Fig. 4. Work and ATP utilization vary with respect to the phase of Ca²⁺ activation (_Ca). Data points (connected with a line) represent average values (mean ± s.d.) from 245 individual work loops at a duty cycle (_Ca) of 0.5. The _Ca values producing maxima and minima varied for work (A) and ATP (B), therefore yielding a unique _Ca that maximized efficiency (ratio of work to ATP; C). Strain amplitude was 0.05 (left) and 0.025 (right), and primarily affected work magnitude. Note the difference in ordinate scales between the left and right panels in A and C when =0.05 versus 0.025. Mechanical parameters for the filaments and cross-bridges are listed in Fig. 2.