, he says.

“Just because you are older does not mean that you cannot still engage in higher-intensity exercises, you just may need to push yourself a little harder,” he says.

The bottom line is that you need both muscle types to optimize your activity level and ability to function at a healthy level throughout your life, Wellsandt explains. A regular exercise routine that engages both muscle fiber types will help keep you strong and healthy. “It’s always a good idea to consult with your health care provider before you begin a regular exercise routine to ensure you don’t have other health conditions that could put you at risk for complications,” says Wellsandt.