If 2025 proved one thing, it’s that Americans’ interest in maximal fitness is higher than ever.

Participation in running races broke records. The New York City Marathon in November had almost 60,000 finishers, breaking the previous year’s record by several thousand, and ultrarunning events, which involve running longer-than-marathon distances, recorded historic participation levels. Meanwhile, extreme workout regimens are going viral. That includes 75 Hard, which dictates 75 consecutive days of drinking a gallon of water, forgoing alcohol, following a new diet of your choice, reading 10 pages of nonfiction, and twice-daily 45-minute workouts (one of which must be outside).

While there is also undoubtedly a social component to this exercise maximalism, the aim of all this exertion is to be “fitter,” a word we tend to use as a synonym for “healthier.” We all know that getting regular exercise is critically important for our physical and mental well-being, but what’s less clear is just how fit we need to be for health. Should we all be striving to be ultra-fast marathoners, or is there a middle ground in fitness, where we can be just as healthy without those lofty exercise goals? After all, far from all of us will become elite athletes. When it comes to fitness, is more really more?

The answer is a little complicated. Improving your fitness through exercise affects health in a number of ways. It reduces the risk of diseases like heart failure or dementia, but it also makes your body more functional, so you’re able to open heavy doors or carry your groceries. Researchers have found that the biggest improvement to health — for longevity but also your quality of life — appears when a non-exerciser begins to add some light exercise in their routine. Going from only light exercise to more intense or prolonged exercise does have additional health benefits, but there do seem to be diminishing health returns on the investments of your time and efforts. Complicating the issue is the fact that fitness can’t be boiled down to just one thing, or one measure.

What exactly is fitness?

Experts typically divide fitness into two categories you are probably familiar with: muscular strength and cardiorespiratory (or cardiovascular) fitness.

Muscular strength is simply how strong you are, and can be developed by resistance training like lifting weights. The primary connection between muscular strength and health is in the fact that maintaining good muscle mass prevents frailty and allows people to do the activities they want to do without injury.

The benefits that resistance training in young adulthood might provide for long-term health and longevity are a little unclear. But what is clear is that maintaining muscular strength is incredibly important for the health of older adults, since muscles weaken as you age. There’s also some mixed scientific evidence suggesting........

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