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Many of us only think about our muscles when they are sore from being a “weekend warrior” or shoveling out after a snowstorm. However, without skeletal muscles we would not be able to breathe, stand upright, walk or run. The fact that our muscles make up between 40 and 50 percent of our body mass speaks to their importance in normal function and health.

The amazing diversity of our muscles is what allows them to let us move in different ways, from sitting up to slam-dunking a basketball. All of our muscles are made up of individual muscle fibers that contain two major proteins, actin and myosin. These proteins—called contractile proteins—slide past one another to produce muscle force (known as “contraction”). Our ability to get up and go results from our muscle fibers contracting and relaxing, but we need energy to do that. The body continuously makes energy in the form of a chemical called ATP. ATP takes energy from the digestion of food and stores it for cells throughout the body to use. All muscles contain actin and myosin and use ATP for energy, but to understand how they work, think of them as economy cars or powerful sports cars.

Everybody has a combination of two major types of muscle fibers: fast and slow. Slow muscles are like economy cars, and fast muscles are like sports cars. Our genes primarily determine whether each of us is more economical or powerful. If we have more fast muscle fibers—like a sports car—we can go very fast, but use up energy very quickly. If we have mostly slow muscles—like an economy car—we may not go as fast, but we have lots of endurance and use energy more efficiently. For example, a marathoner would have a large percentage of slow muscle fibers, while a sprinter would have a large percentage of fast fibers.

While our genetics play a role in deciding if we are a Prius or a Corvette, with regular exercise training we can make both our fast and slow muscle fibers stronger and more efficient. Specifically, strength training makes our muscle fibers larger, which is why our muscles get bigger and stronger when we lift weights. If we enjoy endurance exercise, such as training for a marathon, our muscle fibers become more efficient at producing and using energy, which allows us to run for long distances without getting tired. Overall, our muscles are uniquely designed to let us complete all of our everyday activities while also allowing us to slam-dunk the occasional basketball or run a 5K.

Kimberly A. Huey, PhD, FAPS, is professor of physiology in the department of health sciences at Drake University in Des Moines, Iowa.