Proceedings of The Physiological Society
King's College London (2009) Proc Physiol Soc 14, PC7
Voluntary activation and force variation during maximal voluntary contraction depend on muscle temperature
M. Brazaitis1, A. Skurvydas1, A. Ratkevicius2, L. Daniuseviciute1,3
1. Department of Applied Physiology and Physiotherapy, Lithuanian academy of physical education, Kaunas, Lithuania. 2. School of Medical Sciences, College of Life Sciences and Medicine, Institute of Medical Sciences, University of Aberdeen, Aberdeen, Scotland, United Kingdom. 3. Department of Physical Education, Kaunas Technological University, Kaunas, Lithuania.
Fig 1. Peak torque (A) voluntary activation index (VA) (B) coefficient of variation (CV) (C) for torque and half-relaxation time (D) during the 2-min maximal voluntary contraction (MVC) in control (CON) experiment and experiments with body heating (HT) and cooling (CL) respectively. Values for time point 0 are taken from initial measurements using 5-s MVC efforts in these experiments. Data are means ± SEM.
It is well known that high environmental temperatures are associated with impaired exercise performance and accelerated muscle fatigue (Nybo & Nielsen 2001). However, the mechanism of this phenomenon is not entirely clear. In general, exercise-induced fatigue can be due to limitations in the skeletal muscles or in the nervous system (Bigland-Ritchie et al 1986; Ratkevicius et al 1998; Streckis et al 2007). It is possible that muscular temperature plays a special role in voluntary activation of skeletal muscles. Muscle relaxation accelerates at high temperatures and could impair force generation as higher motor unit (MU) firing rates will be required to produce the same force compared to exercise at lower muscle temperatures (Todd et al 2005). However, there is little evidence to support this hypothesis and we decided to examine it in greater detail. However, force variation has not been studied during MVC at different muscle temperatures. Impairment in voluntary activation is observed during a continuous MVC (Streckis et al 2007). It can be hypothesized that force variation will be elevated during MVC at high muscle temperatures as ability to maintain voluntary activation of skeletal muscles becomes compromised. On the other hand, force variation should decrease in pre-cooled muscles as their relaxation rate would decrease facilitating force maintenance during continuous contractions. The aim was to investigate if voluntary activation (VA) and force variation during maximal voluntary contraction (MVC) depend on muscle temperature. Ten volunteers performed a 2-min MVC of the knee extensors under the control conditions as well as after the muscle heating and cooling in CON, HT and CL experiments, respectively. Peak torque, torque variation as well as muscle voluntary activation and half-relaxation time (HRT) were assessed during the exercise. Lower body heating increased muscle and core temperatures while cooling lowered muscle temperature, but did not affect core temperature. At 30-s MVC, peak torque was lower in HT compared to CON and CL experiments (52.6 ± 2.3 vs 69.0 ± 2.3 and 65.6 ± 1.9% of initial MVC, respectively, P < 0.001)(Figure 1A). From 30-s to 2-min MVC, torque remained lower, torque variation (Figure 1C) larger and VA more depressed (P < 0.01) (Figure 1B) in HT compared to CON and CL experiments. In CL experiment, VA was higher compared to CON experiment when assessed from 60-s to 2-min MVC which corresponded to a significant slowing in HRT (Figure 1D) of the cooled muscles. It is concluded that muscle heating impairs VA of the exercising muscles while cooling has an opposite effect. This was at least partially mediated by changes in muscle relaxation rate.
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