It's not enough to build muscle and achieve aerobic fitness. You need to think about flexibility, too. Stretching can help.

You may think of stretching as something performed only by runners or gymnasts. But we all need to stretch in order to protect our mobility and independence. A lot of people don't understand that stretching has to happen on a regular basis. It should be daily.

Why stretching is important

Stretching keeps the muscles flexible, strong, and healthy, and we need that flexibility to maintain a range of motion in the joints. Without it, the muscles shorten and become tight. Then, when you call on the muscles for activity, they are weak and unable to extend all the way. That puts you at risk for joint pain, strains, and muscle damage.

For example, sitting in a chair all day results in tight hamstrings in the back of the thigh. That can make it harder to extend your leg or straighten your knee all the way, which inhibits walking. Likewise, when tight muscles are suddenly called on for a strenuous activity that stretches them, such as playing tennis, they may become damaged from suddenly being stretched. Injured muscles may not be strong enough to support the joints, which can lead to joint injury.

Where to start

With a body full of muscles, the idea of daily stretching may seem overwhelming. The areas critical for mobility are in your lower extremities: your calves, your hamstrings, your hip flexors in the pelvis and quadriceps in the front of the thigh. Stretching your shoulders, neck, and lower back is also beneficial. Aim for a program of daily stretches or at least three or four times per week.

Proper execution

We used to believe that stretching was necessary to warm up the muscles and prepare them for activity. However, mounting research has shown that stretching the muscles before they're warmed up can actually hurt them. When everything is cold, the fibers aren't prepared and may be damaged. If you exercise first, you'll get blood flow to the area, and that makes the tissue more pliable and amenable to change. All it takes to warm up the muscles before stretching is 5 to 10 minutes of light activity, such as a quick walk. You can also stretch after an aerobic or weight-training workout.

source - https://www.health.harvard.edu/staying-healthy/the-importance-of-stretching