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Although force-velocity (P-V) relation is important to obtain insights into kinetics of actin-myosin interaction in muscle, determination of P-V relation in skinned muscle fibers are difficult mainly due to gradual deterioration at the two cut fiber ends, which makes it impossible to obtain enough data points to construct P-V curve over the whole range of forces. In addition, determination of the maximum shortening velocity Vmax by back extrapolation of limited data points to velocity axis is ambiguous because of the steep force versus velocity relation. To overcome these difficulties, we developed a novel method to obtain continuous P-V curves over the whole range of forces from 0 to the maximum isometric force Po, by applying length changes, with time course L=-kt2, to Ca2+-activated skinned fibers, so that time derivative of length change, i.e. shortening velocity V=-dL/dt=-2kt; namely, V increased linearly with time t. By recording the resulting chane in force P, we could obtain continuous P-V curve in one shot. Using the above method, we examined effect of high inorganic phosphate (Pi) concentration and low pH (6.5) on P-V relation of skinned rabbit psoas muscle fibers at 20°C and at 5°C with the following results: (1) The P-V curves obtained exhibited a distinct hump at high force region >0.5-0.6Po. The P-V curves, deviated from rectangular hyperbola with forces > ~0.2Po, did not change their shape appreciably by Pi and low pH.; (2) Pi (30 mM) reduced Po by ~30% at 20°C and ~45% at 5°C; (3) Low pH (6.5) showed no significant effect on Vmax at both 20 and 5°C; and (4) Pi (30 mM) at low pH (6.5) reduced Po by ~43% at 20°C and ~45% at 5°C. These results are discussed in relation of other published results.