There are three muscles in the legs that are collectively referred to as the Hamstrings – the Biceps Femoris, Semitendinosus and Semimembranosus. All three Hamstrings attach to the Ischial Tuberosity of the pelvis – the sit-bone. At the knee, the Biceps Femoris attaches to the outside of the Femur and the knee and the Semitendinosus and Semimembranosus together are attached to the inside of the knee.
The Hamstrings can be visualized as forming a tripod with the knee as the base. It is useful to think of the hamstrings this way if you consider their function as knee stabilisers – they control and create rotary forces in the Femur. The inner Hamstrings, i.e. the Semitendinosus and Semimembranosus act as internal rotators of the Femur and the outer Hamstring, the Biceps Femoris, acts as an external rotator together with the Gluteus Maximus.
The inner Hamstrings are often injured in yoga in wide-legged asanas like Prasarita Padottanasana, Upavista Konasana or Kurmasana and the Hamstring tightens up after injury. Yogis that don’t take rest time for healing but keep right on practising might find that the Adductor muscles begin to compensate for the injured Hamstring, which gets ‘switched off’ neurologically. What happens then is that the Femur begins to rotate internally because the Adductors function as internal rotators of the Femur, and so does the tightened inner Hamstring.
The hip stabiliser muscles react to the increase in internal rotation with increased tension in the external rotator muscles – mostly in the Piriformis. A tight Piriformis is a major cause of sciatic pain. When a person goes to a doctor for sciatic pain, they will be told to stretch the Piriformis but their problem worsens because stretching a tense Piriformis weakens a muscle that is already stressed from trying to stabilise the excessive internal rotation of the Femur. A tight Piriformis on one side also exerts a twisting force on the Sacrum, causing Sacroiliac joint problems.
If you develop sciatic pain after a Hamstring injury it makes more sense to focus on eccentric stretching of the inner hamstring with standing balancing asanas like Ardha Chandrasana or Dighasana and by making sure that the foot is pointing forwards or slightly outwards – when the Adductors are being substituted for the hamstrings, the foot turns inwards – and to avoid stretching the hips excessively in asanas like Raja Kapotasana, Garudhasana, Gomukhasana or Parvritta Ardha Chandrasana.
Hamstrings are also important as Pelvic extensors and it is the balance between the Hamstrings and the various Hip Flexor muscles that determines pelvic tilt. Tight, injured hamstrings will cause lower back pain, because the Hip flexors, especially the Iliopsoas, will tighten up as a reflex response.
The outer Hamstring has a direct connection to the Sacrotuberous ligament, which is a crucial support ligament for the Sacroiliac joint. Abnormal tension in the hamstrings will be transferred into the Sacroiliac joint, creating pain and instability in the joint.
If you wish to avoid hip problems after a hamstring injury, it is vital to rest for 2-3 weeks to allow healing to take place. Thereafter, stretch the hamstrings slowly and gently and avoid hip-opening asanas if the hips begin to feel tight or painful. Hip strengthening with standing balancing asanas will be more helpful.
Reading sources: Brunnstrom’s Clinical Kinesiology, 1996, 5th edition Ellenbecker, De Carlo, DeRosa, 2009, Effective Functional Progressions in Sport Rehabilitation Cash, 1996, Sport and Remedial Massage Therapy