During an unfused tetanic contraction the contractile component stretches and then is stretched by the series elasticity in the muscle fibre during each tension oscillations. This causes the heat rate to increase, from increased metabolic rate, during the time when the contractile component is shortening. During the time when the contractile component is being stretched there is heat produced within the contractile component from dissipation of the work stored in the contractile component. A simulation is used to show that these effects are not negligible when the effects of shortening velocity on energy output rate is determined using unfused contraction. The overall effects resemble those that would be produced in a muscle if the effect of shortening velocity in accelerating the rate of cross-bridge cycling were reduced at low activation levels.