The most commonly sprained ligament is the medial collateral ligament (MCL). This ligament can be sprained by a blow to the outside of the knee, particularly if your foot is planted on the ground when you are hit. The blow causes the knee to move toward the inside of the body and stretches the ligament. You will feel tenderness and pain on the inside of the knee, and the knee will feel like it may buckle or give way to the inside. Anything more than minimal pain should be treated by a doctor.