The person with myofascial dysfunction in these muscles usually experience pain that is increased by sitting, and walking this pain often disturbs sleep.
The pain may even cause the person to limp, because loading this group of muscles is so painful and muscle muscle inhibition compromises hip stability. When sitting, the pressure on the stiff/affected parts may radiate pain to upper thigh, back of the knee and the buttock. Myofascial trigger points in the biceps femoris muscle often lead to waking up during the night. Often described "nonrestful sleep".
Hamstrings pain need to be distinguished from forms of sciatica, osteoarthritis, and possible entrapment...

Articular dysfunction, particularly lack of mobility L4-L5-S1 vertebral joints are associated with hamstring spasm. This is easily seen on the restriction of the strait leg raising test (SLR). Surgical fusion of the L5-S1 articulations aggravate hamstring tension and makes it a critical factor for myofascial pain.
When sudden voluntary trunk flexion is done, hamstrings are recruited firs to counteract and produce breacing action. Which may also contribute to the tender, soreness of these muscles.

Entrapment of sciatic nerve has been observed in some cases in which the nerve was constricted where it passed fibrotic bands near attachment to the TBI (Tuber Ishiadicum). And the symptoms was relieved by surgically releasing the fibrotic bands.

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The restricted movement in the trunk especially when fused causes the movement to be taken out in a bigger extent in other areas (compensating with the pelvis, hip joint, and other vertebra that is mobile) As mentioned, the pelvis is the transition structure between the trunk and the lower extremity.

Compressed/Entrapment. The sciatic nerve (n.ishiadicus) is a nerve that reaches from the trunk all the way to the foot. And there is several structures that may be causing symptoms.

• It can be compressed by the vertebra(no more since it fused, unless there is osteofytes(ossification), etc)
• Entrapped by the piriformis muscle in the buttock area (the nerve sometimes penetrate this muscle)
• Scar/fibrotic bands tissue after trauma or surgery may cause entrapment of the nerve just between the hamstring muscles near the ishial tuberocity (Tuber Ishiadicum), this has been observed in some cases.
• Hamstrings may have some influence in some cases, the nerve passes in between.

Now you are pretty sure it is not entrapment. But to differentiate the cause of pain muscular or nerve related, painkillers can be used. NSAID works if it is muscle pain, morphine related drug is needed to subdue the neurological pain.

Muscle pain due to dysfunction in muscles(tension) can be treated with direct palpation and injection to the myofaschial tightening of the muscle. When using this technique one side of the body is treated on one visit. Since the injection causes temporary pain in the treated limb during weight bearing.

Intermittent cold with stretch can also be used. First the range of motion is measured, later it can be realized how much release of muscular shortening was achieved. This may be technique may be used by some therapists.

Corrective actions include avoiding working these muscles in a shortened position without opportunity for full stretch, as when bicycling with a low seat so that the knees never straighten. When sitting down, avoid having your feet placed in a shortened position for a long time and make sure there is enough clearance(one finger to slip) between the thigh and the front edge of the chair seat. This prevents under thigh compression.

There is a variety of exercises that effectively stretch open the tissues. These are done with and without weights to strengthen and to pull them into balanced position. Physical therapists are specially trained in the instruction of biomechanical exercises and in the and their evaluation of the patient can be helpful. Exercises is introduced and can be done at home or at work to relax the muscles...