Dynamic force responses in electrically stimulated triceps surae muscles: effects of fatigue and temperature

To elicit dynamic force responses (unfused tetani) in isometric triceps surae muscles, low frequency electrical stimulation ranging from 12.5 to 30.0 Hz was applied. The fusing frequency (FF) and the relative dynamic force amplitude (DF) at the 20% and 40% maximum voluntary contraction (MVC) levels were calculated as parameters to determine effects of muscle fatigue (n = 6) and local muscle cooling. In the fatigued muscle (15 min plantar flexion at a 20% MVC level), the FF and DF increased when the fatigue was induced by voluntary contraction (FF increased from 19.6 to 22.5 Hz at 20% MVC) and also when induced by electrical stimulation (FF increased from 19.2 to 23.3 Hz). Cooling of the muscles showed an inverse effect on both parameters, indicating contractile slowing. The responsible physiological mechanisms as well as practical applications, using low frequency stimulation to monitor degenerative changes in muscles, are discussed.