Cooling‐effects on variable frequency train (VFT) stimulation in isolated skeletal muscle

The addition of two closely spaced stimulating pulses (doublets) at the beginning of a low‐frequency train of stimuli increases isometric tension and maximizes the force‐time integral. This study examined the effects of doublet timing and ambient temperature on subtetanic VFT potentiation. Sartorii muscles (60–70 mg) from R. pipiens were dissected free and mounted vertically in water‐jacketed chambers containing oxygenated (100%) Ringer solution (pH 7.2). After recovery, muscles were exposed serially to incubation temperatures of 10 and 20°C. Twitch characteristics were examined via field stimulation consisting of ten supramaximal square wave pulses (1 ms) delivered at 20 Hz. Average tension (AT, g/g) at 20°C was 87.4 ± 7.8 (mean ± SE). Decreasing incubation temperature to 10°C increased AT by 63.8% (143.1 ± 21.0 (10°C) vs. 87.4 ± 7.8 (20°C), n = 8, p < .05). This protocol was repeated with the exception that an extra pulse (P) was inserted between the third and fourth (P 3‐4 ) or eighth and ninth (P 8‐9 ) stimulating pulses (doublet ISI = 10 ms). At 20°C, AT in the 75 ms before and after P 3‐4 was 95.8 ± 15.3 and 140.7 ± 19.9 (Δ 46.9%, p<.05), respectively. AT before and after P 8‐9 was 100.1 ± 4.8 and 135.2 ± 16.4 (Δ 35.1%, p<.05), respectively. After cooling, AT in the 75 ms before and after P 3‐4 was 88.7 ± 16.1 and 154.2 ± 30.1 (Δ 73.8%, p<.05), respectively and after P 8‐9 was 161.8 ± 17.2 and 170.7 ± 17.2 (Δ 5.5%, p>.05), respectively. These findings indicate that doublet potentiation is thermally‐sensitive and suggest that the efficacy of paired stimulation in vivo, as a potential neural control strategy to augment force production, may be somewhat mediated by changes in muscle temperature.