Fig. 5. Effect of altering stimulation rate and number of periods of force redevelopment on the force depression in two muscle fibres. `Control', same conditions as in the rest of the study with shortening 1500 ms into activation so that there were two periods of force development; `+10Hz', same as `Control' except for a 10 Hz increase in stimulation frequency; `ES' (early-shortening), shortening performed 20 ms into activation so that there was only one period of force development; `DS' (double-shortening), shortening broken into two equal steps with a 1000 ms gap so that there were three periods of force development; `TS' (triple-shortening), shortening broken into three equal steps with 500 ms gaps so that there were four periods of force development (only performed on fibre 2). For all trials performed on each fibre, total shortening distance and final length were the same. However, shortening distance and final length were different in the two fibres (see Materials and methods), which may explain the different magnitudes of force depression observed. Force depression increased when stimulation frequency was increased, in contrast to what would be expected for the `movement effect' (Edman, 1975; Edman, 1980). Force depression increased as the number of periods of force development was increased but the relationship was not linear.