Employee was at Skyline Materials working on a 966G. EE had pulled the cab off so EE could replace the shift solenoid and speed sensors. EE was down on EE's knees working when EE's right knee slipped on one of the hoses causing EE to hit EE's knee on the head of the bolt that holds the top center pin in. EE's knee has been hurting and swelling from EE's knee down to EE's ankle.