A muscle imbalance occurs when there is an alteration in the latest tension relationship surrounding a given joint. Charles Sherington first describes the neurological concept of reciprocal innervation. Basically, when a muscle contracts for example the quads they receive a nerve impulse to contract. The antagonist which is the hamstrings receive a nerve impulse to relax. This explains how opposing muscle groups become neurologically inhibited as tight agonist inhibit their functional antagonist.
We also have synergistic dominance. Synergistic can be defined as acting together. When the synergistic muscles are recruited to help balance the connective tissue surrounding an injured structure the normal neurologic firing order is thrown off and muscle substitution patterns occur. As a result, the synergistic stabilizers designed to help in the movement are forced to perform the task that the weekend prime mover is unable to do.
The synergistic dominance eventually can lead to a more serious condition called arthrokinetic. This is from prolong length tension forces at a joint, result in an abnormal movement and a loss of joint play. Going back to the hamstring example, the hamstrings are recruited to compensate for a weakened glute max to help aid in pelvic stability. This constant pull on the ischial tuberosity weakens the sacrotuberous and sometimes the iliolumbar ligaments, and causing the pelvis to lose alignment and stability. The brain will usually put the piriformis muscle in a constant contraction to help stabilize the SI joint.
So, the question is, is the piriformis tight or is it a neurological protective mechanism? These are the questions you want to ask yourself and assess before you go straight into where the client says they have pain or tightness. What is causing that pain and is it a truly restricted muscle or joint?