All muscle fibers are mainly comprised of two types, Type I, slow-twitch, and Type II, fast-twitch; they’re intermingled within our muscles.
Slow-twitch fibers are able to contract repeatedly without much fatigue. Slow-twitch muscle fibers have a high aerobic capacity, meaning they can produce energy for a long time provided sufficient oxygen is available. The ability to go on a long walk or bike ride requires lots of slow-twitch muscles.
Fast-twitch fibers have a high anaerobic capacity — they can produce power without oxygen for a relatively short period of time. Sprinters have lots of fast-twitch muscle fibers because they’re executing short-term, intense activity using stored muscle sugar (glycogen).
We’re all born with a fixed percentage of slow-twitch and fast-twitch muscle fibers. Across a broad population, muscle composition is forty percent to fifty percent Type I, and fifty percent to sixty percent Type II. To some extent we can change these proportions. Endurance exercise will decrease the proportion of Type II and increase proportion of Type I, but there’s less evidence demonstrating changing Type I to Type II. Our physical activities determine what happens, including increasing muscle mass and strength by engaging in activity, or losing mass and strength due to inactivity.
We lose more fast-twitch muscle fibers than we do slow-twitch muscle fibers as we age simply because we don’t use them enough. We need the fast-twitch fibers for golf, lifting weights, accelerating while cycling or running. Slow-twitch fibers are for endurance; long cycling trips, hiking or walking. The loss of fast-twitch muscle fibers due to lack of use is why golfers complain about not being able to hit the ball as far as they used to. Strength training is the best way to activate the underused fast-twitch muscle fibers.
An increase in muscle mass in response to muscle use is called muscle hypertrophy. It’s the opposite of muscle atrophy, which results from muscle inactivity and aging. Usually both occur simultaneously. The good news with aging loss is that the loss is not as great in older individuals as it is in younger individuals because there is less to lose. The bad news is that reversing this in older individuals is more difficult. It takes longer to produce hypertrophy, but it can be done.