Possible knee injuries!

• Knee injuries. Ligament injuries to the knee are very common in sports that require stopping and starting or quickly changing directions. These extreme forces on the knee can result in torn ligaments. The anterior cruciate ligament (ACL) and the medial collateral ligament (MCL) are the most often injured, but the posterior cruciate ligament (PCL) and the lateral collateral ligament (LCL) can also be injured. Cruciate ligament injuries don't always cause pain, but typically cause a loud pop. Most of these injuries are confirmed with an MRI. Arthroscopic surgery is sometimes the best way to find a partial tear.

• Torn knee cartilage (meniscus injuries). Torn knee cartilage is usually a torn meniscus. These small, "c" shaped pieces of cartilage act as cushions between the thigh bone (femur) and the tibia (shin bone). There's one on the outside (lateral meniscus) and one on the inside of the knee (medial meniscus). Meniscus tears are often the result of twisting, pivoting, decelerating, or a sudden impact. These tears can be identified by various manual tests a physician can perform to detect torn cartilage.
• Chondromalacia. This term refers to the softening and deterioration of the underside of the kneecap. In young athletes, this is typically an injury from trauma, overuse, poor alignment of the knee joint, or muscle imbalance. This leads to friction and rubbing under the kneecap that causes damage to the surface of the cartilage. The sensation is a dull pain around or under the kneecap that worsens when walking down stairs or hills, climbing stairs, or other weight-bearing activity.   
• work cite:  Essential Shoulder Exercises for Tennis Players, www.itftennis.com/scienceandmedicine/injury-clinic/tennis-injuries/jumpers-knee.aspx.