It is the sinew.

Tendon meat comes from the thigh, wrapped in muscle membrane, containing sinews inside, with moderate hardness and regular texture lines, making it suitable for braised dishes. Precisely speaking, it should be a small strip of half meat and half sinew in the middle of the hind leg—of course, this is the terminology used by businesses; generally in the marketit is considered part of the hind leg.and it contains a large amount of connective tissue distributed among the muscles.