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PubMed1978

Influence of posture on isometric fatigue.

The isometric strength of four trained subjects was unaltered by changes in posture. But the endurance of an isometric contraction held to fatigue at 25 and 40% of the maximum voluntary contraction (MVC) was 20% greater in the sitting than in the recumbent posture. This difference was abolished when the exercise was performed with the arm's circulation arrested. At rest, the blood flow through the forearm was greater when the subjects were in the recumbent than in the sitting position but the reverse was true during isometric contractions. In these two postures, there was no difference in the right atrial pressure during the contraction, suggesting that the low-pressure baroreceptors are not responsible for the differences in blood flow during exercise. To date no mechanism is available to explain these observations.

Adult↗

PubMed1991

Differential effects of fatty acids on glycolysis and glycogen metabolism in vascular smooth muscle.

The effects of fatty acids of different chain lengths on aerobic glycolysis, lactic acid production, glycogen metabolism and contractile function of vascular smooth muscle were investigated. Porcine carotid artery segments were treated with 50 microM iodoacetate and perchloric acid tissue extracts were then analyzed by 31P-NMR spectroscopy to observe the accumulation of phosphorylated glycolytic intermediates so that the activity of the Embden-Myerhof pathway could be tracked under various experimental paradigms. Aerobic glycolysis and lactate production in resting arteries were almost completely inhibited with 0.5 mM octanoate, partially inhibited with 0.5 mM acetate and unaffected by 0.5 mM palmitate. Inhibition of glycolysis by octanoate was not attributable to inhibition of glucose uptake or glucose phosphorylation. Basal glycogen synthesis was unchanged with palmitate and acetate, but was inhibited by 52% with octanoate incubation. The characteristic glycogenolysis which occurs upon isometric contraction with 80 mM KCl in the absence of fatty acid in the medium was not demonstrable in the presence of any of the fatty acids tested. Glycogen sparing was also demonstrable in norepinephrine contractions with octanoate and acetate, but not with palmitate. Additionally, norepinephrine-stimulated isometric contraction was associated with enhanced synthesis of glycogen amounting to 6-times the basal rate in medium containing octanoate. Contractile responses to norepinephrine were attenuated by 20% in media containing fatty acids. Thus, fatty acids significantly alter metabolism and contractility of vascular smooth muscle. Fatty acids of different chain lengths affect smooth muscle differentially; the pattern of substrate utilization during contraction depends on the contractile agonist and the fatty acid present in the medium.

Animals↗

PubMed2002

Between-days reliability of electromyographic measures of paraspinal muscle fatigue at 40, 50 and 60% levels of maximal voluntary contractile force.

OBJECTIVE: To ascertain which percentage of maximal voluntary contractile force of the paraspinal muscles, when tested in a functional position, is most reliable for assessing electromyographic (EMG) fatigue changes. SUBJECTS: Ten healthy volunteers with no history of low back pain (six males). MAIN OUTCOME MEASURES: The surface EMG signal during 60-second isometric contractions of the paraspinal muscles at 40, 50 and 60% levels of maximal voluntary contractile force was captured and analysed. Each contraction level was assessed on two occasions, at least three days apart. The initial median frequency, the decline in median frequency slope and the increase in root mean square values were assessed for between-days reliability, using intraclass correlation coefficients (ICCs) and standard errors of measurements (SEM). Normalized median frequency and root mean square values were also assessed. RESULTS: At 40% of maximal voluntary contraction, little or no EMG fatigue changes occurred in any of the observed parameters. At 50% maximal voluntary contraction the initial mean frequency and root mean square changes proved highly reliable, with ICCs ranging from 0.74 to 0.86 and 0.75 to 1.00 respectively. Normalizing the root mean square data reduced the reliability, but this was still acceptable with ICCs 0.70-0.83. The median frequency decline slope proved less reliable with ICCs 0.24-0.74 for raw and 0.26-0.77 for normalized data. At 60% maximal voluntary contraction the initial mean frequency proved as reliable as initial median frequency at 50% with ICCs 0.70-0.89. The raw and normalized root mean squares (ICCs 0.43-0.89 and 0.30-0.87 respectively) and raw and normalized median frequency (ICCs 0.27-0.51 and 0.24-0.53 respectively) changes were less reliable than at 50% MVC. Overall, the reliability is better at the L4/5 than at the L2/3 level. CONCLUSION: Outcome measures taken at 50% maximal voluntary contraction are the most reliable in functional testing the paraspinal muscles of healthy volunteers. With initial median frequency and root mean square values being more reliable parameters than median frequency decline. At the L4/5 level, however, all parameters were acceptably reliable at 50% of maximum effort. However the between-subject variability of the median frequency decline and root mean square incline slopes suggest that these parameters are not yet fully suitable for monitoring fatigue changes during prolonged isometric contraction.

Adult↗

PubMed1980

Mechanical and electrical correlates of isometric muscle fatigue in skeletal muscle in the cat.

The amplitude and duration of motor unit action potentials, the rise time, peak tension, and half relaxation time of an isometric twitch, the force-velocity relationship, and tetanic tension were measured at the beginning (fresh muscle) and at the end of a fatiguing isometric contraction at a tension of either 40 or 70% of the initial strength in the soleus (a slow twitch muscle) and the medial gastrocnemius (a fast twitch muscle) of the cat. These same parameters were also measured at set intervals following these contractions to assess their rate of recovery to pre-exercise values. At The end of a fatiguing contraction at either tension examined, Vmx, twitch tension and tetanic tension, were all reduced while there was a prolonged twitch duration and duration of the motor unit action potential for both types of muscle. The height of the motor unit action potential was only marginally effected by muscle fatigue. Following the fatiguing contraction, the endurance required several hours to recover in the medial gastrocnemius muscle but recovered fully within 15 min after either tension in the soleus muscle. Tetanic tension and twitch tension both required less than 10 min for full recovery in the medial gastrocnemius muscle but recovered fully to the pre-exercise values within 3 min following fatiguing isometric contractions in the soleus muscles. Vmx, and the height and duration of the motor unit action potential both recovered within 1 min following the end of the exercise.

Animals↗

PubMed1988

The effect of verapamil on potentiated rest contractions in the rat ventricular myocardium.

The effects of verapamil (0.3 to 30 microM) on potentiated rest contractions (PRC) were studied in isometric contractions of left ventricular papillary muscles of the rat. The post-rest potentiation was studied after rest periods of 5, 10, 15, 30, 60 and 120 s. Verapamil reduced the steady-state force and PRC in a dose-dependent manner. However, PRC were less depressed and the magnitude of this depression decreased as the rest period increased. The time constant of the recovery of steady state force after rest potentiation was shortened. These results indicate that the first PRC are less dependent on calcium entering the cell through the slow channels and apparently more dependent on an intracellular calcium store.

Animals↗

PubMed2000

Geometrical factors in surface EMG of the vastus medialis and lateralis muscles.

Surface EMG signals detected in dynamic conditions are affected by a number of artefacts. Among them geometrical factors play an important role. During movement the muscle slides with respect to the skin because of the variation of its length. Such a shift can considerably modify sEMG amplitude. The purpose of this work is to assess geometrical artefacts on sEMG during isometric contractions at different muscle lengths. The average rectified value (ARV) of 15 single differential signals was obtained by means of a linear array of 16 bar electrodes from the vastus medialis and lateralis muscles. The knee angle was changed from 75 degrees to 165 degrees in steps of 30 degrees and voluntary isometric contractions at a low, medium and high force level were performed for each angle. The ARV pattern was normalized with respect to the mean activity to compare signals from different joint angles. From the data collected it was possible to separate the geometrical changes from the changes due to different intensities of activation. In three out of five subjects, we found (within the resolution of our measures) a 1 cm shift for the vastus medialis muscle while no shift was observed for the other two subjects. For the vastus lateralis muscle a 1 cm shift was found in two out of four subjects. Such a shift produces the main contribution to geometrical artefacts. To avoid such artefacts the innervation zones should be located and the EMG electrodes should not be placed near them.

Adult↗

PubMed2004

Effects of experimental muscle pain on mechanical properties of single motor units in human masseter.

OBJECTIVE: Muscle pain is known to influence muscle activity but the details of its effects on the mechanical properties of single motor units (SMU) have not been described. We have recently reported a decreased firing rate of SMU in the human masseter muscle during painful contractions with a constant force output. Force output can be modulated by the SMU discharge rate in relation to the contractile properties of SMU. Therefore, the objective of the present study was to measure the mechanical properties of SMU in the masseter to clarify the mechanism which underlies the decrease in SMU firing rate during jaw-muscle pain. METHODS: A spike-triggered averaging (STA) technique was used to determine the mechanical properties of low-threshold SMU in the masseter muscle recorded with fine wire electrodes during a voluntary isometric contraction. The twitch amplitude, contraction time, and half-relaxation time were determined from the averaged force records before and during experimental jaw-muscle pain induced by injection of 0.2 ml (100 microg/ml) capsaicin in 8 healthy subjects. Injections of 0.2 ml isotonic saline served as a non-painful control in 11 healthy subjects. RESULTS: The twitch amplitude was significantly increased during capsaicin-evoked muscle pain (P<0.001) without significant changes of half-relaxation time and contraction time. No significant changes in SMU twitch properties were observed during the control injections. CONCLUSIONS: Potentiation of twitch force could be a possible compensatory mechanism to maintain a constant force output during painful isometric contractions when SMU firing decreases. This finding therefore provides new information on the adaptation of motor function by muscle pain.

Adult↗

PubMed1985

Electrical stimulation of smooth muscle strips from the urinary bladder of the pig.

Strips of smooth muscle from pig urinary bladders were electrically stimulated to contract. Stimulation parameters and conditions were optimized so as to obtain a maximum number of isometric contractions with maximal force. It was found that the contractions could be described mathematically by a simple model. In the model there is a constant probability for cells to pass from the non-contractile to the contractile state during stimulation; this leads to a linearly decreasing phase plot (a plot of the rate of rise of a variable as a function of the variable) for the force. 'Activation' of the cells is described by a physical step function. Isometric contractions were thus characterized by a set of three parameters: U, the time derivative of the force, extrapolated to zero force, Fiso the value of the isometric force which is approached asymptotically after infinitely long periods of stimulation and t1, the activation time. The sensitivity of these three parameters to variation of the stimulus parameters was investigated. It was found that the parameter U was consistently correlated with the stimulus parameters, suggesting that this parameter can be used to describe the effectiveness of electrical stimulation of such strips.

Animals↗

PubMed1985

Angle of active site of myosin heads in contracting muscle during sudden length changes.

The change in orientation of myosin crossbridges in contracting muscle during sudden length changes was examined by fluorescence polarization. This study used a fluorescent ATP analogue, 1,N6-etheno-2-aza-ATP(epsilon-2-aza-ATP) as a probe. Its fluorescence is considerably enhanced upon binding with myosin and is dependent on the chemical state of the myosin-nucleotide complex in muscle. The results showed that nucleotides bound to crossbridges in the intermediate attached state (presumably AM-epsilon-2-aza-ADP-Pi) during isometric contraction are highly oriented at the same angle as that of AM in rigor with bound epsilon-2-aza-ADP. Furthermore the orientation of nucleotides bound to crossbridges in the attached state is not altered during sudden changes in length of isometrically contracting muscle. The results of this time-resolved measurement support the conclusion obtained from a previous steady-state experiment that change in axial orientation of the active site of the myosin head is not involved in force generation.

Animals↗

PubMed2002

Prevention of histopathologic changes from 30 repeated stretches of active rat skeletal muscles by long inter-stretch rest times.

This study was performed to examine the protective effect of a long rest between stretches of activated rat plantar flexor muscles on immediate functional and delayed structural signs of injury. Plantar flexor muscles activated by nerve stimulation by ankle rotation from 1.57 to 0.70 rad under anesthesia (brevital) were stretched 30 times. Three groups of female Sprague Dawley rats were tested: (1) long inter-stretch (180 s, S180), (2) short inter-stretch (40 s, S40), and (3) short inter-isometric contraction (40 s, IC40) times. Isometric and peak stretch forces during the stretch protocols and force-frequency relationships before and 1 h after the stretching protocols were measured. Histopathologic changes were evaluated in medial gastrocnemius muscle samples using antibodies to cellular (actin, desmin, dystrophin, fast myosin) and extracellular (laminin, albumin, complement C3) proteins and markers for neutrophils (W3/13) and macrophages (ED1). Both stretch protocols produced the same force deficits [mean (SEM)] [S40 56.7 (2.1)%, S180 51.3 (3.7)%] with little recovery within 1 h. After 2 days, histopathologic changes were present only in samples from the S40 protocol including desmin negative fibers [162 (32)] which stained for albumin, complement C3, laminin, actin and fast myosin. Many of the necrotic fibers also contained infiltrating neutrophils and macrophages. In summary, histopathologic changes in rat gastrocnemius muscle following repeated stretch injury was prevented by long inter-stretch rests.

Animals↗

PubMed2005

Effects of cold and hot water immersion on the mechanical properties of human muscle and tendon in vivo.

BACKGROUND: Cooling and heating have been shown to affect the contractile properties of muscles. However, the reasons for these changes remain unclear. The present study aimed to quantify the mechanical properties of muscle and tendon during passive stretch and active contraction, and to investigate the effects of cooling and heating on the mechanical properties of muscle and tendon. METHODS: Before and after these conditions, the elongation of the muscle fascicle, tendon and aponeurosis of the medial gastrocnemius muscle was directly measured by ultrasonography, while the ankle joint was passively moved within the joint range of +15 to -30 deg (0 deg = neutral anatomic position; positive values for plantar flexion) and subjects performed ramp isometric plantar flexion up to the voluntary maximum. FINDINGS: While the muscle fascicle, tendon and aponeurosis stretched during passive dorsi-flexion, the elongation of the tendon was significantly greater than that of the aponeurosis. During isometric contraction, the maximal elongation of the tendon was significantly greater than that of the aponeurosis. After cooling and heating, no significant changes in the elongation of muscle fascicle, tendon and aponeurosis were found during passive stretch. Similarly, after both the immersions there were no changes in the relationship between the estimated muscle force and elongation of each structure (tendon-aponeurosis complex, tendon) during isometric contraction. INTERPRETATION: These results implied that the general application of icing and hot pack did not change the mechanical properties of muscle and tendon.

Adult↗

PubMed1993

Kinetics of binding and hydrolysis of a series of nucleoside triphosphates by actomyosin-S1. Relationship between solution rate constants and properties of muscle fibers.

We have measured the steady state kinetics of hydrolysis and presteady state kinetics of binding of the nucleoside triphosphate GTP, CTP, aza-ATP (1-N6-etheno-2-aza-ATP), and ATP by rabbit skeletal actomyosin-S1. The maximum rates of steady state hydrolysis at 10 degrees C at low ionic strength are: CTP, 1.9 s-1 > ATP, 1.3 s-1 > aza-ATP, 0.19 s-1 > GTP, 0.03 s-1. A similar dependence of the rate of steady state hydrolysis upon nucleotide structure has been observed in isometrically contracting muscle fibers in the accompanying paper (Pate, E., Franks-Skiba, K., White, H., and Cooke, R. (1993) J. Biol. Chem. 268, 10046-10053) which strongly suggests that the same biochemical step that limits the maximum rate of hydrolysis of nucleoside triphosphates by actomyosin-S1 in solution also limits the rate of hydrolysis by isometrically contracting muscle fibers. The apparent second order rate constants for the dissociation of actomyosin-S1 by nucleoside triphosphates at 10 degrees C are: ATP, 2.7 x 10(6) M-1 s-1 > aza-ATP, 3.4 x 10(5) M-1 s-1 > GTP, 2.5 x 10(5) M-1 s-1 > CTP, 1.4 x 10(5) M-1 s-1. There is an excellent correlation between the second order rate constant for the dissociation of actomyosin-S1 in solution and the dependence of shortening velocity in glycerinated muscle fibers upon the concentration for ATP, aza-ATP, and CTP (as per accompanying article; Pate et al., 1993). We have used the second order rate constants obtained in solution for the dissociation of actomyosin-S1 by these nucleotides and shortening velocity data obtained with the same nucleoside triphosphates in glycerinated psoas fibers in the accompanying article (Pate et al., 1993) to determine the average distance over which cross-bridges remain attached during unloaded shortening to be 5-12 nm.

Actins↗

PubMed1997

Effect of ingested sodium bicarbonate on muscle force, fatigue, and recovery.

The influence of acute ingestion of NaHCO3 on fatigue and recovery of teh quadriceps femoris muscle after exercise was studied in six healthy male subjects. A bicycle ergometer was used for exercising under three loading conditions: test A, load corresponding to maximal oxygen consumption; test B, load in test A + 17%; test C, load in test B but performed 1 h after acute ingestion of NaHCO3. Functional electrical stimulation (FES) was applied to provoke isometric contraction of the quadriceps femoris. The resulting knee torque was monitored during fatigue (2-min chronic FES) and recovery (10-s FES every 10 min, for 40 min). Quadriceps torques were higher in the presence of NaHCO3 (P < 0.05): with NaHCO3 the peak, residual, and recovery (after 40 min) normalized torques were, respectively, 0.68 +/- 0.05 (SD), 0.58 +/- 0.05, and 0.73 +/- 0.05; without NaHCO3 the values were 0.45 +/- 0.04, 0.30 +/- 0.06, and 0.63 +/- 0.06. The increased torques obtained after acute ingestion of NaHCO3 indicate the possible existence of improved nonoxidative glycolysis in isometric contraction, resulting in reduced fatigue and enhanced recovery.

Administration, Oral↗

PubMed1996

Mechanical and physical responses to stretching with and without preisometric contraction in human skeletal muscle.

OBJECTIVE: To examine electromyography (EMG) activity, passive torque, and stretch perception during static stretch and contract-relax stretch. DESIGN: Two separate randomized crossover protocols: (1) a constant angle protocol on the right side, and (2) a variable angle protocol on the left side. SUBJECTS: 10 male volunteers. INTERVENTION: Stretch-induced mechanical response in the hamstring muscles during passive knee extension was measured as knee flexion torque (Nm) while hamstring surface EMG was measured. Final position was determined by extending the knee to an angle that provoked a sensation similar to a stretch maneuver. Constant angle stretch: The knee was extended to 10 degree below final position, held 10sec, then extended to the final position and held for 80 sec. Variable angle stretch: The knee was extended from the starting position to 10 degrees below the final position, held 10sec, then extended to the onset of pain. Subjects produced a 6-sec isometric contraction with the hamstring muscles 10 degrees below the final position in the contract-relax stretch, but not in the static stretch. MAIN OUTCOME MEASURES: Passive torque, joint range of motion, velocity, and hamstring EMG were continuously recorded. RESULTS: Constant angle contract-relax and static stretch did not differ in passive torque or EMG response. In the final position, passive torque declined 18% to 21% in both contract-relax and static stretch (p<.001), while EMG activity was unchanged. In the variable angle protocol, maximal joint angle and corresponding passive torque were significantly greater in contract-relax compared with static stretch(p<.01), while EMG did not differ. CONCLUSION: At a constant angle the viscoelastic and EMG response was unaffected by the isometric contraction. The variable angle protocol demonstrated that PNF stretching altered stretch perception.

Adult↗

PubMed1996

Effect of a brief stretch on time course of shortening after a quick release in guinea pig tenea coli.

To examine the origin of velocity deceleration in the shortening of smooth muscle, guinea pig tenea coli which contracts isometrically in a high-potassium solution was released quickly (1st release), restretched to the original length, kept at this length for 0.06-0.2s and quickly released again (2nd release). The muscle shortened isotonically under the same very-light load after the 1st and 2nd releases. The length change of the muscle during shortening was examined using a video camera. The rapid shortening phase, seen after the 1st release, which followed the instantaneous, elastic recoil and lasted for about 0.3s, disappeared after the 2nd release. This effect was prominent when the stretch duration was short. The time course of the slow phase of shortening subsequent to the rapid one was not affected by stretching. These results suggest that the rapid shortening phase is due largely to adjust of the cross-bridges between the thick and thin filaments to the change from an isometric to an isotonic contraction, while the slow phase reflects a steady turnover in the cross-bridges. The velocity of the slow shortening phase reached a maximum before the isometric force peaked and decreased as the activation level was lowered by reducing the external Ca2+ concentration from 2.5 to 0.5 mM. This suggests that the steady turnover rate of the cross-bridges depends on the phase of contraction and level of activation. The cause of the gradual velocity decrease in the slow phase of shortening was discussed.

Animals↗

PubMed2002

Force enhancement following stretching of skeletal muscle: a new mechanism.

We investigated force enhancement following stretching in the in situ cat soleus muscle on the ascending and descending limb of the force-length relationship by varying the amount and speed of stretching and the frequency of activation (5 Hz, 30 Hz). There was a small but consistent (P<0.05) amount of force enhancement following muscle stretching on the ascending limb of the force-length relationship for both stimulation frequencies. The steady-state active isometric forces following stretches of 9 mm on the descending limb of the force-length relationship were always equal to or greater than the corresponding forces from the purely isometric contractions at the length at which the stretch was started. Therefore, force production for these trials showed positive stiffness and was associated with stable behavior. Following active stretching of cat soleus on the descending limb of the force-length relationship, the passive forces at the end of the test were significantly greater than the corresponding passive forces for purely isometric contractions, or the passive forces following stretching of the passive muscle. This passive force enhancement following active stretching increased with increasing magnitude of stretch, was not associated with structural damage, and only disappeared once the muscle was shortened. For stretches of 6 mm and 9 mm, the passive force enhancement accounted for more than 50 % of the total force enhancement, reaching a peak contribution of 83.7 % for the stretches of 9 mm at a speed of 3 mm s(-1). The results of this study suggest that a passive structural element provides a great part of the force enhancement on the descending limb of the force-length relationship of the cat soleus. Furthermore, the results indicate that mechanisms other than sarcomere length non-uniformity alone are operative.

Animals↗

PubMed1995

Modulation of the human nociceptive reflex by cyclic movements.

During static conditions the nociceptive reflex is known to vary as a function of, for example, the stimulus position, stimulus intensity, and muscle contraction. The aim of the present human study was to investigate whether the reflex and the corresponding perception of pain are modulated by cyclic movements of the limb involved. Reflexes, evoked by nociceptive electric stimulation of the sural nerve, were recorded from the biceps femoris and the rectus femoris muscles in eight volunteers. Four different experiments were performed to compare the nociceptive reflex and pain score elicited during active isometric/dynamic flexion/extension of the knee joint. The amplitudes of the reflexes were largest for the dynamic conditions. The reflexes, evoked during dynamic extension and isometric contraction of the rectus femoris muscle, had the shortest latencies but the recordings from the biceps femoris muscle were larger than from the rectus femoris muscle. Knee joint angle recordings showed that the largest angle variations occurred for the dynamic conditions and were only marginally disturbed for the isometric conditions. A given stimulus intensity evoked the highest pain intensity during isometric contractions. This indicates that there would seem to be no causal relationship between the size of the nociceptive reflex and the pain intensity.

Adult↗

PubMed2001

Conformation of myosin interdomain interactions during contraction: deductions from muscle fibers using polarized fluorescence.

Myosin cross-bridge subfragment 1 (S1) is the ATP catalyzing motor protein in muscle. It consists of three domains that catalyze ATP and bind actin (catalytic), conduct energy transduction (converter), and transport the load (lever arm). Force development during contraction is thought to result from rotary lever arm movement with the cross-bridge attached to actin. To elucidate cross-bridge structure during force development, two crystal structures of S1 were extrapolated to working "in solution" or oriented "in tissue" forms, using structure-sensitive optical spectroscopic signals from two extrinsic probes. The probes were located at two interfaces containing the catalytic, converter, and lever arm domains of S1. Observed signals included circular dichroism (CD) and absorption originating from S1 in solution in the presence and absence of actin and fluorescence polarization from cross-bridges in muscle fibers. Theoretical signals were calculated from S1 crystal structure models perturbed with lever arm movement from swiveling at three conserved glycines, 699, 703, and 710 (chicken skeletal myosin numbering). Best agreement between the computed and observed signals gave structures showing that actin binding to S1 causes movement of the lever arm. A three-state model of S1 conformation during contraction consists of three actin-bound cross-bridge states observed from muscle fibers in isometric contraction, in the presence of MgADP, and in rigor. Structures best representing these states show that most of the lever arm rotation occurs between isometric contraction and the MgADP states, i.e., during phosphate release. Smaller but significant lever arm rotation occurs with ADP dissociation. Structural changes within the S1 interfaces studied are discussed in the accompanying paper [Burghardt et al. (2001) Biochemistry 40, 4834-4843].

Animals↗