Force depression following muscle shortening was investigated in cat soleus (n=6) at 37 degrees C for a variety of contractile conditions with the aim to test the hypotheses that force depression was independent of the speed of shortening and was directly related to the mechanical work produced by the muscle during shortening. Force depression was similar for tests in which the mechanical work performed by the muscle was similar, independent of the speed of shortening (range of speeds: 4-256mm/s). On the other hand, force depression varied significantly at a given speed of shortening but different amounts of mechanical work, supporting the hypothesis that force depression was not speed - but work dependent. The variations in the mechanical work produced by the muscle during shortening accounted for 87-96% of the variance observed in the force depression following shortening further supporting the idea that the single scalar variable work accounts for most of the observed loss in isometric force after shortening. The results of the present study are also in agreement with the notion that the mechanism underlying force depression might be associated with an inhibition of cross-bridge attachments in the overlap zone formed during the shortening phase, as proposed previously (Herzog and Leonard, 1997. Journal of Bimechanics 30 (9), 865-872; Maréchal and Plaghki, 1979.