There are several conditions that will make the knees hurt when jumping on a trampoline: (1) Edema in the knee. (2) Arthritis in the knee. (3) Degeneration of the cartilage in the knee. (4) There may be compression of the bones in the knee at landing on the canvas. This compression is sometimes known as a shock. (5) Knee bones may be bumping together on impact because of the thinning knee cartilage. (6) In knee surgery to remove half of the knee cap, a common surgical practice, the knee becomes more susceptible to edema and arthritis in the knee.