I have an old hamstring injury that doesn't bother me, but sometimes it feels tight when I run. I try to stretch out and warm up each time. What causes this sensation?

Reports of muscle tightness after injury are common even years later. It's possible that scar tissue from the healing process causes this sensation. It may be that some muscle fibers get linked up with scar tissue instead of reconnecting with the aponeurosis
where they belong.

The aponeurosis is a strong sheet of fibrous connective tissue. It acts as a tendon to attach some muscles to the bone or to bind muscles together. When muscle fibers link to scar tissue instead of the aponeurosis, the muscle gets shorter. This may cause the
tightness you are having.

Research shows that structural changes in the muscle after injury remain for a long time. The same is true for any changes made during rehab. Much of rehab is focused on contracting the muscle to regain strength. This can add to the problem of muscle fiber shortening.