Contract EE was attempting to set Heinzman jacks at a belt drive. EE bumped another jack that was leaning against the rib & EE attempted to prevent it from falling, straining back. Cause of injury was not paying attention to the task. Compliance with rules/regs, PPE, Mining equip/systems not a factor. Miner proficiency may be a factor. EE did not start to miss work until 10/3/16.