EFFECTS OF MUSCLE LENGTH, FREQUENCY OF STIMULATION, AND FATIGUE ON THE ISOMETRIC TENSION IN CANINE LATISSIMUS-DORSI

An experimental protocol was designed to study the mechanical response of the canine latissimus dorsi muscle stimulated to contract isometri cally. Active and passive tensions were measured with the muscle's ini tial length varying within 10% of its physiologic length in situ. The force-frequency relationship was obtained at frequencies of stimulatio n of 1, 10, 15, 30, and 60 Hz. Muscle fatigability was assessed during 3 min of successive contractions, Tests were performed in pedicled mu scles of anesthetized mongrel dogs (n = 10). Force-length characterist ics were found to affect evoked tension markedly. Maximal active tensi on was generated near the muscle length in situ. Specific isometric te nsion measured with unit pulse stimulation was 2 +/- 0.4 N/cm(2), and the time to peak twitch was 92.8 +/- 2.67 ms. With 60 Hz of stimulatio n frequency, the tension was 9.31 +/- 0.32 N/cm(2), and the time to pe ak tension was 216.05 +/- 16.28 ms. After the fatigue test, the tensio n generated decreased to 62.5% of its initial value, and this decline was paralleled by the rate of tension development and tension relaxati on.