The efficiency of human muscle has been measured (in the context of rowing and cycling ) at 18% to 26%. The efficiency is defined as the ratio of mechanical work output to the total metabolic cost, as can be calculated from oxygen consumption. This low efficiency is the result of about 40% efficiency of generating ATP from food energy , losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of exercise and the type of muscle fibers being used (fast-twitch or slow-twitch). For an overall efficiency of 20 percent, one watt of mechanical power is equivalent to kcal per hour. For example, one manufacturer of rowing equipment calibrates its rowing ergometer to count burned calories as equal to four times the actual mechanical work, plus 300 kcal per hour,  this amounts to about 20 percent efficiency at 250 watts of mechanical output. The mechanical energy output of a cyclic contraction can depend upon many factors, including activation timing, muscle strain trajectory, and rates of force rise & decay. These can be synthesized experimentally using work loop analysis .
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Please note, that this calculator predicts your maximum possible natural weight, key of course is which definition of natural I am using. In the context of this prediction calculator, “natural” means no steroids, no pro-hormone supplements, no hormone replacement therapy, and no testosterone boosters – that is, no products that artificially raise testosterone levels. I realize that this is a very, very loose definition of natural and its one that personally I don’t agree with. If you use every product that GNC has to offer that doesn’t boost testosterone, I would consider you natural and the predictions of this calculator would be applicable to you.