Contractile properties of the human triceps surae with some observations on the effects of temperature and exercise
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The contractile properties of the triceps of five healthy male subjects (mean age 22 years) during electrically stimulated and voluntary isometric muscle contractions were investigated and some observations made on the effects of muscle heating and cooling and dynamic exercise.
The times to peak twitch tension (TPT) and half relaxation time (1/2RT) were 111±20 ms and 83±13 ms respectively. Heating and prior exercise decreased, and cooling severely prolonged, TPT and 1/2RT. Exercise and heating had no effect on supramaximal twitch tension (Pt0) but cooling the muscle to a temperature of 24.3‡ C reduced it by 52%. The effects of repetitive stimuli on Pt0 were dependent on frequency; at 0.2 Hz potentiation was observed but at 2 Hz, Pt0 was depressed under control conditions. Heating had no effect on these responses but cooling reversed the 2 Hz and abolished 0.2 Hz response. Post-tetanic potentiation of the twitch was observed under all conditions of measurement. At submaximal stimulation voltages, heating and exercise enhanced twitch and tetanic tensions, but at supramaximal voltages heating reduced tetanic tension (P0) at 10 Hz (by 115N), though not at 20 Hz. Exercise decreased P0 at both frequencies of stimulation. Cooling significantly reduced the maximal voluntary contraction and P0 at 20 Hz. At submaximal voltages, heating enhanced and cooling severely depressed tetanic tensions at high frequency (100 Hz) stimulation. A 2-min fatigue test was unaffected by heating but cooling reduced force generation at the onset of, and the decline of force during, the test.
It was concluded that studies of the contractile properties of human muscle in vivo should be based on supramaximal stimulation, and that temperature and prior exercise should be carefully standardised in order to obtain reliable and meaningful results.
Key wordsMuscle Electrical stimulation Gastrocnemius Soleus Mechanical and dynamic responses to work Thermal effects
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