Sir, in athletic people most of the time its due to ligament tear due to excessive strain on the ligament or due to some unhealed injury while doing that activity. In some cases its due to bursitis due to repetitive kneeling or bending of the knee. Or it might be due to injury to tendon. This things generally heal by taking Complete rest for few weeks, apply hot water compression with a cloth, avoid strenuous activities. You can also use knee pads if you kneel frequently. Other causes like osteoarthritis, infections,patellar dislocation etc might also cause a knee pain.consult a orthopedician for further management.