The total metabolism, as we said a moment ago, varies greatly day by day because the extent to which we use our muscles is so different. The basic metabolism is very steady and the functional metabolism of the vital processes like breathing, the heartbeat, etc., is also pretty steady, so that the metabolism for one who makes no use of his muscles is fairly uniform. Of course it differs in large people as compared with small, although curiously enough the difference is not proportional to the weight but to the body surface. Just why this is so we do not know. The average figure for the total metabolism of a resting man is about 1,900 calories a day; this includes the motions that are necessary for eating and swallowing food, since life cannot go on indefinitely without making those motions, but supposes that all other activities are done away with. This figure, as we said a moment ago, holds pretty steady day in and day out. To find the total metabolism on any day we have simply to add to it the figure for the energy expenditure from the use of the muscles, which will vary with the amount of work that is done. In figuring this we have to reckon with the fact that the muscles are like other machines in working at what is called in engineering a low efficiency; by that we mean that the amount of energy that can be gotten out in the form of useful work is less than the energy that is actually consumed; there is a waste of energy which takes the form of heat. Our muscles are under ordinary circumstances about twenty per cent efficient, which means that for every calory-worth of actual muscular work we do we use up five calories-worth of fuel; the energy of four wasted calories takes the form of heat and we all know from experience that the amount of heat thus produced inside our bodies warms us up very quickly, when we are using our muscles actively. The actual muscular work done in a day by individuals varies from almost nothing in the case of invalids confined to bed through about forty calories for a person of decidedly sedentary habits and about 120 calories for the average clerical or professional man up to from 300 to 400 in the case of manual laborers. These figures are for the actual muscular work done; to obtain the energy expenditure we have to multiply each of them by five on account of the inefficiency of the muscles. If we do this and then add to each product the constant figure of 1,900 for the metabolism of rest we obtain for the total metabolism of a decidedly sedentary person an average of about 2,100 calories; for an average clerical or professional man 2,500 calories, for manual laborers from 3,200 to 4,000 calories. Of course, individuals may exceed even these latter figures. It is believed that athletes in extreme competitions such as for example a six-day bicycle race may liberate energy at the rate of 10,000 calories a day, although they probably cannot keep this up long enough actually to do that amount in a single day.