Fig. 1— The ATP-ADP-Pi-actomyosin crossbridge cycle. The crossbridge cycle was subdivided into six different conformational states, with three detached states (D1, D2 and D3) and three attached states (A1, A2 and A3). The A3→D1 transition was the ATP-binding step; crossbridge detachment occurred when ATP bound to the actin (A)–myosin (M) complex (AM) and the rate constant for detachment was g2; AM→A+M. The D1→D2 transition was the ATP hydrolysis; M+ATP→M-ADP-Pi. The D2→D3 transition was M-ADP-Pi→M*-ADP-Pi, where M* is myosin in refractory state 24. The D3→A1 transition was the attachment state: the myosin head (M*-ADP-Pi) was weakly bound to A and the rate constant for attachment was f1; M-ADP-Pi+A→AM-ADP-Pi. The A1→A2 transition was the power stroke, a strongly bound, high-force state which was triggered by Pi release: AM-ADP-Pi→AM-ADP+Pi; ts was the duration of the power stroke, step size was the amplitude of the myosin head movement and π was the crossbridge unitary force. The A2→A3 transition was the release of the hydrolysis product ADP: AM-ADP→AM+ADP; tc was the duration of the overall crossbridge cycle and kcat = 1/tc, where kcat was the maximum turnover rate of myosin ATPase under isometric conditions.