straight leg raising (SLR) test :
This is a test for lumbosacral nerve root irritation for example, due to disc prolapse.

With the patient laid on their back:

raise one leg - knee absolutely straight - until pain is experienced in the thigh, buttock and calf
record angle at which pain occurs - a normal value would be 80-90 degrees - higher in people with ligament laxity
perform sciatic stretch test - dorsiflex foot at this point of discomfort - test is positive if additional pain results
flexing the knee will relieve the buttock pain - but this is restored by pressing on the lateral popliteal nerve
Severe root irritation is indicated when straight raising of the leg on the unaffected side produces pain on the affected side. A central disc prolapse is likely with risk to the cauda equina and consequently, of bladder dysfunction.

Pain upon straight leg raising before the leg is raised 30 degrees cannot be due to disc prolapse as the nerve root is not stretched within this range. Another explanation of nerve root irritation must then be sought.

Lasčgue's sign is said to be positive if the angle to which the leg can be raised (upon straight leg raising) before eliciting pain is <45°.