Employees were working on an excavation to expose underground gas lines for repair. The backhoe operator pulled up the digging stabilizers and turned his chair around to drive the machine away from the excavation. As the operator turned his chair, the headrest on the back of the chair caught the boom joystick, causing the boom to swing to the right. The injured employee who was near the backhoe was struck in the chest by the backhoe boom and pushed up against the dump truck, resulting in 5 rib fractures, a punctured lung, and heart arrythmia.