http://www.bringithomepersonaltraining.com/11-components-of-physical-fitness-in-action/
Agility
"is the ability to move and change direction and position of the body quickly and effectively while under control."
It requires quick reflexes, coordination, balance, speed, and correct response to the changing situation.
https://www.verywellfit.com/understanding-agility-in-sports-3120338
Balance
"balance is the ability to stay upright or stay in control of body movement, and coordination is the ability to move two or more body parts under control, smoothly and efficiently."
There are two types of balance: static and dynamic. Static balance is maintaining equilibrium when stationary, while dynamic balance is maintaining equilibrium when moving. We use our eyes, ears and 'body sense' to help retain our balance.
https://www.topendsports.com/fitness/balance.htm
Body Composition
"Is the percentage of body weight that is fat compared to other body tissue, such as bone and muscle. "
People who have a high percentage of fat are more likely to be ill and have a higher death rate than lean people. Exercise and eating the right foods in the proper amounts can improve body composition. Body composition can be measured using an instrument called calipers, a specialized scale, or it can be calculated by using the body mass index (BMI) which uses height and weight to determine your BMI.
https://www.randall.k12.wi.us/cms/lib/WI01001877/Centricity/Domain/52/Fitness%20Focus.pdf
Cardiovascular Endurance
"Is the ability to exercise the entire body for long periods of time. It requires a strong heart, healthy lungs, and clear blood vessels to supply the body with oxygen." Activities to improve fitness in this area include running, swimming and aerobic dance. A person must do the activity continuously for a minimum of 20 minutes within their target heart rate zone. Endurance/cardiovascular activity should be done a minimum of 3 days per week. Every other day is preferable. The mile or the pacer will measure fitness testing in this area.
https://www.randall.k12.wi.us/cms/lib/WI01001877/Centricity/Domain/52/Fitness%20Focus.pdf
Coordination
"Coordination is the body’s ability to perform smooth and efficient movements."
Good coordination requires the athlete to combine multiple movements into a single movement that is fluid and achieves the intended goal. This is contrary to what many people first think of, which is hand-eye coordination which refers to the relationship between eye movements and hand movements, so that our hands make an intended movement in response to our eye movement, usually tracking a ball or something similar.
https://www.pdhpe.net/the-body-in-motion/what-is-the-relationship-between-physical-fitness-training-and-movement-efficiency/skill-related-components-of-physical-fitness/coordination/
Flexibility
"Is the ability to use your joints fully." You are flexible when the muscles are long enough and the joints are free enough to allow movement. People with good flexibility have fewer sore and injured muscles. Stretching before and after activities will help to improve flexibility. The sit-and-reach and the trunk lift are two tests used to measure flexibility.
https://www.randall.k12.wi.us/cms/lib/WI01001877/Centricity/Domain/52/Fitness%20Focus.pdf
Muscular Endurance
"Is the ability to use the muscles, which are attached to the bones, many times without getting tired. "
People with good muscular endurance are likely to have better posture, have fewer back problems, and be better able to resist fatigue than people who lack muscular endurance. You can improve muscular endurance by lifting weights with many repetitions or doing sit-ups. Measuring the number of sit-ups you can do correctly is used for fitness testing.
https://www.randall.k12.wi.us/cms/lib/WI01001877/Centricity/Domain/52/Fitness%20Focus.pdf
Muscular Strength
"Is the amount of force you can put forth with your muscles."
It is often measured by how much weight you can lift. People with strength have fewer problems with backaches and can carry out their daily tasks efficiently. Examples of muscular strength include push-ups, weight lifting heavy weight with few repetitions, and pull-ups. Fitness testing will be measured by doing push-ups.
https://www.randall.k12.wi.us/cms/lib/WI01001877/Centricity/Domain/52/Fitness%20Focus.pdf
 Power
"Is the ability to move the body parts swiftly while applying the maximum force of the muscles."
Power is a combination of both speed and muscular strength. For example, fullbacks in football muscling their way through other players and speeding to advance the ball and volleyball players getting up to the net and lifting their bodies high into the air.
http://www.glencoe.com/sites/common_assets/health_fitness/gln_health_fitness_zone/pdf/heart_rate_monitor_activities/health_skill_related_itness/health_skill_related_fitness_activity_4.pdf
Reaction Time
"is the ability to reach or respond quickly to what you hear, see, or feel."
For example, an athlete quickly coming off the blocks early in a swimming or track relay, or stealing a base in baseball.
http://www.glencoe.com/sites/common_assets/health_fitness/gln_health_fitness_zone/pdf/heart_rate_monitor_activities/health_skill_related_itness/health_skill_related_fitness_activity_4.pdf
Speed
"Speed is the ability to move your body or parts of your body swiftly." Many sports rely on speed to gain advantage over your opponents. For example, a basketball player making a fast break to perform a layup, a tennis player moving forward to get to a drop shot, a football player out running the defense to receive a pass.
http://www.glencoe.com/sites/common_assets/health_fitness/gln_health_fitness_zone/pdf/heart_rate_monitor_activities/health_skill_related_itness/health_skill_related_fitness_activity_4.pdf

Skeletal muscles are composed of type I, type IIa and type IIb fibers. These classifications refer to the speed with which they can contract and their aerobic endurance capacity.

A type I fiber contracts slowly and has the greatest endurance, whereas type IIb fibers contract rapidly and have the lowest endurance capacity. Type IIa fibers contract rapidly as well, but they have a higher aerobic endurance capacity than type 11b fibers.

Endurance training increases the aerobic capacity of type IIa and IIb fibers in particular, resulting in more fibers with fast-contracting, fatigue-resistant properties and thus enabling you to run longer distances.

During endurance exercise, your muscles need a greater supply of oxygen than they do at rest. Therefore, they have a large network of capillaries that supply oxygen-rich blood. The oxygen diffuses across the capillary into the muscle fiber, where it supports sustained energy production.

Endurance training increases the number of capillaries per area of muscle, thus increasing oxygen supply to the muscle. Oxygen supply to the muscles is critical for maintaining endurance, as muscles fatigue very rapidly without sufficient oxygen supply.

Your muscles primarily rely on the breakdown products of carbohydrates — stored as glycogen — and fats — stored as triglycerides for fuel during exercise. Carbohydrates are the most efficient source of energy, and their usage proportionally increases with increased exercise intensity.

However, your body has a very limited supply of stored carbs as compared to fat — about 1,800 to 2,000 calories worth of carbohydrates versus 100,000 calories worth of stored fat. Therefore, it is advantageous to spare muscle glycogen usage as much as possible in the early stages of endurance exercise.

Glycogen depletion is a major factor in the onset of fatigue, particularly in endurance exercise lasting longer than one hour. Endurance training enables your body to use proportionally more fat at a given exercise intensity, sparing the prized muscle glycogen and allowing you to exercise longer.

Whether your muscle uses carbohydrates or fats for energy, it must be able to convert these energy sources into usable cell energy, or ATP. Your mitochondria are energy powerhouses of the muscle cell — they use oxygen and the activity of several enzymes to produce the majority of ATP that the muscle cell needs to fuel endurance exercise.

Endurance exercise increases the amount of mitochondria per area of muscle, increasing the ATP-producing capacity. In addition, endurance training increases the number of enzymes in the mitochondria, which speeds up energy formation.

Myoglobin is a special protein in your muscles that binds the oxygen that enters the muscle fiber. When oxygen becomes limited during exercise, myoglobin releases the oxygen to the mitochondria.

Although scientists do not know the degree to which myoglobin content contributes to the muscle’s oxidative capacity, endurance exercise training increases myoglobin content, likely increasing the oxygen reserve in the muscle.

Other training types such as hypertrophy training can also result in these adaptations occurring in the cardiovascular system however the degree of adaptation will be less significant than the adaptation caused by aerobic fitness, anaerobic fitness and muscular endurance training.
https://www.ptdirect.com/training-design/anatomy-and-physiology/chronic-cardiovascular-adaptations-to-exercise

Let’s take a look at each of the components.
Frequency

Frequency is a key component of the FITT Principle. Remember that it’s important to know why you’re exercising and what you want to achieve before rushing into any exercise program.

Adjust the number of times you exercise per week to reflect your current fitness level, the time you realistically have available, your other commitments like family and work, and the goals you’ve set for yourself.

This is an extremely important aspect of the FITT principle and is probably the hardest factor to monitor. The best way to gauge the intensity of your exercise is to monitor your heart rate.

There are a couple of ways to monitor your heart rate but the best way by far is to purchase an exercise heart rate monitor. These can be purchased at most good sports stores and retail from $50 to $400. They consist of an elastic belt that fits around your chest and a wrist watch that displays your exercise heart rate in beats per minute.

If you don’t want to spend the money on a heart rate monitor, simply count your heart rate over a 15 second period. All you need is a wrist watch that has a “seconds” display. Feel for your heart beat by either placing your hand over your heart or by feeling for your pulse in your neck or on your wrist. Count the beats over a 15 second period and then multiply by 4. This will give you your exercise heart rate in beats per minute.

The time you spend exercising is also an important part of the FITT Principle. The time dedicated to exercise usually depends on the type of exercise undertaken.

For example, it’s recommended that to improve cardio-vascular fitness you’ll need at least 20 to 30 minutes of non stop exercise. For weight loss, more time is required; at least 40 minutes of moderate weight bearing exercise. However, when talking about the time required for muscular strength improvements, time is often measured as a number of “sets” and “reps.” A typical recommendation would be 3 sets of 8 reps.

Like time, the type of exercise you choose will have a big effect on the results you achieve. That’s why it’s important to know what you want to gain from your efforts.

For example, if you’re looking to improve your cardio-vascular fitness, then exercises like walking, jogging, swimming, bike riding, stair climbing, aerobics and rowing are very effective. For weight loss, any exercise that using a majority of your large muscle groups will be effective. To improve muscular strength the best exercises include the use of free weights, machine weights and body weight exercises like push-ups, chin-ups and dips.
https://stretchcoach.com/articles/fitt-principle/