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At least 973 records · Page 54Linked to original sources

PubMed1992

Time course of rise of muscle stiffness at onset of contraction induced by photorelease of ATP.

Isometric contraction (9-10 degrees C) of skinned fibres from rat psoas muscle was elicited by photorelease of ATP (1.3-1.4 mM), and force (F), in-phase (I) and quadrature (Q) stiffness were monitored. The sinusoidal length change for the stiffness measurement was approximately 0.09% fibre length and 200-1000 Hz. On photolysis, F and I initially fell, and then rose. The half-time of the rise of I was shorter than that of F. Contrary to the previous assumption, Q rose even earlier than I. Although the time courses of the rise of F and I were greatly affected by the presence of phosphate ion (Pi; 4 mM), its effect on Q was modest. We propose the following hypothesis: the rigor crossbridges, after binding ATP, initially enter a state characterized by high Q with low I and none or little F, and then, releasing Pi during passing the state with the increased I, reach the final force generating state.

Adenosine Triphosphate↗

PubMed2006

Knee extensor muscle oxygen consumption in relation to muscle activation.

Recently, fatigability and muscle oxygen consumption (mVO(2)) during sustained isometric contractions were found to be less at shorter (30 degrees knee angle; 0 degrees = full extension) compared to longer knee extensor muscle lengths (90 degrees ) and, at low torques, less in the rectus femoris (RF) muscle than in the vastus lateralis and medialis. In the present study we hypothesized that these findings could be accounted for by a knee angle- and a muscle-dependent activation respectively. On two experimental days rectified surface EMG (rsEMG) was obtained as a measure of muscle activation in nine healthy young males. In addition, on day 1 maximal torque capacity (MTC) was carefully determined using superimposed nerve stimulation on brief high intensity contractions (> 70%MVC) at 30, 60 and 90 degrees knee angles. On day 2, subjects performed longer lasting isometric contractions (10-70%MTC) while mVO(2) was measured using near-infrared spectroscopy (NIRS). At 30 degrees , maximal mVO(2) was reached significantly later (11.0 s +/- 6.5 s) and was 57.9 +/- 8.3% less (average +/- SD, across intensities and muscles) than mVO(2) at 60 and 90 degrees (p < 0.05). However, rsEMG was on average only 18.0 +/- 11.8% (p = 0.062) less at the start of the contraction at 30 degrees . At 10%MTC at all knee angles, maximal mVO(2) of the RF occurred significantly later (28.8 +/- 36.0 s) and showed a significantly smaller increase in rsEMG compared to both vasti. In conclusion, it is unlikely that the tendency for less intense muscle activation could fully account for the approximately 60% lower oxygen consumption at 30 degrees , but the later increase in RFmVO(2) seemed to be caused by a less strong activation of the RF.

Adult↗

PubMed1997

Sarcomere dynamics and contraction-induced injury to maximally activated single muscle fibres from soleus muscles of rats.

1. The focal nature of contraction-induced injury to skeletal muscle fibres may arise from heterogeneities in sarcomere length that develop during contractions. We tested the hypothesis that when a maximally activated single permeabilized fibre segment is stretched and a deficit in maximum isometric force (force deficit) is produced, the regions of sarcomeres with the longest lengths of prior to the stretch contain the majority of the damaged sarcomeres when the fibre is returned to optimum length (Lo) after the stretch. 2. Single fibre segments (n = 16) were obtained from soleus muscles of rats. Average sarcomere length at five discrete positions along the length of each fibre was determined by lateral deflection of a diode laser spot. Diffraction patterns were obtained while fibres were relaxed and immediately before, during and after a single stretch of 40% strain relative to Lo. Following the stretch, the regions of each fibre that potentially contained damaged sarcomeres were identified by an increased scatter of the first-order diffraction patterns. The damage was confirmed by light and electron microscopy. 3. While single fibre segments were in relaxing solution, the mean value for all of the average sarcomere lengths sampled (n = 80) was 2.53 +/- 0.01 microns (range, 2.40-2.68 microns). During the maximum isometric contraction before each stretch, the mean sarcomere length decreased to 2.42 +/- 0.02 microns and the range increased to 2.12-3.01 microns. 4. During the stretch of 40% strain, all regions of sarcomeres were stretched onto the descending limb of the length-force curve, but sarcomere lengthening was non-uniform. After the stretch, when the maximally activated fibres were returned to Lo, the force deficit was 10 +/- 1%. Microscopic evaluation confirmed that the regions with the longest sarcomere lengths before the stretch contained the majority of the damaged sarcomeres after the stretch. We conclude that when heterogeneities in sarcomere length develop in single permeabilized fibre segments during a maximum isometric contraction, the sarcomeres in the regions with the longest lengths are the most susceptible to contraction-induced injury.

Animals↗

PubMed1995

Twitch response of striated muscle in patients with progressive external ophthalmoplegia, mitochondrial myopathy and focal cytochrome c-oxidase deficiency.

Some aspects of the contractile properties of skeletal muscle in patients with progressive external ophthalmoplegia (PEO), mitochondrial myopathy and focal cytochrome c-oxidase deficiency were investigated by studying the twitch response (TR) of the tibialis anterior muscle both at rest and after maximum isometric contraction. The results of needle electromyography were normal in four of the six examined patients, and myopathic in the remaining two. A slowing in muscle relaxation was the most frequently observed abnormality; significantly prolonged muscle contraction times and reduced twitch torque potentiation values after isometric contraction were also detected. TR abnormalities in PEO patients may be due either to a dysfunction of the contractile machinery depending upon impaired muscle energy supply or to altered muscle fiber characterized by the predominance of type I slow fiber. In addition to conventional electromyographic investigations, TR study may be a useful diagnostic tool in PEO patients.

Adult↗

PubMed1977

Effect of isometric handgrip exercise on left ventricular function in the patient with artificial cardiac pacemaker.

The cardiovascular effects of isometric handgrip exercise were determined with a nonivasive method in the patient with artificial pacemaker. The arterial blood pressure was measured with mercury manometer as well as cardiac output by the dye dilution method with Indocyanin green before and after handgrip exercise. Isometric handgrip exercise produced a significant increase in the left ventricular ejection time, the left ventricular ejection time/the preejection period, the ejection time index/the isometric contraction time, the arterial blood pressure and cardiac output and a significant decrease in the preejection time and the isometric contraction time. Cardiac output was increased more remarkably in the negative Master two step test group than in the positive Master two step test group. There was found a significant positive relationship between the increase in cardiac output and P wave rate upon handgrip exercise.

Adolescent↗

PubMed1986

The effect of length range on heat rate and power during shortening near in situ length in frog muscle.

Heat rate and tension were measured during the steady state of isovelocity shortening in frog sartorius muscles at two speeds (7.5 and 2.0 mm s-1) through two ranges of the length-tension curve: 1.11 to 1.01 L0 and 1.01 to 0.91 L0. Both heat rate and mechanical power were higher at short length than at long length. The ratio of mechanical power to total energy rate was smaller at short length. The difference in heat rates in the two length ranges was greatest during shortening at 7.5 mm s-1 and least during isometric contractions. Calculations were made for two extreme interpretations of the results, based on the assumption that 30% of the isometric maintenance heat rate at optimum length is produced by processes related to activation: that all the difference in heat rate results from effects of length on activation processes and that all the difference results from effects on contractile processes. The rate of heat production by activation processes would have to be 1.5 times higher at short than at long length in an isometric contraction, 1.7 times higher during shortening at 2mm s-1 and 2.0 times higher at 7.5 mm s-1 to explain the differences in heat rate. The rate of contractile processes would have to be only 1.2 times higher at short than at long length. The results favour the interpretation that the contractile process itself is sensitive to sarcomere length near the plateau of the length-tension curve.(ABSTRACT TRUNCATED AT 250 WORDS)

Actomyosin↗

PubMed2004

An EMG fractal indicator having different sensitivities to changes in force and muscle fatigue during voluntary static muscle contractions.

During a sustained contraction, electromyographic signals (EMGs) undergo a spectral compression. This fatigue behaviour induces a shift of the mean and the median frequencies to lower frequencies. On the other hand, several studies conclude that the mean/median frequency can increase, decrease or remain constant with an increasing force level. Such inconsistency is embarrassing since the fatigue state may be influenced by the force level. In this paper, we propose a frequency indicator which is sensitive to the force level independently of the fatigue state evaluated at 70% of the maximal voluntary contraction. Ten healthy volunteers participated in the study and both surface EMGs (from the short head of the biceps brachii) and force signals were measured. This study compared force and fatigue effects on the EMGs during short (3-s) isometric contractions at different strength intensities and during a sustained isometric contraction until exhaustion. The EMGs partly show 1/falpha spectral behaviours since their power spectral densities may experimentally fit with two linear segments in a log-log representation. The measured "right" slope produces variations of force as 20 times the variations of fatigue. 1/falpha Behaviour may be related to stochastic fractals. This fractal indicator is a new frequency indicator that is thus complementary to other known classical frequency indicators when studying force during unknown fatigue states.

Adult↗

PubMed2002

Intrinsic activation of human motoneurons: possible contribution to motor unit excitation.

The main purpose of this study was to estimate the contribution of intrinsic activation of human motoneurons (e.g., by plateau potentials) during voluntary and reflexive muscle contractions. Pairs of motor units were recorded from either the tibialis anterior or soleus muscle during three different conditions: 1) during a brief muscle vibration followed by a slow relaxation of a steady isometric contraction; 2) during a triangular isometric torque contraction; and 3) during passive sinusoidal muscle stretch superimposed on a steady isometric contraction. In each case, the firing rate of a tonically firing control motor unit was used as a measure of the effective synaptic excitation (i.e., synaptic drive) to a slightly higher-threshold test motor unit that was recruited and de-recruited during a contraction trial. The firing rate of the control unit was compared at recruitment and de-recruitment of the test unit. This was done to determine whether the estimated synaptic drive needed to recruit a motor unit was less than the amount needed to sustain firing as a result of an added depolarization produced from intrinsic sources. After test unit recruitment, the firing rate of the control unit could be decreased significantly (on average by 3.6 Hz from an initial recruitment rate of 9.8 Hz) before the test unit was de-recruited during a descending synaptic drive. Similar decreases in control unit rate occurred in all three experimental conditions. This represents a possible 40% reduction in the estimated synaptic drive needed to maintain firing of a motor unit compared with the estimated amount needed to recruit the unit initially. The firing rates of both the control and test units were modulated together in a highly parallel fashion, suggesting that the unit pairs were driven by common synaptic inputs. This tight correlation further validated the use of the control unit firing rate as a monitor of synaptic drive to the test motor unit. The estimates of intrinsically mediated depolarization of human motoneurons ( approximately 40% during moderate contractions) are consistent with values obtained for plateau potentials obtained from intracellular recordings of motoneurons in reduced animal preparations, although various alternative mechanisms are discussed. This suggests that similar intrinsic conductances provide a substantial activation of human motoneurons during moderate physiological activity.

Adult↗

PubMed1993

Absence of phosphocreatine resynthesis in human calf muscle during ischaemic recovery.

Changes in the metabolites phosphocreatine (PCr), Pi and ATP were quantified by 31P n.m.r. spectroscopy in the human calf muscle during isometric contraction and recovery under ischaemic conditions. Time resolution of the measurements was 10 s. During a 30-60 s ischaemic isometric contraction, PCr decreased linearly at a rate of 1.17%/s (relative to the resting value) at a contraction strength equivalent to 70% of the maximal voluntary contraction (MVC) and at a rate of 2.43%/s at 90% MVC. There was a corresponding increase in Pi but the concentration of ATP did not change. pH decreased linearly during contraction by 4.22 and 8.23 milli-pH units/s at 70 and 90% MVC respectively. During a subsequent 5 min interval of ischaemic recovery, PCr, Pi, ATP, phosphomonoesters and calculated free ADP, free AMP and pH retained the value they had attained by the end of contraction with no significant recovery. Thus it is concluded that anaerobic glycolysis and glycogenolysis is halted momentarily on termination of contraction and that PCr is not resynthesized during ischaemic recovery. This paradoxical arrest of glycolytic flow in spite of the very significantly elevated concentration of potent activators such as Pi and free AMP clearly indicates that parameters other than PCr, ATP, Pi, calculated pH, free ADP and free AMP regulate glycolysis and glycogenolysis of human skeletal muscle very efficiently under ischaemic conditions.

Adenosine Diphosphate↗

PubMed1977

Effects of cholecystokinin, gastrin, and related peptides on coho salmon gallbladder contraction in vitro.

UNLABELLED: Isometric contractions of coho salmon (Oncorhynchus kisutch) gallbladder longitudinal muscle strips were recorded in response to porcine cholecystokinin (CCK), octapeptide of CCK (OP-CCK), desulfated octapeptide of CCK (ds-OP-CCK), porcine heptadecapeptide gastrins I and II, and pentagastrin. Peak tonic contraction amplitude and peak rhythmic contraction frequency were used to construct concentration-response curves for each peptide. Observed maximal responses to each peptide were not significantly different; therefore, all peptides tested were equal in efficacy. CCK, OP-CCK, and gastrin II were equipotent, and ds-OP-CCK and gastrin I were equipotent. CONCLUSIONS: 1) porcine CCK can stimulate contraction of the coho salmon gallbladder; 2) tonic and rhythmic contractile responses are elicited in coho salmon gallbladder by agonists; 3) coho salmon gallbladder muscle cannot distinguish between agonists that differ in the location of a sulfated tyrosyl residue at position 6 versus position 7 from the C-terminus; and 4) the results of this study are consistent with the hypothesis that coho salmon gallbladder muscle contains a relatively primitive CCK receptor.

Acetylcholine↗

PubMed1987

[Does guanabenz have a non-specific effect on spontaneous contractions of isolated guinea pig atria?].

Guanabenz was found to produce a concentration-dependent depression of the isometric contractility of the isolated, spontaneously beating atria of the guinea-pig. It also depressed the atrial rate of the isolated, spontaneously beating atria of the guinea-pig. The effect of the increasing concentrations of guanabenz on the heart rate was weaker than its effect on the isometric contraction. A time-dependent depression of both the isometric contraction and of the atrial rate after the addition of a single dose of guanabenz was also found up to 10th min. Guanabenz did not change the maximal driving (following) frequency of the atria. Aminophylline partially, isoprenaline almost completely and calcium completely antagonized the negative inotropic action of guanabenz. They, however, did not antagonize the negative chronotropic action of guanabenz. It seems, regardless of what the precise mechanism(s) of action of guanabenz may be (probably nonspecific on the isolated guinea-pig atria), that all these substances (aminophylline, isoprenaline and calcium) restore the contractility of the isolated atria by compensating the calcium balance which has been previously changed by guanabenz.

Aminophylline↗

PubMed1988

Electro-mechanical response times and muscle strength after sleep deprivation.

This study examined the effect of 60 h of sleep deprivation (SD) on electromechanical response times (EMRT), maximal voluntary isometric contraction (MVC), rate of force development (RFD), and times required to reach various percentages of MVC, during a maximal voluntary isometric contraction of both the forearm flexors and leg extensors. Eleven male subjects were either sleep deprived for 60 h (E) or performed similar daily activities and slept 7 h per night (C). Performance variables were evaluated at the same time intervals during both conditions. No significant differences were observed between the E and C conditions for EMRT (pre-motor time, electro-mechanical delay, total reaction time) or muscular performance (MVC, RFD). The results suggest that subjects who have undergone 60 h of SD can react as fast, and with as much force, as those who have had 7 h of sleep per night.

Adult↗

PubMed2006

Time under tension and blood lactate response during four different resistance training methods.

Mechanical stimuli have often been suggested to be the major determinant of resistance training adaptations; however, some studies suggested that metabolic changes also play an important role in the gains of muscle size and strength. Several resistance training methods (RTM) have been employed with the purpose of manipulating mechanical and metabolic stimuli; however, information about their physiological effects are scarce. The objective of this study was to compare the time under tension (TUT) and blood lactate responses among four different RTM reported in the literature. The four RTM were performed in a knee extension machine at 10 repetition maximum (RM) load by 12 recreationally trained young men. The RTM tested were: 10RM, super-slow (SL-subjects performed one 60-second repetition with 30 seconds for eccentric and 30 seconds for concentric phase), functional isometrics (FI-in each repetition, a five-second maximal isometric contraction was executed with the knees fully extended) and adapted vascular occlusion (VO-subjects performed a 20-second maximal isometric contraction with the knees fully extended and immediately proceeded to normal isoinertial lifts). According to the results, all RTM produced significant increases in blood lactate levels. However, blood lactate responses during FI (4.48+/-1.57 mM) and VO (4.23+/-1.66 mM) methods were higher than the SL method (3.41+/-1.14 mM). The TUT for SL (60 s), FI (56.33+/-6.46 s), and VO (53.08+/-4.76 s) methods were higher than TUT for 10RM (42.08+/-3.18 s). Additionally, TUT for the SL method was higher than TUT during the VO method. Therefore, the SL method may not be recommended if one wants to provide a high metabolic stimulus. The FI method appeared to be especially effective in promoting both type of stimuli.

Adaptation, Physiological↗

PubMed1983

Studies on conformation of F-actin in muscle fibers in the relaxed state, rigor, and during contraction using fluorescent phalloidin.

F-actin in a glycerinated muscle fiber was specifically labeled with fluorescent phalloidin-(fluorescein isothiocyanate) FITC complex at 1:1 molar ratio. Binding of phalloidin-FITC to F-actin affected neither contraction of the fiber nor its regulation by Ca2+. Comparison of polarized fluorescence from phalloidin-FITC bound to F-actin in the relaxed state, rigor, and during isometric contraction of the fiber revealed that the changes in polarization accompanying activation are quantitatively as well as qualitatively different from those accompanying transition of the fiber from the relaxed state to rigor. The extent of the changes of polarized fluorescence during isometric contraction increased with decreasing ionic strength, in parallel with increase in isometric tension. On the other hand, polarized fluorescence was not affected by addition of ADP or by stretching of the fiber in rigor solution. It is concluded from these observations that conformational changes in F-actin are involved in the process of active tension development.

Actins↗

PubMed2000

Force-frequency relationship and potentiation in mammalian skeletal muscle.

Repetitive activation of a skeletal muscle results in potentiation of the twitch contractile response. Incompletely fused tetanic contractions similar to those evoked by voluntary activation may also be potentiated by prior activity. We aimed to investigate the role of stimulation frequency on the enhancement of unfused isometric contractions in rat medial gastrocnemius muscles in situ. Muscles set at optimal length were stimulated via the sciatic nerve with 50-micros duration supramaximal pulses. Trials consisted of 8 s of repetitive trains [5 pulses (quintuplets) 2 times per second or 2 pulses (doublets) 5 times per second] at 20, 40, 50, 60, 70, and 80 Hz. These stimulation frequencies represent a range over which voluntary activation would be expected to occur. When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction - rest tension) of doublet contractions increased from 2.2 +/- 0.2, 4.1 +/- 0.4, and 4.3 +/- 0.5 to 3.1 +/- 0.3, 5.6 +/- 0.4, and 6.1 +/- 0.7 N, respectively. Corresponding measurements for quintuplet contractions increased from 2.2 +/- 0.2, 6.1 +/- 0.5, and 8.7 +/- 0.7 to 3.2 +/- 0.3, 7.3 +/- 0.6, and 9.0 +/- 0.7 N, respectively. Initial peak active force values were 27 +/- 1 and 61.5 +/- 5% of the maximal (tetanic) force for doublet and quintuplet contractions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increased significantly for frequencies up to 60 Hz. Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the pattern of stimulation during the 8 s, and twitch peak active force returned to prestimulation values by 5 min. These experiments confirm that activity-dependent potentiation is evident during repeated, incompletely fused tetanic contractions over a broad range of frequencies. This observation suggests that, during voluntary motor unit recruitment, derecruitment or decreased firing frequency would be necessary to achieve a fixed (submaximal) target force during repeated isometric contractions over this time period.

Animals↗

PubMed2005

Relationship between force and stiffness in muscle fibers after stretch.

The purpose of this study was to evaluate the relationship between force and stiffness after stretch of activated fibers, while simultaneously changing contractility by interfering with the cross-bridge kinetics and muscle activation. Single fibers dissected from lumbrical muscles of frogs were placed at a length 20% longer than the plateau of the force-length relationship, activated, and stretched by 5 and 10% of fiber length (speed: 40% fiber length/s). Experiments were conducted with maximal and submaximal stimulation in Ringer solution and with the addition of 2 and 5 mM of the myosin inhibitor 2,3-butanedione monoxime (BDM) to the solution. The steady-state force after stretch of an activated fiber was higher than the isometric force produced at the corresponding length in all conditions investigated. Lowering the frequency of stimulation decreased the force and stiffness during isometric contractions, but it did not change force enhancement and stiffness enhancement after stretch. Administration of BDM decreased the force and stiffness during isometric contractions, but it increased the force enhancement and stiffness enhancement after stretch. The relationship between force enhancement and stiffness suggests that the increase in force after stretch may be caused by an increase in the proportion of cross bridges attached to actin. Because BDM places cross bridges in a weakly bound, pre-powerstroke state, our results further suggest that force enhancement is partially associated with a recruitment of weakly bound cross bridges into a strongly bound state.

Animals↗

PubMed2001

The relative stress on the Achilles tendon during ambulation in an ankle immobiliser: implications for rehabilitation after Achilles tendon repair.

BACKGROUND: After Achilles tendon repair, immediate weightbearing and immobilisation closer to neutral plantarflexion are thought to limit atrophy and stiffness, but may place deleterious stress on the repair. OBJECTIVES: To estimate the relative stress on the Achilles tendon during weightbearing with immobilisation in varying degrees of plantarflexion. METHODS: Electromyographic (EMG) activity from the plantarflexors was recorded during walking in 10 subjects (six men, four women) without ankle pathology. Four walking conditions were examined: (a) normal walking; (b) immobilised (cam-walker) in neutral plantarflexion; (c) immobilised with a 0.5 inch heel lift; (d) immobilised with a 1 inch heel lift. EMG activity relative to plantarflexor torque was determined for each subject during isometric contractions at 25%, 50%, 75%, and 100% of maximal voluntary contraction (MVC). EMG activity during walking was calculated as a percentage of MVC based on the EMG-torque relation during graded isometric contractions. RESULTS: During normal walking, the plantarflexor torque was estimated to be 30 (12)% (mean (SD)) of MVC, compared with 21 (15)% MVC for immobilisation in neutral (p<0.05), 17 (15)% MVC with the addition of a 0.5 inch heel lift (p<0.01), and 12 (12)% MVC with the addition of a 1 inch heel lift (p<0.01). The 1 inch heel lift resulted in less than 10 degrees plantarflexion in all subjects. CONCLUSIONS: When the ankle is immobilised, stress on the Achilles tendon is determined by the degree of plantarflexion and the contractile activity of the plantarflexors. In the immobilised ankle, the addition of a 1 inch heel lift was sufficient to minimise plantarflexor activity during walking.

Achilles Tendon↗

PubMed1991

The effects of isotonic contractions on the rate of fatigue development and the resting membrane potential in the sartorius muscle of the frog, Rana pipiens.

The goal of this study was to characterize how isotonic contractions affect the rate of fatigue development. Muscle bundles dissected from frog sartorius muscles were stimulated with 100-ms long train of pulses (0.5 ms, 6 V, 140 Hz). To measure the effect of the isotonic contractions, isometric tetanus were elicited at regular time intervals during the stimulation to fatigue. In general, isotonic contractions caused a faster decrease in tetanic force than isometric contractions. The difference in tetanic force between an isotonic and isometric fatigue increased gradually over a 20-min period to 7.9 and 13.5% at 0.04 and 0.1 trains/s (TPS), respectively. At 0.2, 0.5, and 1.0 TPS, the decrease in tetanic force was also faster during an isotonic fatigue, which resulted in an initial difference in tetanic force between the two types of fatigue. The difference did not exceed 18.5% and did not persist throughout the stimulation period; i.e., the difference disappeared before the end of the fatigue stimulation. The half-relaxation time was prolonged during fatigue development, and the prolongation was greater during an isotonic fatigue, except at 0.04 TPS. The increases in the half-relaxation time at 0.2, 0.5, and 1.0 TPS were followed by a decrease, and the decreases were especially pronounced during an isotonic fatigue at 0.5 and 1.0 TPS. The results showed for the first time that isotonic contractions cause a faster rate of fatigue development in frog sartorius muscles, and this effect depends on the frequency of stimulation.

Animals↗