All of these lists give people a well-rounded approach to fitness that supports longevity, disease prevention, quality of life, and the ability to reach personal athletic goals.

To help you understand each of these five components of fitness and how they help your overall health and longevity, as well as your running performance, we spoke with experts to break down ACSM’s list. According to ACSM, each component ensures that your body operates at its optimal level during exercise and daily living.

What to Know About the 5 Components of Physical Fitness

1. Cardiovascular Endurance

The term endurance basically means, “how long can you keep doing the thing you’re doing?” Milica McDowell, D.P.T., physical therapist, certified exercise physiologist, and vice president of operations at Gait Happens, tells Runner’s World. Cardiovascular endurance specifically refers to the ability of the heart, blood vessels, and blood to continuously deliver necessary oxygen and nutrients throughout the body when the demand is elevated—like during a run.

Improved cardiovascular endurance is marked by specific physiological adaptations. McDowell explains that this includes you heart’s stroke volume, or the amount of blood it pumps out with each beat.

Your body also produces more capillaries, the small blood vessels that deliver oxygen to the muscles and help remove waste products, as well as mitochondria, the parts of the cell that convert oxygen to adenosine triphosphate (ATP), the body’s primary source of energy. Essentially, you have more gas in the tank, and you’re better able to utilize it.

What runners should know: To continuously put one foot in front of the other, you need a solid baseline of cardiovascular endurance. Then, to increase your mileage and feel less fatigue on your runs, you need to continue to improve this component.

Zone 2 training (steady-state exercise during which you can hold a conversation) is the foundation of any endurance-building program, but you also need to stress the cardiovascular system if you want it to evolve. McDowell encourages runners to prioritize variability and overload in their training.

In other words, if you run the same three-mile loop at the same pace every day, your endurance will plateau. But if you add in speed intervals or do another lap of that loop, you’re increasing intensity or volume, which leads to more cardiovascular endurance gains.

2. Muscular Endurance

Muscular endurance refers to a muscle or muscle group’s ability to “repeatedly fire over time in a coordinated pattern,” McDowell says. Everything from walking and running to carrying objects and maintaining an upright posture requires some level of muscular endurance.

For a muscle to continuously contract and relax, it needs adequate hydration, electrolytes, and oxygen. It also must be able to withstand sustained neuromuscular firing, meaning your brain doesn’t succumb to fatigue and signal your body to stop exercising.

When you have solid muscular endurance, “your brain keeps telling your body to rhythmically do the pattern, and your body is adapted and used to that because you’ve trained for it,” McDowell says.

What runners should know: To build and maintain muscular endurance as a runner, you’ll need to accumulate time on your feet. Cross-training on the bike or the elliptical is a solid option if you have an injury. (“You’ll lose some muscular endurance, but not a ton,” McDowell says.)

Generally speaking, one of the most effective ways for runners to develop lower-body muscular endurance is to run, but strength training can also help—that includes in your upper body. Unlike hypertrophy or maximal strength-focused lifting, which uses heavy weights, programming designed to build muscular endurance typically prescribes higher repetitions with a lighter load.

3. Muscular Strength

Muscular strength is measured by how much force you can generate. Muscle size, placement, and physiological makeup are among the factors that determine muscle strength. Like muscular endurance, strength varies from one muscle to the next.

For athletes, the need for muscular strength is sport-specific. A powerlifter, for example, needs to generate more force than a horseback rider. That said, non-athletes also need muscular strength for everyday activities such as lifting groceries, opening doors, and getting up from the floor.

For an older person, muscle strength can mean the difference between independent living and needing assistance, according to the National Institute on Aging, which says that building strength also helps maintain muscle mass, improve mobility, and increases the healthy years of life.

Building muscular strength requires consistent strength training, proper nutrition (with a focus on protein), and adequate recovery (muscles become stronger after exercise when they repairs themselves).

Also, you need to lift heavy weights, which requires doing fewer reps and lifting loads close to your one-rep max or the max amount of weight you can lift for one rep.

What runners should know: Runners often deprioritize strength training in favor of more cardio, but Will Baldwin, USATF and VDOT.O2-certified running coach argues that strength gains deliver game-changing returns in terms of performance.

In fact, a 2024 study published in Sports Medicine, found that a variety of strength training methods, particularly lifting with high loads and plyometric training, improve running economy in middle- and long-distance runners.

Your running will improve as your muscular strength goes up, Baldwin tells Runner’s World. If it’s one-percent easier to lift your knee, you’re going to come off the ground easier and glide further, he explains, making your stride more efficient. Stronger muscles also make it easier and safer to absorb ground reaction forces, which means you can better withstand the impact of running.

Don’t focus solely on the lower body, however. “I think one of the levers people forget to pull is everything above the legs,” McDowell says, noting the positive effect on running of a powerful arm swing and a strong core, and backed up by a 2019 study published in Sports.

4. Flexibility