The claimant, employed as a cylinder press feeder in the appellant-employer's printing plant, testified that while he was engaged (together with a fellow employee) in putting a fifteen to twenty pound roller back into a printing press, he placed his left foot on a step alongside the press and then attempted to bring his right foot on to the step, but he missed the step, causing his left foot to slip off it and throwing his entire weight upon his right foot. This resulted in a sprain of his back. The board found that a pre-existing symptomless osteomyelitis of the spine had been aggravated by the sprain and twisting of the claimant's back and that this precipitated the disabling symptoms. There was ample medical evidence that the accident was a competent precipitating cause of the claimant's disability. The appellants' brief raises only questions of fact upon which the board's decision is final, since it is supported by substantial evidence. Decision and award unanimously affirmed, with costs to the Workmen's Compensation Board.