A common overuse syndrome in young soccer players is Osgood-Schlatter. The symptoms are pain and often some swelling about two inches below the kneecap in the front of the knee. The patellar tendon attaches here on a bump of bone called an apophysis. Irritation of this attachment site and the apophysis can occur with repetitive running and jumping activities. It's no surprise, then, that adolescent soccer players suffer from this problem.
It is typically a self-limiting process, but can be bothersome for weeks to months. Long-term problems with this are very uncommon, although some individuals will end up with a more prominent bump below their knee.
Treatment is ice and anti-inflammatories. Sometimes, a neoprene knee sleeve will help with direct blows to the area. Players may continue to play if their symptoms allow, as long as they are mild and are not limiting their play. If pain is affecting play, it may be best to rest for a few weeks to settle it down. If pain is worsening, leading to limping, or if swelling is increasing, then a physician evaluation is indicated.