Abstract
The aim of this study was to investigate the effect of heating and cooling on time course of voluntary and electrically induced muscle force variation. Ten volunteers performed 50 maximal voluntary and electrically induced contractions of the knee extensors at an angle of 120 degrees under the control conditions and after passive lower body heating and cooling in the control, heating, and cooling experiments. Peak torque, torque variation, and half-relaxation time were assessed during the exercise. Passive lower body heating increased muscle and core temperatures, while cooling lowered muscle temperature, but did not affect core temperature. We observed significantly lower muscle fatigue during voluntary contraction compared with electrically induced contractions. Body heating (opposite to cooling) increased involuntarily induced muscle force, but caused greater electrically induced muscle fatigue. In the middle of the exercise, the coefficient of correlation for electrically induced muscle torque decreased significantly as compared with the beginning of the exercise, while during maximal voluntary contractions, this relation for torque remained significant until the end of the exercise. It was shown that time course of voluntary contraction was more stable than in electrically induced contractions.
Highlights
It has been established that muscle temperature changes muscle contractile properties; core temperatures can alter the activation pattern from the central nervous system (CNS)
We observed significantly lower muscle fatigue during voluntary contraction compared with electrically induced contractions
In the middle of the exercise, the coefficient of correlation for electrically induced muscle torque decreased significantly as compared with the beginning of the exercise, while during maximal voluntary contractions, this relation for torque remained significant until the end of the exercise
Summary
It has been established that muscle temperature changes muscle contractile properties; core temperatures can alter the activation pattern from the central nervous system (CNS). Force variation increases during fatigue in submaximal isometric contractions [5,6,7], but its behavior during brief electrically induced contractions (EIC) and maximal voluntary contractions (MVC) at different muscle temperatures is less clear. It was suggested that during voluntarily induced contractions, muscle force could be maintained by modulating the firing rates of active motor units, i.e., by recruiting different MUs as those initially recruited become fatigued, and/ or activating additional motor units at lower firing frequencies [8]. This recruitment strategy is no longer in operation during electrically induced muscle contractions
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