Temperature effects on the force‐frequency relationship in isolated frog rectus muscles

Maximal force production in skeletal muscle is generally perceived to be thermally‐independent. Nevertheless, peak twitch tension (P T , g/g) and peak tension at sub‐tetanic stimulation frequencies (e.g., 15 Hz (P 15 , g/g)) in fast‐twitch muscle frequently decreases at higher temperatures (R 10 < 1.0), while peak tetanic tension (P 0 , g/g) remains relatively unaffected (R 10 ≈ 1.0). This study examined the contractile effects of temperature on predominantly slow‐twitch skeletal muscle. Rectus abdomini muscles (200–300 mg) from small male 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 three different incubation temperatures (10°, 20°, and 30°C), while contractile characteristics were examined via field stimulation with supramaximal square wave pulses (1 ms). Increasing temperature from 10 to 20°C caused a 22.8 ± 7.8% decrease in P T , a 48.5 ± 2.3% decrease in P 15 and an 18.6 ± 2.1% increase in P 0 (means ± SE). Corresponding thermal ratios (i.e., R 10 ) were 0.73 (P T ), 0.52 (P 15 ), and 1.19 (P 0 ). Increasing temperature from 20 to 30°C caused a 24.1 ± 3.7% decrease in P T , a 21.2 ± 3.5% decrease in P 15 , and a 4.6 ± 2.5% decrease in P 0 . Thermal ratios were 0.74, 0.78, and 0.94 for P T , P 15 , and P 0, respectively. The contraction tension‐time integral (g·s) was similarly affected. These results indicate that, at physiologically relevant stimulation frequencies, fluctuations in muscle temperature might alter force production and perhaps affect some aspects of motor control and behavior.