HUMAN ENERGY

To utilize human energy to create power, you have to fuel the human. Food is the fuel--how much fuel is necessary for basic human functioning, and how much extra fuel is necessary to create energy "to burn"? The following excerpts describe how cellular energy works, and approximately how much power humans can produce.

adenosine triphosphate
from http://hyperphysics.phy-astr.gsu.edu/hbase/biology/atp.html

"Adenosine triphosphate (ATP) is considered by biologists to be the energy currency of life. It is the high-energy molecule that stores the energy we need to do just about everything we do. It is present in the cytoplasm and nucleoplasm of every cell, and essentially all the physiological mechanisms that require energy for operation obtain it directly from the stored ATP. As food in the cells is gradually oxidized, the released energy is used to re-form the ATP so that the cell always maintains a supply of this essential molecule. ...In animal systems, the ATP is synthesized in the tiny energy factories called mitochondria .

The structure of ATP has an ordered carbon compound as a backbone, but the part that is really critical is the phosphorous part - the triphosphate. Three phosphorous groups are connected by oxygens to each other, and there are also side oxygens connected to the phosphorous atoms. Under the normal conditions in the body, each of these oxygens has a negative charge, and as you know, electrons want to be with protons - the negative charges repel each other. These bunched up negative charges want to escape - to get away from each other, so there is a lot of potential energy here.

  If you remove just one of these phosphate groups from the end, so that there are just two phosphate groups, the molecule is much happier. This conversion from ATP to ADP is an extremely crucial reaction for the supplying of energy for life processes. Just the cutting of one bond with the accompanying rearrangement is sufficient to liberate about 7.3 kilocalories per mole = 30.6 kJ/mol. This is about the same as the energy in a single peanut.

Living things can use ATP like a battery. The ATP can power needed reactions by losing one of its phosphorous groups to form ADP, but you can use food energy in the mitochondria to convert the ADP back to ATP so that the energy is again available to do needed work. In plants, sunlight energy can be used to convert the less active compound back to the highly energetic form. For animals, you use the energy from your high energy storage molecules to do what you need to do to keep yourself alive, and then you "recharge" them to put them back in the high energy state. The oxidation of glucose operates in a cycle called the Krebs cycle in animal cells to provide energy for the conversion of ADP to ATP."

...humans need food energy to cover the basal metabolic rate; the metabolic response to food; the energy cost of physical activities; and accretion of new tissue during growth and pregnancy, as well as the production of milk during lactation. Energy balance is achieved when input (or dietary energy intake) is equal to output (or energy expenditure), plus the energy cost of growth in childhood and pregnancy, or the energy cost to produce milk during lactation.

Not all combustible energy is available to the human for maintaining energy balance (constant weight) and meeting the needs of growth, pregnancy and lactation. First, foods are not completely digested and absorbed, and consequently food energy is lost in the faeces. The degree of incomplete absorption is a function of the food itself (its matrix and the amounts and types of protein, fat and carbohydrate), how the food has been prepared, and - in some instances (e.g. infancy, illness) - the physiological state of the individual consuming the food. Second, compounds derived from incomplete catabolism of protein are lost in the urine. Third, the capture of energy (conversion to adenosine triphosphate [ATP]) from food is less than completely efficient in intermediary metabolism (Flatt and Tremblay, 1997). Conceptually, food energy conversion factors should reflect the amount of energy in food components ... that can ultimately be utilized by the human organism, thereby representing the input factor in the energy balance equation....The energy that remains after accounting for the important losses is known as "metabolizable energy."

Not all metabolizable energy is available for the production of ATP. Some energy is utilized during the metabolic processes associated with digestion, absorption and intermediary metabolism of food and can be measured as heat production; this is referred to as dietary-induced thermogenesis (DIT), or thermic effect of food, and varies with the type of food ingested. This can be considered an obligatory energy expenditure and, theoretically, it can be related to the energy factors assigned to foods. When the energy lost to microbial fermentation and obligatory thermogenesis are subtracted from ME, the result is an expression of the energy content of food, which is referred to as net metabolizable energy (NME)."

  "Where humans worked against gravity, as they did inside cage wheels and upon treadmills, we can calculate the power outputs. As a benchmark, we might use data, first obtained in the eighteenth century by British scientists Jean Desaguliers and John Smeaton, of the power a human laborer could produce if working steadily all day: 90 to 100 watts. In contemporary terms, this means that if you attach a generator to an exercise machine, you can watch TV as long as you climb, pedal, or row.

  By the same token, a Roman cage wheel sixteen feet in diameter and eight feet across accommodated six to eight men, who could, forty times per hour, jointly lift one ton a distance of twenty-seven feet--which equals a power output of 600 foot-pounds per second. Dividing that among eight workers, we calculate a power output per person of just over 100 watts. That figure attests both to respectable efficiency for the machine and to considerable effort for the workers (who may have worked in relays).

  On the Bellevue Penitentiary treadmill, prisoners climbed on treads protruding from a wheel that was slightly over five feet in diameter and turned three times each minute. If one assumes that a typical prisoner weighed 132 pounds, then the prisoner must have worked at a power of almost 140 watts. Since the normal duty cycle allowed each prisoner to rest one-third of the time, the sustained output would have been a little over 90 watts--sustained, according to the report, for up to ten hours a day. That figure of 90 watts confirms the reported unpleasantness of the task. A similar output was demanded of nineteenth-century Australian convicts, who worked up to twelve hours per day; some said they'd rather hang than work their mill.

We can view that 90 watts in yet another context. At best, only about one-fourth of the energy in food emerges as useful mechanical work. Thus, laboring on the treadmill--sustaining 90 watts for ten hours--itself requires more than 3,000 Calories. So Bellevue's inmates worked hard enough and long enough to require double the food intake of a normally active adult male.

A worker doing hard physical labor all day long--or a felon turning a treadmill--can put out about 100 watts of power. That's the output, in the form of a little light and a lot of heat, of the familiar light-bulb. It's a little more than the rate at which an inactive human heats a room. But to understand 100 watts of muscle power, one needs to turn to a quantifiable everyday task that humans do with reasonable efficiency. Climbing stairs fits that bill.

When you climb a flight of stairs, what's your power output? Just multiply your weight in pounds by the height of each step in inches and by your climbing rate in steps per second, and then divide by 9. The last number takes care of gravity and converts the figure into watts. I weigh 140 pounds. When climbing seven-inch steps at two per second, I put out about 220 watts--a rate that I, an age-challenged man, can sustain only briefly. Ascending a down escalator, I work at 140 watts.

But what climbing rate corresponds to an output of 100 watts? Divide 900 by your weight in pounds and by the step height in inches; the resulting figure is how many steps per second you would have to climb. On seven-inch stairs, I'd have to ascend them at a little less than one step per second--trivial for the first couple of flights but a tiring regimen to keep UP for even an hour.

  What about fuel? We're at best only about 25 per cent efficient, so an output of 100 watts requires a minimum input of 400 watts, which translates (when we multiply by 0.86) into about 350 Calories per hour. Burning a tenth of a pound of good fuel--fat--yields 350 Calories, so working at 100 watts for eight hours costs less than a pound of body fat--still nearly double a human male's normal energy use.

For a difficult task of only a few seconds' duration, a person can put out thousands of watts--many times the 746 watts in one official horsepower. For tasks lasting a few minutes, a fit human can generate perhaps 1,000 watts. For an activity that must be sustained for an hour, output drops to around 300; for an activity kept up all day, 150 watts is about the maximum."