Embodiments of valve operating mechanisms for operating the poppet valve of an internal combustion engine. Each embodiment includes a rotating cam member having a cam lobe surface engaged with a follower for actuating the poppet valve. The profile of the cam surface is such so that the absolute value of the jerk of the valve acceleration in the vicinity of maximum valve lift is smaller than the absolute value of the jerk of the valve in areas adjacent to the area of maximum valve lift. The load between the cam surface and the follower at the point of maximum lift is greater than the load during the time of at least one of the approach to maximum lift and the closing of the valve after the maximum lift because the tip of the nose of the cam has a greater effective radius than on the sides adjacent the tip. This reduces stress and permits greater engine speeds without valve float.