Sports physiologists have attributed the advantage that small stature holds in cycling up steep ascents to the way in which body mass and body surface area scale according to height (see square-cube law). As a hypothetical cyclist's height increases, the surface areas of his body increase according to the square of his height whereas the mass of his body increases according to the cube of his height. The surface area relation applies not only to the total surface area of the body, but also to the surface areas of the lungs and blood vessels, which are primary factors in determining aerobic power. Thus, an equally proportioned cyclist who has 50% more body mass (i.e. is 50% heavier) will generate only about 30% more aerobic power. On a steep climb most of the cyclist's energy goes into lifting his own weight, so the heavier cyclist will be at a competitive disadvantage. There is, of course, a lower limit to the benefit of small stature because, among other factors, the cyclist must also lift the weight of his bicycle. The additional power is proportional to the grade or slope of the road and the speed of the rider along the slope (or along the level line). For a 5% grade, each meter of road requires lifting the body weight by 5 cm. The power (watts) is equal to change in gravitational potential energy (joules) per unit time (seconds). For a rider, the additional power needed is about 30 watts per meter/second of road speed (about 8 watts per km/hour).