When there has been repetitive stress, the knee structures may start to fail despite there having been no single major injury:
The bone where a tendon attaches can become stressed with repetitive strain, and it and its attached tendon can break off from the main part of the bone.
With repetitive stress the inside of a tendon can degenerate ('mucoid degeneration)' and render the tendon weak and painful.
The knee has a number of lubricating pockets, the surfaces of which slide over one another during movement. With overuse these pockets or 'bursae' can become inflamed and painful and fluid can collect inside them to form a swelling.
These are the more common overuse problems: