Some tightness is a nerve and stretching only makes it worse

A hamstring that resists every attempt to lengthen may be protecting an irritated nerve rather than reporting a short muscle.

An athlete has been stretching a hamstring for four months. It has not changed. The sensation is a pulling that runs down the back of the thigh, sometimes into the calf, and it is worse in the morning and worse when they sit for a long time. That is not a muscle length problem and it will not respond to being pulled on, because the structure objecting is the sciatic nerve.

Nerves are not slack cables. They run through tunnels of muscle and fascia, they glide relative to surrounding tissue as joints move, and they have their own blood supply and their own sensitivity. Straighten the knee and flex the hip and the sciatic nerve and its branches are placed under considerable tension by that combination alone. A standard hamstring stretch tensions the nerve and the muscle at the same time, which is why the test to tell them apart is so simple. Get to the point of tightness, then flex the ankle to pull the toes toward the shin. That movement adds nothing to the hamstring. If the symptom sharpens, you have your answer.

When a nerve is irritated, whether by compression somewhere along its path, by scarring in the tissue it passes through, or by inflammation, the nervous system responds by increasing protective muscle tone in the region. That tone is the tightness. It is an output, not a cause. Stretching into it applies tension to an already sensitised structure and the system responds by protecting harder, which is the mechanism behind an athlete who feels tighter after every stretching session and cannot work out why.

The right handling is gentler and looks like almost nothing. Movements that slide the nerve rather than stretching it, by tensioning at one end while releasing at the other, plus finding whatever is irritating it, which is often a lot further up than the symptom.

I bring this up because hamstring tightness gets treated as a single entity in nearly every gym I have been in, and it is at least three different problems wearing the same description.

Short muscle. Protective tone after injury. Irritated nerve.

Only one of those wants to be stretched, and it is the least common of the three.