These two types of muscles control whether you can run a marathon or a sprint, and you likely have more of one than the other.
Have you ever set out in earnest to train for a marathon, but find yourself way better at short sprints? Your inability to become a master of endurance could actually be because of your genetics, not laziness.
It turns out that everybody is born with differing amounts of the two main types of muscles, called slow-twitch and fast-twitch. You may have seen some people online call this pseudoscience, but the research backs it up. Slow- and fast-twitch muscles operate in vastly different ways and are in charge of various athletic functions. But don't worry -- you can train to change the amount of slow- vs. fast-twitch muscles you have, so all hope is not lost for your future marathoning career.
Muscle fibers can be generally split into two categories based on how quickly they produce tension, though all fibers generate the same amount of force. Slow-twitch muscles contract more slowly (hence the name) and can work for long periods of time without running out of energy. Fast-twitch muscles are stronger, but they tire out faster.
When you do aerobic endurance activities -- think long-distance running, cycling and swimming -- you're relying on slow-twitch muscles. They're more efficient at using oxygen to generate ATP, the energy our cells use to operate.
More explosive movements -- sprinting, jumping and heavy weightlifting -- use fast-twitch muscles. Fast-twitch muscles contract using an anaerobic process, meaning that they don't use oxygen. They also produce lactic acid, which is why you get that burning feeling in your legs after a hard sprint.
There's actually a third type of fiber, aptly named "couch-potato" muscle fibers. They're also called super-fast-twitch, and are even stronger than regular fast-twitch, but they fatigue much faster. If you start exercising, these fibers will convert to the more useful fast-twitch. Conversely, if you've spent a little too much time sitting on the couch, they'll revert back to couch-potato status.
Think of couch-potato fibers as an evolutionary failsafe -- even if you're generally inactive, you'll need their quick bursts of strength in an emergency situation.
As a general population, our muscle fibers are split about 50/50 down the middle, but between each person there's some pretty wide variation. There's no exact way to tell which type you have more of, unless you're an elite athlete and take part in some scientific testing. But, you can make a pretty good guess by thinking about what types of activities you're naturally better at.
For example, I love doing slow cardio for long periods of time. I can't sprint to save my life, but I'm always game for a 10-mile hike. I'd venture to guess that my distribution is pretty weighted towards slow-twitch muscles.
Your baseline distribution is determined by genetics. So, if you've always wanted to complete a crazy endurance event but seem to be naturally better at lifting heavy weights, you have Mom and Dad to blame.
The short answer is yes, and the medium-length answer is also yes -- but researchers are unclear about the exact science behind the phenomenon. Our muscle fiber distribution seemingly changes on a day-to-day level, and scientists don't have a formula for what intensity of which activities will produce an exact result.
However, it's been widely observed that focusing your workouts on either endurance or explosive movements will result in an increase of slow- or fast-twitch muscle, respectively. So, if you've been hitting the treadmill hard lately, the percentage of your muscle that is slow-twitch is almost certainly growing.
For optimal overall muscle growth, you'll want to do both kinds of fitness training -- after you're done pounding out a long, slow jog, don't forget to throw in some sprints or bodyweight exercises.