There are many factors which contribute to the fitness of a person. Some of these factors are discussed below
1)What is the composition of your body?
2)Sufficient cardiac and respiratory supply
3)Muscle strength
4)Diet
5)Body flexibility
6)Age
7)Weight
8)Gender
9)Smoking
10)Psychological stress
11)Heredity
12)Regular exercise
13)Hydration
What is the composition of your body?
Fitness is greatly dependent upon the compounds from which your body is conposed. The relative amount of fats carbohydrates and protein give an estimation of the health and fitness of a person. Also, the relative position of these compounds in the body has a significant role in determining the fitness level of a person. For example, increased levels of fats in the blood (in the form of cholesterol) cause cardiovascular diseases which are the most common cause of death in this era. Also increased fat in the abdomen (in the adipocytes) causes obesity which also leads to cardiovascular disease and hypertension. Increased protein in the muscles (in the form of muscle mass) is usually beneficial in terms of fitness.

Sufficient cardiac and respiratory supply
To achieve fitness, your cardiac and respiratory systems must be working normally. Heart activity must be normal to cause proper oxygenation of the blood and consequently of tissues. If you are suffering from some respiratory disease it becomes difficult to exercise and increase your muscle strength resulting in decreased level of fitness.

Muscle strength
Muscle strength is a term which refers to the force which is required to accomplish a given task. Muscle strength can decrease or increase depending on the level of daily purposes and routine activities. If you do an active job which requires force and power your muscle strength will be normally good. on the other hand, if you have a sedentary lifestyle and do not exercise, muscle strength will be decreased. It can be increased by activities like swimming and cycling.
Body flexibility
Body flexibility refers to the range of motion across a joint i.e. how far a limb can move from its normal position while pursuing a movement. You must have seen the flexible bodies of sportsmen and especially athletes. They are flexible because their bodies are quite fit. Workouts can help you attain a flexible body and high fitness level.
Diet
The level of fitness is also dependent on diet. Eating junk food and having bad eating habits can ruin your health and fitness level. Having a diet plan and strung your routine can help you overcome this problem. A balanced dietetic proper amount of proteins fats and carbohydrates keeps you energetic and healthy. Starvation leads to decreased body mass and muscle strength which hurts your fitness.
Age
Age is also a factor which affects your fitness. Of course, an older person can’t be more fit than the young one. The strength and power your muscles carry when you are young cannot be compared to the ones when you become older.
Weight
Weight can also be a fitness-affecting factor. It is not the weight but the relative height and weight of a person which helps determine the fitness of a person. Increased weight due to increased lean body mass but not due to fat deposition and obesity can be helpful in increasing the fitness.
Gender
As, the body of women contains more fat and those of men contains more lean muscle mass, men can be considered as more fit than women.
Smoking
Smoking is not only injurious to the person himself but also to the environment. It causes lung cancer and leads to respiratory insufficiency which causes lower oxygenation of blood hence decreasing the fitness of the person.
Psychological stress
Psychological stress is a risk factor for obesity which causes low levels of fitness.
Heredity
Heredity is also a factor which helps in determining the fitness of a person. There are families with increased body mass and consequently high fitness. There may be families with short stature and consequently low fitness.
Regular exercise
Exercise helps to increase skeletal muscle mass and strength, thus increasing the fitness level of a person.
Hydration
To maintain fitness, drink more water. Keep your body hydrated.