There are several great seated stretches that can help improve flexibility in your hamstrings. Your hamstrings are the muscles on the back of your upper leg that help bend your lower leg back at the knee. When these muscles are tight, it can cause lower back pain and other problems.
The simplest sitting hamstring stretch when you are on the floor is to sit with your legs together, all the way from your hips to your ankles. Bend from the hips, reaching both hands toward your feet with your head down. Stop and hold the stretch for about 30 seconds, when you begin to feel it. As you practice this, you should be able to reach farther.
You can also focus on the inside and outside portions of your hamstring by isolating one leg at a time. Sit with one leg straight and the other bent with its foot against the thigh of your straight leg. Point the toe on the straight leg to the left and stretch toward the toe with both arms, holding it for 30 seconds. Repeat the stretch with the toe pointed to the right. Switch legs and repeat the stretch two more times, once with the toe pointing each direction.
You don't have to be seated on the floor to stretch your hamstrings. When you are sitting on a chair or stability ball, move to the edge of the chair, leave one leg bent with your foot flat on the floor and extend the other leg straight with your heel on the floor and your toe pointed up. Put each hand on the leg under it. Slide the hand on your straight leg down toward your toe, stop when you reach a comfortable stretch and hold the position for about 30 seconds. Switch so your other leg is the straight one and repeat the stretch.