Fig. 1.

A: active force development increases in isolated rabbit cardiac trabeculae when frequency of stimulation is increased, resulting in a positive force-frequency relationship. Both diastolic and systolic calcium concentrations increase when frequency increases, whereas the calcium transient amplitude (systolic minus diastolic values) also increases. B: time from peak force to 50% relaxation (RT₅₀) declined when frequency of stimulation is increased, whereas myofilament calcium sensitivity decreases with frequency. Data were recorded at optimal length and at 37°C and modified from Varian and Janssen (54).