We are all guilty at one time or another of prescribing a hamstring stretch prematurely. You saw a patient bend forward to touch their toes and they abruptly stop short. This must be their hamstrings stopping their motion! Well unfortunately it is not that simple anymore. Modern literature tells us the reason they cannot touch their toes may be due to an array of possibilities: tight posterior chain muscles, adverse neural tensioning, or poor stability/motor control of the core musculature just to name a few. For the sake of this post we want to address hamstring tightness vs. adverse neural tension.
To begin this conversation, let's take that same patient above and place him/her in long sitting to reduce the effects of spinal loading. Now, postural demands are less than that in standing. We can focus specifically on muscle tightness vs. neural tension. The patient leans forward to touch their toes, but again nothing changes. We are no closer to finding our solution than we were in standing. Now, we bring the patient into supine, and the therapist performs a passive straight leg raise (PSLR). The patient cannot raise beyond 60 degrees of hip flexion. We still do not know, but we are getting close to the answer. Is it muscle tightness that is stopping this patient at 60 degrees or is the act of passively straightening the leg causing strain on the neural tissues. To answer this question, simply palpate the ASIS as you perform the PSLR. If you notice that hip flexion stops prior to movement at the ASIS, the answer is Neural Tension. If the ASIS begins to move prior to resistance, then hamstring tightness is the answer.
Since the hamstrings attach to the Ischial Tuberosity, the body will naturally begin to posterior pelvic tilt once all the available range is taken up in the hamstring muscles. This posterior pelvic tilt will cause movement at the ASIS, letting you know the hamstring muscles have reached their end range. If no movement was noted at the ASIS, you can shift your hypothesis towards adverse neural tension. An additional component you can consider is the use of cervical flexion/extension to change the tension of the nerves and thus potentially alter SLR ROM.
Just like any examination test, it is important to cluster your findings with other tests and measures. For example, if you suspect adverse neural tension, perform a SLUMP test as well. Additionally, always check side to side symmetry.