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

To elicit dynamic force responses (unfused tetani) in isometric triceps sur ae muscles, low frequency electrical stimulation ranging from 12.5 to 30.0 Hz was applied. The fusing frequency (FF) and the relative dynamic force am plitude (DF) at the 20% and 20% 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 b y 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, i ndicating contractile slowing. The responsible physiological mechanisms as well as practical applications, using low frequency stimulation to monitor degenerative changes in muscles, are discussed.