What is fitness? What does being physically fit mean?


Maintaining a good level of physical fitness is something that we should all aspire to do. But what does fitness actually entail? What does being physically fit mean?

According to the United States Department of Health and Human Services,1 physical fitness is defined as “a set of attributes that people have or achieve that relates to the ability to perform physical activity.”

This description goes beyond being able to run long distances or lift heavy weights at the gym. Despite being important, these attributes only address single areas of fitness. Fitness is more than simply a question of listing which activities you do or how long you do them for.

  • Maintaining physical fitness can help ward off a number of diseases.
  • Different types of muscle fiber develop depending on the type of activity completed.
  • Body composition can change drastically without an associated change in weight.
  • Athletes’ hearts show different changes dependent on their chosen sport.
  • Muscle strength increases by fiber hypertrophy and neural changes.
  • Body composition can be ascertained in a number of ways.
  • Stretching to increase flexibility can ease a number of medical complaints.


Cardiorespiratory fitness

Cardiorespiratory endurance indicates how well our body can supply fuel during physical activity via the body’s circulatory and respiratory systems.

Activities that help improve your cardiorespiratory endurance are those that cause an elevated heart rate for a sustained period of time.

These activities include swimming, brisk walking, jogging, and cycling. It is important to begin these activities slowly and then gradually increase the intensity.

Exercising increases cardiorespiratory endurance in a number of ways. Essentially, the heart muscle is strengthened so that it is able to pump more blood per heartbeat.

At the same time, additional small arteries are grown within muscle tissue so that blood can be delivered to working muscles more effectively when needed.

Heart changes with exercise

The fact that the heart changes and improves its efficiency after persistent training is well known. However, more recent research shows that different types of activity change the heart in subtly different ways.

All types of exercise increase the heart’s overall size, but there are significant differences between endurance athletes, like rowers, and strength athletes like footballers.

Endurance athletes’ hearts show expanded left and right ventricles, whereas strength athletes show thickening of their heart wall, particularly the left ventricle.2

Changes within the lungs

While the heart steadily strengthens over time, the respiratory system does not adjust to the same degree. Lung function does not drastically change, but oxygen that is taken in by the lungs is used more effectively.3

In general, exercise encourages the body to become more efficient at taking on, distributing and using oxygen. This improvement, over time, increases endurance and overall health.

The American College of Sports Medicine recommends aerobic exercise 3-5 times per week for half an hour to an hour, at an intensity that keeps the heart rate at 65-85% of the maximum heart rate.4

Cardiorespiratory fitness has been found to help ward off the risk of heart disease, lung cancer, type 2 diabetes and stroke, among other diseases.

Muscular strength

The US Department of Health and Human Services defines muscular strength as “the ability of muscle to exert force during an activity.”

There are a number of ways to measure muscular strength in an individual. Generally, they involve lifting or pushing something of a set weight in a prescribed position and comparing the results against any given population.

In general, if a muscle is worked consistently and regularly, it will eventually increase in strength. There are various ways of putting your muscles through rigorous activity, but anything that works a muscle until it is tired will increase muscle strength over time.

Muscle structure

Muscles consist of elongated muscle cells. Each muscle cell contains contractile proteins – actin and myosin – that give the muscle its strength. These fibers contract en masse, producing the so-called power stroke. The total force depends on the number of these units contracting in unison.5

If muscles are regularly exercised, and if the individual has taken in enough protein, muscles can be made larger and stronger.

The exact mechanism of muscle building is not fully understood, but the general principles are well known.

Training causes the muscle cells to hypertrophy (expand). This hypertrophy is caused by an upscaling of protein manufacture within the cells. This increase in actin and myosin means there are more units contracting together and, therefore, more power.

In parallel with this additional firepower, there are neural changes that occur through training. These work in two ways.

Firstly, in untrained muscles, fibers tend to fire in an asynchronous manner. As they become trained they learn to fire together as one, increasing maximum power output.

Secondly, the body starts to disinhibit the muscles’ activation. Normally, there is a stopcock mechanism that prevents individual muscles from overworking and becoming injured. As the muscle is trained, this inhibitory neural feedback is limited, allowing more power to be exerted.

Alberta Education6 recommend starting with a resistance of around 80% of the maximum weight you can lift at one time and doing 3-9 repetitions of this weight through 3-5 sets for effective muscular benefits.

Muscular endurance

Alongside muscular strength, fitness can include muscular endurance, which is the ability of a muscle to continue exerting force without tiring.

As mentioned above, resistance training induces muscle hypertrophy. In other words, strength training builds bigger muscles.

Endurance training, on the other hand, does not necessarily generate muscles of a larger size. This is because the body focuses more on the cardiovascular system, ensuring that the muscles receive the oxygenated blood they need to keep functioning at the required level.

Another important change in muscles that are specifically trained for endurance concerns the two types of muscle tissue – fast twitch and slow twitch fibers:

  • Fast twitch fibers: contract quickly but get tired quickly. They use a lot of energy and are useful for sprints. They are whitish in color as they do not require blood to function
  • Slow twitch fibers: best for endurance work, they can carry out tasks without getting tired. They are found in postural muscles and in elevated proportions in muscles necessary to keep exerting power over longer periods. These fibers appear red as they rely on a strong supply of oxygenated blood and contain stores of myoglobin.

Different exercises will promote fast twitch fibers, slow twitch fibers or both. A sprinter will have comparatively more fast twitch fibers whereas a long distance runner will have more slow twitch fibers.

Body composition

Body composition measures the relative amounts of muscle, bone, water and fat. An individual can potentially maintain the same weight but radically change the ratio of each of the components that make up the body.

For instance, people with a high muscle (lean mass) ratio weigh more than those with the same height and waist circumference who have less muscle. Muscle weighs more per cubic inch of volume than fat.

In general, a lower fat ratio is generally beneficial, but the ideal body composition depends on the activity that the individual is being trained to carry out.7 For instance, the average percentage of fat in the body varies from sportsperson to sportsperson.

These measurements of body fat content were taken from high-level sportsmen and women of different disciplines

  • Basketball: men 9% and women 13%
  • Cross-country skiing: men 5% and women 11%
  • Golf: men 13% and women 16%
  • Kayaking/Canoeing: men 13% and women 22%
  • Swimming: men 12% and women 19%
  • 100-, 200- and 400-meter racers: men 6.5% and women 14%
  • Boxing: men 7%
  • Wrestling: men 8%8

Calculating body composition

Working out the body composition of an individual to any degree of accuracy can be a painstaking task. There are a number of accurate measures; this is just one:

First, weight is measured on standard scales. Next, volume is defined by submerging the individual in water and measuring the displacement.

The proportions of water, protein and mineral in the body can be ascertained by various chemical and radiometric tests. The densities of water, fat, protein and mineral are either measured or estimated. The numbers are then entered into the following equation:

1/Db = w/Dw + f/Df + p/Dp + m/Dm

Where: Db = overall body density, w = proportion of water, f = proportion of fat, p = proportion of protein, m = proportion of mineral, Dw = density of water, Df = density of fat, Dp = density of protein, Dm = density of mineral.

Other methods of ascertaining body composition include dual energy X-ray absorptiometry, air displacement plethysmography, bioelectrical impedance analysis, total body imaging (MRI and CT) and ultrasound.


Flexibility is the range of movement across a joint. Flexibility is important because it improves the ability to link movements together smoothly and can help prevent injuries.

Flexibility is defined by the Centers for Disease Control and Prevention (CDC) as:

“A health- and performance-related component of physical fitness that is the range of motion possible at a joint.

Flexibility is specific to each joint and depends on a number of specific variables, including but not limited to the tightness of specific ligaments and tendons.

Flexibility exercises enhance the ability of a joint to move through its full range of motion.”

Flexibility is increased by various activities, all designed to stretch joints, ligaments and tendons.

There are three types of exercise that are generally utilized to increase flexibility:

  • Dynamic stretching: the ability to complete a full range of motion of a particular joint. This type of flexibility is used in standard “warming up” exercises as it helps ready the body for physical activity
  • Static-active stretching: holding the body or part of the body in a stretched position and maintaining that position for a period of time. One example of static-active stretching is the splits
  • Ballistic stretching: only to be used when the body is already warmed up and limber from exercise, it involves stretching in various positions and bouncing. Some bodies, including the American Academy of Orthopedic Surgeons, believe that ballistic stretching can cause injuries.

To improve flexibility, there are a number of methods. A daily stretching regimen can be the simplest and most efficient way of achieving whole body flexibility. The following video demonstrates one such routine:

In general, fitness means different things to different people. The important take home message is that embarking on any regular exercise will be of benefit to your health. The more exercise that is carried out, the healthier an individual will look and feel.

You’ve probably heard countless times how exercise is “good for you.” But did you know that it can actually help you feel good, too? Getting the right amount of exercise can rev up your energy levels and even help improve your mood.

Rewards and Benefits

Experts recommend that teens get 60 minutes or more of moderate to vigorous physical activity each day. Here are some of the reasons:

  • Exercise benefits every part of the body, including the mind. Exercising causes the body to produce endorphins, chemicals that can help a person to feel more peaceful and happy. Exercise can help some people sleep better. It can also help some people who have mild depression and low self-esteem. Plus, exercise can give people a real sense of accomplishment and pride at having achieved a certain goal — like beating an old time in the 100-meter dash.
  • Exercising can help you look better. People who exercise burn more calories and look more toned than those who don’t. In fact, exercise is one of the most important parts of keeping your body at a healthy weight.
  • Exercise helps people lose weight and lower the risk of some diseases.Exercising to maintain a healthy weight decreases a person’s risk of developing certain diseases, including type 2 diabetes and high blood pressure. These diseases, which used to be found mostly in adults, are becoming more common in teens.
  • Exercise can help a person age well. This may not seem important now, but your body will thank you later. Women are especially prone to a condition called osteoporosis (a weakening of the bones) as they get older. Studies have found that weight-bearing exercise — like jumping, running, or brisk walking — can help girls (and guys!) keep their bones strong.

The three components to a well-balanced exercise routine are: aerobic exercise, strength training, and flexibility training.

Aerobic Exercise