Fig. 2. Work loop simulations oscillated muscle length and intracellular Ca²⁺ concentration as a function of time, while monitoring force and ATP utilization. These panels show the initial half second of a simulation where muscle strain amplitude (, normalized peak to peak) was 0.025 and the Ca²⁺ transient had a 0.2 duty cycle (_Ca) at a 0.033 phase of activation (_Ca). Force and ATP utilization were calculated at each time step (dt=1 ms). Our standard mechanical parameters apply to these simulations: k_xb=5, k_a=5229 and k_m=6060 pN nm^–1 (Tanner et al., 2007).