Manipulation of Rest Period Length Induces Different Causes of Fatigue in Vertical Jumping

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Increase of skeletal muscle relaxation speed by direct injection of parvalbumin cDNA.

Parvalbumin (PV) is a high affinity Ca(2+)-binding protein found at high concentration in fast-contracting/relaxing skeletal muscle fibers of vertebrates. It has been proposed that PV acts in the process of muscle relaxation by facilitating Ca2+ transport from the myofibrils to the sarcoplasmic reticulum. However, on the basis of metal-binding kinetics of PV in vitro, this hypothesis has been challenged. To investigate the function of PV in skeletal muscle fibers, direct gene transfer was applied in normal and regenerating rat soleus muscles which do not synthesize detectable amounts of PV. Two weeks after in vivo transfection with PV cDNA, considerable levels of PV mRNA and protein were detected in normal muscle, and even higher amounts were detected in regenerating muscle. Twitch half-relaxation time was significantly shortened in a dose-dependent way in transfected muscles, while contraction time remained unaltered. The observed shortening of half-relaxation time is due to PV and its ability to bind Ca2+, because a mutant protein lacking Ca(2+)-binding capacity did not promote any change in physiology. These results directly demonstrate the physiological function of PV as a relaxing factor in mammalian skeletal muscle.

The effects of varying time under tension and volume load on acute neuromuscular responses

The purpose of this study was to examine the effects of different methods of measuring training volume, controlled in different ways, on selected variables that reflect acute neuromuscular responses. Eighteen resistance-trained males performed three fatiguing protocols of dynamic constant external resistance exercise, involving elbow flexors, that manipulated either time-under-tension (TUT) or volume load (VL), defined as the product of training load and repetitions. Protocol A provided a standard for TUT and VL. Protocol B involved the same VL as Protocol A but only 40% concentric TUT; Protocol C was equated to Protocol A for TUT but only involved 50% VL. Fatigue was assessed by changes in maximum voluntary isometric contraction (MVIC), interpolated doublet (ID), muscle twitch characteristics (peak twitch, time to peak twitch, 0.5 relaxation time, and mean rates of force development and twitch relaxation). All protocols produced significant changes (P

Simulation of surface EMG for the analysis of muscle activity during whole body vibratory stimulation

This study aims to reproduce the effect of motor-unit synchronization on surface EMG recordings during vibratory stimulation to highlight vibration evoked muscle activity. The authors intended to evaluate, through numerical simulations, the changes in surface EMG spectrum in muscles undergoing whole body vibration stimulation. In some specific bands, in fact, vibration induced motion artifacts are also typically present. In addition, authors meant to compare the simulated EMGs with respect to real recordings in order to discriminate the effect of synchronization of motor units discharges with vibration frequencies from motion artifacts. Computations were performed using a model derived from previous studies and modified to consider the effect of vibratory stimulus, the motor unit synchronization and the endplates-electrodes relative position on the EMG signal. Results revealed that, in particular conditions, synchronization of MUs' discharge generates visible peaks at stimulation frequency and its harmonics. However, only a part of the total power of surface EMGs might be enclosed within artifacts related bands (±1. Hz centered at the stimulation frequency and its superior harmonics) even in case of strong synchronization of motor units discharges with the vibratory stimulus. © 2013 Elsevier Ireland Ltd.

Determinação da velocidade de condução nervosa motora dos nervos radial e ulnar de cães clinicamente sadios

O presente trabalho teve como objetivo a padronização dos valores de referência de velocidade de condução nervosa motora dos nervos radial e ulnar em cães clinicamente sadios. Para tanto, foram utilizados 30 cães, 11 machos e 19 fêmeas, sem raça definida, com idade entre dois e seis anos. Os valores médios das medidas do potencial muscular produzidos por meio de estimulação proximal e distal do nervo radial foram, respectivamente: latência inicial, 2,46+0,72ms e 1,58+0,62ms, amplitude de pico a pico, 8,79+2,26mV e 9,52+2,42mV e duração, 2,85+0,76ms e 2,71+0,75ms. Os respectivos valores do nervo ulnar foram: latência inicial, 4,17+0,53ms e 2,67+0,38ms; amplitude de pico a pico, 10,72+2,60mV e 11,72+2,81mV e duração, 2,23+0,38ms e 2,04+0,35ms. Os valores médios das medidas de velocidade de condução nervosa motora dos nervos radial e ulnar foram, respectivamente, 66,18+7,26m/s e 60,50+7,86m/s.

An Isoflavone from Dipteryx alata Vogel is Active against the in Vitro Neuromuscular Paralysis of Bothrops jararacussu Snake Venom and Bothropstoxin I, and Prevents Venom-Induced Myonecrosis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Expressão de microRNAs envolvidos com o crescimento muscular do pacu (Piaractus mesopotamicus): análises in vivo e in vitro

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Comparison between electrically evoked and voluntary isometric contractions for biceps brachii muscle oxidative metabolism using near-infrared spectroscopy

This study compared voluntary (VOL) and electrically evoked isometric contractions by muscle stimulation (EMS) for changes in biceps brachii muscle oxygenation (tissue oxygenation index, ΔTOI) and total haemoglobin concentration (ΔtHb = oxygenated haemoglobin + deoxygenated haemoglobin) determined by near-infrared spectroscopy. Twelve men performed EMS with one arm followed 24 h later by VOL with the contralateral arm, consisting of 30 repeated (1-s contraction, 1-s relaxation) isometric contractions at 30% of maximal voluntary contraction (MVC) for the first 60 s, and maximal intensity contractions thereafter (MVC for VOL and maximal tolerable current at 30 Hz for EMS) until MVC decreased ∼30% of pre-exercise MVC. During the 30 contractions at 30% MVC, ΔTOI decrease was significantly (P < 0.05) greater and ∼tHb was significantly (P < 0.05) lower for EMS than VOL, suggesting that the metabolic demand for oxygen in EMS is greater than VOL at the same torque level. However, during maximal intensity contractions, although EMS torque (∼40% of VOL) was significantly (P < 0.05) lower than VOL, ΔTOI was similar and ΔtHb was significantly (P < 0.05) lower for EMS than VOL towards the end, without significant differences between the two sessions in the recovery period. It is concluded that the oxygen demand of the activated biceps brachii muscle in EMS is comparable to VOL at maximal intensity. © Springer-Verlag 2009.

Changes in muscle sympathetic nerve activity and vascular responses evoked in the spinotrapezius muscle of the rat by systemic hypoxia.

Responses evoked in muscle sympathetic nerve activity (MSNA) by systemic hypoxia have received relatively little attention. Moreover, MSNA is generally identified from firing characteristics in fibres supplying whole limbs: their actual destination is not determined. We aimed to address these limitations by using a novel preparation of spinotrapezius muscle in anaesthetised rats. By using focal recording electrodes, multi-unit and discriminated single unit activity were recorded from the surface of arterial vessels. This had cardiac- and respiratory-related activities expected of MSNA, and was increased by baroreceptor unloading, decreased by baroreceptor stimulation and abolished by autonomic ganglion blockade. Progressive, graded hypoxia (breathing sequentially 12, 10, 8% O2 for 2 min each) evoked graded increases in MSNA. In single units, mean firing frequency increased from 0.2 ± 0.04 in 21% O2 to 0.62 ± 0.14 Hz in 8% O2, while instantaneous frequencies ranged from 0.04–6 Hz in 21% O2 to 0.09–20 Hz in 8% O2. Concomitantly, arterial pressure (ABP), fell and heart rate (HR) and respiratory frequency (RF) increased progressively, while spinotrapezius vascular resistance (SVR) decreased (Spinotrapezius blood flow/ABP), indicating muscle vasodilatation. During 8% O2 for 10 min, the falls in ABP and SVR were maintained, but RF, HR and MSNA waned towards baselines from the second to the tenth minute. Thus, we directly show that MSNA increases during systemic hypoxia to an extent that is mainly determined by the increases in peripheral chemoreceptor stimulation and respiratory drive, but its vasoconstrictor effects on muscle vasculature are largely blunted by local dilator influences, despite high instantaneous frequencies in single fibres.

Histochemical differences in the responses of predominantly fast-twitch glycolytic muscle and slow-twitch oxidative muscle to veratrine

The aim of this study was to investigate if the Na+-channel activating alkaloid veratrine is able to change the oxidative and m-ATPase activities of a fast-twitch glycolytic muscle (EDL, extensor digitorum longus) and slow-twitch oxidative muscle (SOL, soleus) in mice. Oxidative fibers and glycolytic fibers were more sensitive to veratrine than oxidative-glycolytic fibers 15, 30 and 60 min after the i.m. injection of veratrine (10 ng/kg) with both showing an increase in their metabolic activity in both muscles. In EDL, the m-ATPase reaction revealed a significant (p < 0.001) decrease (50%) in the number of type IIB fibers after 30 min while the number of type I fibers increased by 550%. Type I fibers decreased from 34% in control SOL to 17% (50% decrease) in veratrinized muscles, with a 10% decrease in type IIA fibers within 15 min. A third type of fiber appeared in SOL veratrinized muscle, which accounted for 28% of the fibers. Our work gives evidence that the changes in the percentage of the fiber types induced by veratrine may be the result, at least partially, from a direct effect of veratrine on muscle fibers and else from an interaction with the muscle type influencing distinctively the response of a same fiber type. Based on the results obtained in the present study the alterations in EDL may be related to the higher number of Na+ channels present in this muscle whereas those in SOL may involve an action of veratrine on mitochondria. Although it is unlikely that the shift of enzymes activities induced by veratrine involves genotypic expression changes an alternative explanation for the findings cannot be substantiated by the present experimental approach. (C) 2002 Elsevier B.V. Ltd. All rights reserved.

PRESYNAPTIC AND POSTSYNAPTIC MECHANISMS INVOLVED IN TETANIC FADE INDUCED BY PANCURONIUM IN THE ISOLATED RAT MUSCLE

The mechanisms underlying the fade of the tetanic contraction induced by pancuronium were studied in vitro by means of myographical and electrophysiological techniques in the extensor digitorum longus muscle of the rat. Pancuronium (0.5 mu mol/l) induced a complete fade of the tetanic contraction while leaving the twitch unaffected. At the same concentration it decreased the amplitude and increased the tetanic rundown of trains of endplate potentials (e.p.ps) evoked in the frequency of 50 Hz. The electrophysiological changes induced by pancuronium were due to decreases in both quantal sizes and quantal contents of the e.p.ps. The former effect was the result of a postsynaptic competitive action and the latter of a presynaptic inhibitory action of that compound. The decrease in quantal. content affected the e.p.ps starting from the first in the train and became larger during the generation of the sequence of e.p.ps. This intensified their tetanic rundown. It is concluded that the fade of the tetanic contraction induced by pancuronium is due to a summation of pre- and postsynaptic actions and, therefore, not only to an increase in the tetanic rundown of e.p.ps. Possible explanations for the distinct abilities of neuromuscular blockers in affecting tetani and twitches in a differential manner are also discussed.

Guanidine Affects Differentially the Twitch Response of Diaphragm, Extensor Digitorum Longus and Soleus Nerve-Muscle Preparations of Mice

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