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 isometrically. Active and passive tensions were measured with the muscle's initial length varying within 10% of its physiologic length in situ. The force-frequency relationship was obtained at frequencies of stimulation of 1, 10, 15, 30, and 60 Hz. Muscle fatigability was assessed during 3 min of successive contractions. Tests were performed in pedicled muscles of anesthetized mongrel dogs (n = 10). Force-length characteristics were found to affect evoked tension markedly. Maximal active tension was generated near the muscle length in situ. Specific isometric tension measured with unit pulse stimulation was 2 ± 0.4 N/cm2, and the time to peak twitch was 92.8 ± 2.67 ms. With 60 Hz of stimulation frequency, the tension was 9.31 ± 0.32 N/cm2, and the time to peak tension was 216.05 ± 16.28 ms. After the fatigue test, the tension generated decreased to 62.5% of its initial value, and this decline was paralleled by the rate of tension development and tension relaxation.