Figure 2: Reduced three-state crossbridge (XB) model scheme depicting regulatory unit (RU; Tm-Tn) kinetics and XB cycling kinetics. This scheme is adapted from Campbell . and , represent the RU on/off rates and are functions of bound to cTnC. Once turned on by the binding , RU permits the transition of XB from the non-cycling () pool to the cycling pool of XB. The cycling pool of XB includes two states of XB: cycling non-force-bearing () and cycling force-bearing (). The transition between non-cycling and cycling pools is mainly regulated by kinetics of RU. The influence of the force-bearing XB on the RU on/off kinetics is represented by the feedback arrow (dashed line). and represent the rate constants governing forward transition, , and backward transition, , respectively.