Figure 4. Predicting contraction duty cycle and amplitude from neural inputs within stimulated preparations and across semi-intact hearts. A, C, Linear models were fit to the form y = b0 + b1x1 + b2x2 + b3x1x2; where y = muscle contraction duty cycle, x1 = stimulation/neural duty cycle, and x2 = stimulation/neural burst frequency. B, D, Linear models were fit to the form z = b0 + b1x1 + b2x2 + b3x1x2 + b4x12; where z = muscle contraction amplitude. All estimates for regression parameters (b1, b2, etc.) were obtained using ordinary least-squares. Arrows at the top of each panel schematically indicate the sign and significance of each regression parameter using ANOVA; full ANOVA results are given in Tables 1 and 2. Plots at the bottom of each panel indicate the dependent variable on the vertical axis, neural/stimulation duty cycle on the horizontal axis, and neural/stimulation burst frequency on a color scale (see legends to the right of B, D). A, B, Effects of stimulation duty cycle and burst frequency on muscle contraction duty cycle (A) and contraction amplitude (B) within stimulated heart preparations. Error bars indicate mean ± SE (n = 16 preparations). C, D, Effects of spontaneous motor neuron duty cycle and burst frequency on muscle contraction duty cycle (C) and contraction amplitude (D) across semi-intact heart preparations (n = 122).