Often times a lack of dorsifleixon will prevent a full stretch of the biceps femoris muscle. Without the full stretch the biceps femoris can't be recruited as heavily in hip extension. This can lead to a decreased glute max as well. When both are decreased an increase in hip rotator activity may be present. Since the sciatic nerve is present between the superior gemelli and piriformis, compression of this nerve can be present.
Thinking more regionally, this can also lead to an increased activity of the medial hamstrings.
Biceps femoris tends to be a weaker muscle in relation to the other hamstring muscles, but perhaps instead of targeting lateral hamstring exercises, check the dorsiflexion of the ankle.
I often see lack of motion between the tibialis anterior and the peroneal group. This will require some sort of soft tissue intervention. A talus that has shifted anterior. A mobilization from a health practitioner or doing some talar glides against a wall to get some more motion.
Next time you see limited dorsiflexion realize that their can be ramifications into the lateral hamstrings. Next time you see sciatica check the biceps femoris and dorsiflexion.