The change in the ability of frog skeletal muscle fibres to sustain a load was studied during the course of oscillatory length changes or continuous isotonic lengthening following quick increases in load, by applying "test' load steps and measuring the initial velocity of resulting isotonic motion. 2. When quick decreases in load were applied during oscillatory length changes or continuous isotonic lengthening, the fibres were found to shorten against a load above the maximum tension (P0), indicating an increase in load-sustaining ability after quick increases in load. 3. If quick increases in load were applied at various times after preceding quick increase in load, the initial velocity of resulting isotonic lengthening decreased with time, also indicating an increase in load-sustaining ability. 4. An increase in load-sustaining ability was also observed during the course of rapid isotonic lengthening under a load of 1.6-1.7 P0, in which the fibres lengthened with increasing velocity. 5. The increase in load-sustaining ability after quick increases in load was associated with a shift of the force-velocity curve towards higher force values, while no significant change was observed in the maximum shortening velocity at zero load. 6. The stiffness of muscle fibres was estimated by measuring quick length changes coincident with load steps. It decreased with decreasing isotonic load below P0, approaching a certain finite value as the load tended to zero. For isotonic load below P0, approaching a certain finite value as the load tended to zero. For isotonic loads above P0, the stiffness increased with increasing isotonic load up to 1.6-1.7 P0, when step decreases in load were used for stiffness measurements. 7. The mechanism of enhancement of mechanical performance of the fibres after quick increases in load is discussed in relation to the sliding filament/cross bridge hypotheses of muscle contraction.
