Muscles contain elastic material, both intracellular and extracellular, which is mechanically in series with the crossbridges, which are the force generators. When force increases energy is stored in these structures. The work that a muscle produces in a given period is a therefore combination of the power generated by the crossbridges themselves during this time and any change in the amount.of energy stored in the series elastic material. Since the energy stored elastically is determined by the force in the stored elasticity there is no change in stored work when the force is constant. Constant force The simplest case for which the power output might be predicted is a period of constant force. If the period is long enough the classic force velocity curve applies. Maximum power can be expressed as a function of the maximum shortening velocity, the maximum force and the curvature of their relation. Both maximum power and maximum velocity increase with increasing temperature. Maximum power increases very steeply with temperature. There are major differences between vertebrate species in the maximum power at a given temperature. In part these can be understood as adaptations to the different physiological temperatures. Skinned, or permeabilised muscle fibres generally have reduced power output compared to intact muscle fibres. This may be due to raised ADP level. During brief intervals of shortening under constant force following from isometric contraction higher power output is possible than the force velocity curve predicts Presumably this is because work is provided by a change in the distribution of crossbridge states during this period. Series elasticity and inertial load If a muscle is connected to an inertial load and then activated the power output will rise above the maximum predicted by the force velocity curve. This is because work stored in the series elasticity early in the contraction is later released. There is a clear limit to the extra power that can be obtained in this way. This limit can be exceeded if the load is connected to the muscle via a cam, or is restrained by a catch. With these mechanical arrangements there is no clear theoretical limit to how much power can be delivered, although the work that can be delivered is limited simply by the compliance of the series elasticity. Work stored in crossbridges When a muscle is stretched work is stored not only in the series elasticity but also in the contractile machinery itself. This work decays into heat with a time constant of about 50 ms, but may be available to enhance power output while it remains.