An athlete experiencing an injury to the muscles, bones or joints most commonly will complain of:

The Joints. is the answer

An athlete experiencing an injury will first complain of joint injury.

Explanation:

The joints present in the body are composed of connection between the two bones. The injury to the joints can easily recognized because joint involve special functioning in the body. Thus the athlete will first complain about joint injury. The symptoms of joint injuries are sprain, strain, dislocation and fracture.