The area around the sit bone is where the hamstrings originate and insert into the head of the femur or thighbone. This nagging and all-too-common injury typically occurs when the belly of the muscle doesn’t stretch sufficiently, forcing the origin—the point where muscle becomes tendon—to compensate by overstretching.
Always be aware – when you go too far in one direction, the way to fix it is by doing the opposite movement. When a muscle or tendon stretches, it weakens, and when it overstretches to the point of injury it becomes very weak. To strengthen the injured area you need to contract it. Some asanas that strengthen the origin of the hamstrings are Purvottanasana (Intense Front-Body Stretch) and Salabhasana (Locust Pose).
Avoiding forward bends entirely makes it difficult to practice. So, try doing forward bends with the quadriceps firmly engaged and contracted to encourage the lengthening of the belly of the hamstrings. When the quadriceps contract the knee joint extends and the knee is straight, which is why bending your knees has not been helpful. Bending your knees in forward bends makes it impossible for the quadriceps to fully engage and only shortens the belly of the muscle, putting more strain at the origin of the hamstrings.
One way of working with this injury in seated forward bends is to make an eccentric contraction of the hamstrings. Unlike a normal contraction when a muscle shortens, in an eccentric contraction a muscle lengthens as it contracts. An eccentric contraction requires great strength in a muscle. In this case it will strengthen the injured area while maintaining flexibility.
Sit in Paschimottanasana (Seated Forward Bend) with your feet pressed against a wall. Press against the floor with the back side of the injured leg while pressing into a wall with the foot of this leg. As you press the ball of the foot against the wall, contract the quadriceps, and as you press the heel against the wall, contract the hamstrings. Inhale press strongly with the back of the leg against the floor and with the foot against the wall. As you exhale, increase the forward bend slightly while maintaining the resistance.
To do standing poses try cinching a belt tightly around the injured area for support and awareness. You can also help break up the scar tissue at the injured area by placing a small ball under the site and rolling on it.