An employee was working from a lift truck bucket to take 500-pound X-braces down from between the uprights of an H-type power transmission line structure. The employee used a hammer to drive a bolt out so he could connect the X-brace arm to the crane rigging and lower it to the ground. The X-brace arm vibrated off the bolt connection points, causing the cross brace piece to hinge toward the employee's arm. The employee was hospitalized for a muscular injury to the lower left arm and wrist.