I don't think you should consider it as a change in hamstring length, rather and increase in hip flexion with the knee at ?neutral extension and lumbar spine at? . These two conditionas must be standardised or are you going to include lumbar segmental movement in the measure in which case you are measuring anincrease in hip joint and spinal mobility. You really need to be careful when assigning these measures to a hamstring change. Would be nice to also measure the clients lengthe under a general anaesthetic as you will find most have the be increase with this technique. The makes you wonder what are you achieveing, an increase in mobility (YES) but maybe due to a reduction in neural activing or HOLDING, i.e. an induced relaxation rather than a change in anatomical muscle length.

cheers :-) richard