An employee was troubleshooting a 4x4 lever on a bus that was stuck. When EE was unable to engage the lever with EE's arm, EE used EE's leg for additional force/leverage. The knob on the lever is threaded and while force was applied, the knob unthreaded. This caused the foot to slip off the lever and contact the dash of the bus which resulted in a fracture to EE's fibula.