Skeletal muscle shortens faster against a lower load. This force-velocity relationship is the fundamental determinant of muscle performance in vivo and is due to
ATP-driven working
strokes of
myosin II motors, during their cyclic interactions with the actin filament in each half-sarcomere. Crystallographic studies suggest that the working
stroke is associated with the release of
phosphate (Pi) and consists of 70 deg tilting of a light-chain domain that connects the catalytic domain of the
myosin motor to the
myosin tail and filament. However, the coupling of the working
stroke with Pi release is still an unsolved question. Using nanometre-microsecond mechanics on skinned muscle fibres, we impose stepwise drops in force on an otherwise isometric contraction and record the isotonic velocity transient, to measure the mechanical manifestation of the working
stroke of
myosin motors and the rate of its regeneration in relation to the half-sarcomere load and [Pi]. We show that the rate constant of the working
stroke is unaffected by [Pi], while the subsequent transition to steady velocity shortening is accelerated. We propose a new chemo-mechanical model that reproduces the transient and steady state responses by assuming that: (i) the release of Pi from the catalytic site of a
myosin motor can occur at any stage of the working
stroke, and (ii) a
myosin motor, in an intermediate state of the working
stroke, can slip to the next actin monomer during filament sliding. This model explains the efficient action of muscle molecular motors working as an ensemble in the half-sarcomere.