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 [36]. 𝑘 o n and 𝑘 o , represent the RU on/off rates and are functions of C a 2 + bound to cTnC. Once turned on by the binding C a 2 + , RU permits the transition of XB from the non-cycling ( 𝑁 n c ) pool to the cycling pool of XB. The cycling pool of XB includes two states of XB: cycling non-force-bearing ( 𝑁 𝑐 - n f b ) and cycling force-bearing ( 𝑁 𝑐 - f b ). The transition between non-cycling and cycling pools is mainly regulated by 𝑘 o n / 𝑘 o kinetics of RU. The influence of the force-bearing XB on the RU on/off kinetics is represented by the feedback arrow (dashed line). 𝑓 and 𝑔 represent the rate constants governing forward transition, 𝑁 𝑐 - n f b 𝑁 𝑐 - f b , and backward transition, 𝑁 𝑐 - f b 𝑁 𝑐 - n f b , respectively.