What is the true ATPase activity of contracting myofibrils?

The ATPase activity of contracting myofibrils was obtained in a solvent of ionic contents that mimic in vivo conditions: pH 7.4 and 0.1 M potassium acetate. Contracting myofibrils are fleeting structures and their ATPase activity is of short duration: even at 4 degrees C it was over after a reaction time of 1 s and rapid reaction equipment was needed for its study. The ATPase (1.1 s-1) was very similar to that of crosslinked actoS1 (1 s-1). It was 100 x that of relaxed myofibrils.