Exercise.

Being physically active is natural for humans. The sedentary existence of large numbers of people is a historical aberration of the past century. Our sedentary lifestyles, together with the obesity epidemic they help foster, have taken a big toll on our health.

How can you make exercise a part of your daily life? By making it regular. Find what works best for you in terms of type and timing of workouts and where you do them. Exercising with a friend or family member or in a group may motivate you and make it more fun. 

The 4 fundamentals:
There are four main fundamentals of fitness: muscle strength, aerobic (cardiovascular), flexibility and balance. Some people, such as weight-lifters or long-distance runners, excel in just one of these. To be truly fit, you should exercise to develop all of the elements, some of which overlap and work together.

1. Muscle Strength:
Strength (or resistance) training is crucial for everyone, especially women and older people, even those in their eighties and nineties. Most of us start losing muscle (and gain body fat) in our thirties, and by age 50 we’ve lost 10 percent of our muscle mass. After that the losses accelerate, unless we take steps to counteract this.
Muscle strength can help prevent back pain, falls and many other kinds of injuries, which are often caused by muscle weakness and imbalances.
Like any exercise that puts stress on bones, strength training also maintains or increases bone density and helps prevent osteoporosis. And even if it doesn’t result in weight loss (the weight of added muscle can cancel that of lost body fat), strength training can help you look trimmer.
Strength training should be done at least twice a week for all major muscle groups—back, chest, shoulders, arms, legs and abdominals. By strength training we don’t mean lifting very heavy weights to build bulging muscles. As generally recommended, it calls for working out against moderate resistance in order to build muscle strength and endurance.

2. Cardio:
Aerobic (“requiring air”) exercise is any prolonged, moderate-intensity activity that depends primarily on the use of oxygen to generate energy from carbohydrates and fat. These “endurance” or “cardio” activities include cycling, swimming, running, walking fast, jumping rope and any activity that raises heart rate sufficiently for at least 10 minutes. 
Such exercise improves aerobic fitness, which is the sustained ability of the cardio-respiratory system (the heart, lungs and circulatory system) to transport oxygen to cells, especially in muscles. Aerobic fitness decreases by about 10 percent each decade after age 30, on average, but exercise can help prevent or at least slow the decline.
The cardiovascular benefits of aerobic training include a stronger, more efficient heart . It also helps keep blood vessels flexible, thus reducing blood pressure.

3. Balance:
Having good balance involves the integration of various sensory and neuromotors systems, including vision, muscle strength, joint flexibility, reaction time, the vestibular system in the inner ear (which monitors motion and provides orientation clues) and the ability to sense where your body is in space. If any of these systems are not functioning properly, you can lose your balance even while just walking or standing up.
If you have balance problems, it’s best to start with walking and simple exercises. Many kinds of exercise— including running, strength training and most sports—can help improve balance and agility. Any activity that increases strength, especially in your lower limbs, is worthwhile. Try to do some sort of balance training for 10 to 15 minutes, three times a week.

4. Stretching:
Flexibility, the ability to move your joints through their full range of motion, is a key element of fitness that can be improved by doing stretching exercises for muscles and tendons. Being more flexible can enhance physical performance. When done in a slow and controlled manner, as in yoga, flexibility training can be an excellent relaxation method. 
Stretching sessions should last 10 to 20 minutes, be done at least twice a week and focus on all major muscle groups. You should stretch slowly and in a relaxed. You should feel the stretch, but if there’s any pain, stop. Overstretching can increase the risk of injury.