Janu Sirsasana is an excellent stretch to relieve tightness behind the knee.
If you feel tightness in the back of your knee, it may be attributed to a shortening of the muscles that cross behind the knee. These are the three hamstring muscles and one of the calf muscles. When the hamstrings or the gastrocnemius are tight, sensations of achiness and tightness may manifest in the posterior knee. Stretching is an excellent way to calm down the muscles of the back of the knee.
Try the Uttanasana Pose
Uttanasana is a great way to stretch the back of your legs. Stand with your feet hip-width apart and your hands on your hips. Inhale to lift the sternum toward the ceiling. Exhale to hinge at your hips, and lower your chest toward the floor. Inhale and lengthen the spine, reaching the crown of the head toward the floor. Exhale, contract your abdominals and quadriceps and bring your chest toward your knees. Continue to inhale and exhale for a total of six breaths, deepening the stretch with each exhale. If you are super tight, slightly bend your knees.
Do the Downward Facing Dog
Downward facing dog is not as deep a stretch for the hamstrings as Uttanasana, but it is a deeper stretch for the gastrocnemius. Come to a plank position. Place your hands underneath your shoulders. Curl your toes under and make a straight line from your heels to the crown of your head. Exhale and push into your hands to lift your butt up and back. Engage your abdominals and draw your ribcage into your spine. Contract your quadriceps firmly and allow your heels to fall toward the floor. Hold the pose for six long and deep breaths.
Sit for the Janu Sirsasana
For Janu Sirsasana, sit on the floor with your legs in front of you. Bend the left knee and bring the sole of the left foot into the inner thigh of your right leg. Reach your arms straight in front of you. Actively stretch your gastrocnemius and hamstrings by strongly contracting the quadriceps of your right leg to move the back of the right leg down toward the floor, and dorsiflex your ankle by moving the top of your foot toward your face as much you can. Hold the stretch for six breaths. Repeat on the other side.
Stretch with the Paschimottanasana
Paschimottanasana is the ultimate stretch for your hamstrings and gastrocnemius. Sit on the floor with your legs straight in front of you. Inhale and reach your arms up to gain maximum length in your spine before bending forward. Hinge at your hips and reach your hands toward your toes and beyond, if you can. Contract your quadriceps strongly to straighten your legs and stretch your hamstrings. Dorsiflex your ankles and spread your toes as you point them toward your face to stretch the gastrocnemius. Hold the pose for six breaths, deepening the pose with each exhale.