Force-frequency relation. (A, B) Representative single twitches show a decrease of force amplitude and an increase of diastolic tension in both groups. (C) Relative force amplitudes at frequencies between 1 Hz and 3 Hz show a negatively shaped force-frequency relation. (D) Diastolic tension normalized to the values recorded at the lowest frequency revealed no differences between the two groups. (E) Mean values of time to 90% of relaxation do not differ significantly between the two pH groups.