What Fitness Actually Is

Quick Intro

Physical fitness is a set of abilities your body has: how long it can keep going, how much force it can produce, how far it can move, how well it balances and how quickly and accurately it can react. It is not a body shape. A person can look muscular and lack stamina, or look ordinary and be very fit.

This article sets out the parts of fitness, what each does in daily life and why size is only one of them.

The Parts of Fitness

The standard way of describing fitness, used in exercise science for decades, splits it into two groups.

The parts that matter most for health:

Cardiorespiratory endurance, often called stamina. How well your heart, lungs and blood vessels supply energy during sustained activity such as walking, climbing stairs or cycling.
Muscular strength. How much force your muscles can produce at once, for lifting a heavy box or getting up from the floor.
Muscular endurance. How long your muscles can keep working before they tire, for carrying shopping home or holding a position.
Flexibility. How far your joints can move, for reaching a high shelf, tying a shoe or turning to look behind you.
Body composition. The share of your body that is muscle, bone and other lean tissue compared with fat.

The parts that matter most for skill:

Balance. Staying steady, standing still or moving, which keeps you upright on uneven ground or a moving bus.
Coordination. Getting different body parts to work together smoothly, as in catching, dancing or climbing a ladder.
Agility. Changing direction quickly and under control.
Power. Producing force quickly, as in jumping, sprinting for a bus or catching yourself when you slip.
Speed. Moving the body or a limb fast.
Reaction time. How quickly you respond to something you see, hear or feel.

The skill parts are not only for athletes. Balance, power and reaction time are what help a person recover from a trip, and they become more important with age.

Where Size Fits In

Muscle size sits inside one part of this list. It is linked to strength and it affects body composition, but it is a small slice of the whole. Someone who trains only for size can have plenty of muscle and still get breathless on stairs, struggle to touch their toes or wobble on one leg.

This is why “getting fit” and “getting big” are different projects. Most of what fitness does for daily life comes from stamina, strength, balance and mobility, and a person can improve all of these with little change in how they look. The difference is covered in more detail in looking fit and being fit.

Fitness as a Health Measure

Of all the parts, stamina has the strongest link with long-term health. Heart specialists have argued that a person’s aerobic fitness predicts heart disease and death independently of blood pressure, cholesterol and the other usual risk factors, and that doctors should measure it routinely, much as they measure blood pressure. In other words, how much your body can do is a health measure in its own right, not a side effect of looking a certain way.

Strength also matters for health, especially later in life, because it keeps everyday tasks within easy reach. Balance matters because it helps prevent falls.

Different Activities Train Different Parts

No single activity trains everything. Running builds stamina but does little for upper-body strength. Lifting weights builds strength but does less for stamina. Yoga builds flexibility and balance and, depending on how it is done, some strength. Team sports train agility, speed and reaction time alongside stamina.

Every exercise in this site’s guides develops some of these parts more than others. A Plank trains muscular endurance in the trunk, Brisk Walking trains stamina and a Bodyweight Lunge trains leg strength and balance together. Mixing a few kinds of activity is the simplest way to cover most of the list, as being a generalist explains.

Which Parts Matter for You

Everyone benefits from some stamina, some strength, reasonable flexibility and decent balance. How much of each you need depends on your life. A parent of young children, a person with a physical job and someone who plans to keep hiking into old age all need a little more of certain parts. A competitive athlete needs a lot of a few. A useful way to think about this is in fitness as spare capacity: enough of each to handle your days comfortably, with some room to spare.

FAQ

Which part of fitness is most important?

For long-term health, stamina has the strongest evidence, with strength and balance close behind, especially as people age. For daily life, most people need a reasonable amount of all the health-related parts.

Is being thin the same as being fit?

No. Body size says little about stamina, strength, flexibility or balance. A thin person can be unfit and a person in a larger body can be fit.

Can I train all the parts at once?

Many activities train several at a time. A routine that mixes some walking or cycling, some strength work and some balance and mobility work covers most of them.

Are the skill parts of fitness worth training if I do not play sport?

Yes. Balance, power and reaction time help you catch yourself when you trip and keep everyday movement safe, particularly with age.