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Tetanic Force (tetanic + force)
Selected AbstractsHigh temperature does not alter fatigability in intact mouse skeletal muscle fibresTHE JOURNAL OF PHYSIOLOGY, Issue 19 2009Nicolas Place Intense activation of skeletal muscle results in fatigue development, which involves impaired function of the muscle cells resulting in weaker and slower contractions. Intense muscle activity also results in increased heat production and muscle temperature may rise by up to ,6°C. Hyperthermia is associated with impaired exercise performance in vivo and recent studies have shown contractile dysfunction and premature fatigue development in easily fatigued muscle fibres stimulated at high temperatures and these defects were attributed to oxidative stress. Here we studied whether fatigue-resistant soleus fibres stimulated at increased temperature show premature fatigue development and whether increasing the level of oxidative stress accelerates fatigue development. Intact single fibres or small bundles of soleus fibres were fatigued by 600 ms tetani given at 2 s intervals at 37°C and 43°C, which is the highest temperature the muscle would experience in vivo. Tetanic force in the unfatigued state was not significantly different at the two temperatures. With 100 fatiguing tetani, force decreased by ,15% at both temperatures; the free cytosolic [Ca2+] (assessed with indo-1) showed a similar ,10% decrease at both temperatures. The oxidative stress during fatigue at 43°C was increased by application of 10 ,m hydrogen peroxide or tert-butyl hydroperoxide and this did not cause premature fatigue development. In summary, fatigue-resistant muscle fibres do not display impaired contractility and fatigue resistance at the highest temperature that mammals, including humans, would experience in vivo. Thus, intrinsic defects in fatigue-resistant muscle fibres cannot explain the decreased physical performance at high temperatures. [source] Changes in contractile properties of motor units of the rat medial gastrocnemius muscle after spinal cord transectionEXPERIMENTAL PHYSIOLOGY, Issue 5 2006Jan Celichowski The effects of complete transection of the spinal cord at the level of Th9/10 on contractile properties of the motor units (MUs) in the rat medial gastrocnemius (MG) muscle were investigated. Our results indicate that 1 month after injury the contraction time (time-to-peak) and half-relaxation time were prolonged and the maximal tetanic force in most of the MUs in the MG muscle of spinal rats was reduced. The resistance to fatigue also decreased in most of the MUs in the MG of spinal animals. Moreover, the post-tetanic potentiation of twitches in MUs diminished after spinal cord transection. Criteria for the division of MUs into three types, namely slow (S), fast fatigue resistant (FR) and fast fatigable (FF), applied in intact animals, could not be directly used in spinal animals owing to changes in contractile properties of MUs. The ,sag' phenomenon observed in unfused tetani of fast units in intact animals essentially disappeared in spinal rats and it was only detected in few units, at low frequencies of stimulation only. Therefore, the MUs in spinal rats were classified as fast or slow on the basis of an adjusted borderline of 20 ms, instead of 18 ms as in intact animals, owing to a slightly longer contraction time of those fast motor units with the ,sag'. We conclude that all basic contractile properties of rat motor units in the medial gastrocnemius muscle are significantly changed 1 month after complete spinal cord transection, with the majority of motor units being more fatigable and slower than those of intact rats. [source] Surface Action Potential and Contractile Properties of the Human Triceps Surae Muscle: Effect of ,Dry' Water ImmersionEXPERIMENTAL PHYSIOLOGY, Issue 1 2002Yuri A. Koryak The effects of 7 days of ,dry' water immersion were investigated in six subjects. Changes in the contraction properties were studied in the triceps surae muscle. After immersion, the maximal voluntary contraction (MVC) was reduced by 18.9% (P < 0.01), and the electrically evoked (150 impulses s,1) maximal tension during tetanic contraction (Po) was reduced by 8.2% (P > 0.05). The difference between Po and MVC expressed as a percentage of Po and referred to as force deficiency was also calculated. The force deficiency increased by 44.1% (P < 0.001) after immersion. The decrease in Po was associated with increased maximal rates of tension development (7.2%) and relaxation. The twitch time-to-peak was not significantly changed, and half-relaxation and total contraction time were decreased by 5.3% and 2.8%, respectively, but the twitch tension (Pt) was not significantly changed and the Pt/Po ratio was decreased by 8.7%. The 60 s intermittent contractions (50 impulses s,1) decreased tetanic force to 57% (P < 0.05) of initial values, but force reduction was not significantly different in the two fatigue-inducing tests: fatigue index (the mean loss of force of the last five contractions, expressed as a percentage of the mean value of the first five contractions) was 36.2 ± 5.4% vs. 38.6 ± 2.8%, respectively (P > 0.05). While identical force reduction was present in the two fatigue-inducing tests, it would appear that concomitant electrical failure was considerably different. Comparison of the electrical and mechanical alterations recorded during voluntary contractions, and in contractions evoked by electrical stimulation of the motor nerve, suggests that immersion not only modifies the peripheral processes associated with contraction, but also changes central and/or neural command of the contraction. At peripheral sites, it is proposed that the intracellular processes of contraction play a role in the contractile impairment recorded during immersion. [source] Interpolated twitches in fatiguing single mouse muscle fibres: implications for the assessment of central fatigueTHE JOURNAL OF PHYSIOLOGY, Issue 11 2008Nicolas Place An electrically evoked twitch during a maximal voluntary contraction (twitch interpolation) is frequently used to assess central fatigue. In this study we used intact single muscle fibres to determine if intramuscular mechanisms could affect the force increase with the twitch interpolation technique. Intact single fibres from flexor digitorum brevis of NMRI mice were dissected and mounted in a chamber equipped with a force transducer. Free myoplasmic [Ca2+] ([Ca2+]i) was measured with the fluorescent Ca2+ indicator indo-1. Seven fibres were fatigued with repeated 70 Hz tetani until 40% initial force with an interpolated pulse evoked every fifth tetanus. Results showed that the force generated by the interpolated twitch increased throughout fatigue, being 9 ± 1% of tetanic force at the start and 19 ± 1% at the end (P < 0.001). This was not due to a larger increase in [Ca2+]i induced by the interpolated twitch during fatigue but rather to the fact that the force,[Ca2+]i relationship is sigmoidal and fibres entered a steeper part of the relationship during fatigue. In another set of experiments, we observed that repeated tetani evoked at 150 Hz resulted in more rapid fatigue development than at 70 Hz and there was a decrease in force (,sag') during contractions, which was not observed at 70 Hz. In conclusion, the extent of central fatigue is difficult to assess and it may be overestimated when using the twitch interpolation technique. [source] The Characterization of Contractile and Myoelectric Activities in Paralyzed Tibialis Anterior Post Electrically Elicited Muscle FatigueARTIFICIAL ORGANS, Issue 4 2010Nan-Ying Yu Abstract This study aimed to understand the myoelectric and mechanical characteristics of muscle recovering from electrically elicited fatigue. A modified Burke fatigue protocol was delivered to activate the tibialis anterior of 13 spinal cord injured subjects for 4 min. Before and after the fatigue protocol, a series of pulse trains was delivered to induce three twitches and a fused contraction at 0, 1, 3, and 5 min and then followed every 5 min for 60 min. The recovery processes of the ankle dorsiflexion torque and the evoked electromyography (EMG) parameters were analyzed and characterized by a first-order exponential equation. The recovery process was found to be faster in regard to tetanic muscle contraction. Factors relating to low-frequency fatigue, postfatigue potentiation, and the quickly normalized relaxation rate were taken into account for the discussion of this result. During the recovery process, the disassociation was found not only between twitch and tetanic contractions but also between mechanical and myoelectric activities. After the complete normalization of EMG parameters from about 15 min post fatigue, the tetanic force recovered incompletely to an asymptotic level. [source] | ||||||||||