Muscles and tendons are very different structures.

Tendons that have lost elasticity are much more likely to suffer micro tears or become inflamed when stretched and they take much, much longer to heal than muscles when that happens.

This condition is known as tendinopathy,

When a tendon that was elastic has become inelastic, trying to stretch it is likely going to damage it. Try stretching an old, stiff rubber band and you will not make it more elastic. You will likely break it.

I had a plasma-rich-platelet procedure to help reduce painful inflammation in the tendon in my left leg and, subjectively, the procedure provided substantial relief.

Stretching, on the other hand, even very mild and limited stretching, greatly exacerbated my condition, again, because stretching an inelastic tendon (as differentiated from a muscle) will not, unfortunately, make that tendon more elastic.

I wish it would.