Significance of the recovery heat The recovery heat takes no part in the performance of mechanical work, but appears during the passive or resting phase of the cycle. It is produced by the oxidation of a portion of the lactic acid. The energy so set free is, in part, employed in driving the endothermic reaction of resynthesizing the remainder of the lactic acid back into the precursor glycogen and, in part, is lost as heat. Muscle is not a heat engine and once the energy appears in the form of heat it becomes of no further use in the energetics of muscular contraction other than to indicate the influence the rise of temperature, which it will produce, may exert on the inherent biochemical reactions involved. The quantity of heat evolved during the restitution or endothermic phase is approximately equal to that evolved during the tension or exothermic phase when the total energy change of the latter is measured as heat. Of the total loss of energy to the muscle during a single response no more than one-half, that is, that of the exothermic phase, can be employed for the production of work. That of the endothermic phase is totally employed in the process of restitution of lactic acid into the precursor or in the form of heat which is of no use to the muscle machine. Under ordinary conditions, however, from forty to fifty per cent of the energy set free by the breaking down of glycogen (glycolysis) appears as work, fifty to sixty per cent taking the form of heat. This would then give to muscle a net efficiency of from twenty to twenty-five per cent. Experimentally it has been possible to show that skeletal muscle has an efficiency of from twentyone to twenty-fiveper cent. This figure compares favorably with the mechanical efficiency developed in the well-known mechanical engines, as may be seen from the following: Steam engines without condensers  7½%    Steam engines with condensers  9 to 19%    Gas engines  14 to 28%    Diesel engines  29 to 35% A further factor which has to do with the efficiency of a muscle for doing work is its loading. From our physical definition of work it follows that when the load is reduced to zero a contracting muscle does no work. Again when the load becomes so great as to be just beyond the power of the muscle to move it, absolute power of the muscle, no work is done although the muscle is expending considerable energy in developing tension in an attempt to lift the load. Between these two extremes there is a load which, when multiplied by the height to which it is lifted, will give a maximum product of work. This is known as the optimum load and varies from muscle to muscle. The height to which a muscle can contract depends upon its length; its power depends upon its cross-section. The efficiency of a muscle depends, first, upon its quality or energizing power, second, upon its nutritive state, and third, upon such extrinsic factors as loading, rate of stimulation, and others.		Home Classes of levers Varieties of muscle Smooth muscle The blood supply to muscles The neurone Cardiac muscle On the nomenclature of skeletal muscles The nerve supply to muscles Structure of a skeletal muscle Central nervous system The reflex arc and reflex action Physiologic properties of muscle Mechanical Changes The effects of temperature on muscular contraction Muscle glycogen, its source and fate The reaction of muscles Oxygen consumption of muscle Determination of the oxygen consumption of muscle The viscosity factor in muscle and muscle efficiency Significance of the recovery heat Production of Electricity