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PubMed1997

Effects of contract-relax stretching training on muscle performance in athletes.

The effects of an 8-week unilateral contract-relax (CR) stretching training program (passive stretch after isometric contraction) on muscular performance were investigated in a group of 16 athletes. The flexibility, maximum torque and angular position as well as contraction work in movements of the knee joint were determined before training and after 4 and 8 weeks of training. The torque measurements were performed under isokinetic conditions, eccentrically at angular velocities of 60 degrees x s(-1) and 120 degrees x s(-1), isometrically at five different joint positions, and concentrically at angular velocities of 60, 120, 180 and 240 degrees x s(-1) using an isokinetic dynamometer. A surface electromyogram (EMG) of the thigh muscles (quadriceps and hamstrings) was recorded simultaneously. As compared to untrained control limbs, significant improvements in active and passive flexibility (up to 6.3 degrees in range of motion), maximum torque (up to 21.6%) and work (up to 12.9%) were observed, and these were especially pronounced under eccentric load conditions. A comparison between integrated EMG recordings during eccentric and concentric loads, as well as the interpretation of the training-induced changes in the EMG, suggest that muscular activity under eccentric loads may be impaired by mental processes.

Adaptation, Physiological↗

PubMed1986

High voltage versus low voltage electrical stimulation. Force of induced muscle contraction and perceived discomfort in healthy subjects.

High voltage pulsed galvanic stimulation (HVPGS) and low voltage neuromuscular stimulation (LVNMS) techniques were compared for peak torque of an induced isometric contraction, perceived discomfort, and subjective preference of treatment. The high voltage current used a 40-microseconds monophasic waveform, and the low voltage current used a 300-microseconds biphasic waveform. Both currents used a pulse rate of 50 pps. Both HVPGS and LVNMS were administered to one muscle group, either knee extensors or plantar flexors, of 24 healthy subjects. An isokinetic dynamometer was used to assess peak torque. The perceived discomfort experienced with each type of electrical stimulation was quantified by the use of a visual analog scale. For all dependent measures, data first were analyzed for the whole treatment group and then analyzed for each subgroup. Correlated t tests for the whole group and the plantar flexor muscle subgroup demonstrated that HVPGS produced a significantly greater average peak force of muscle contraction than LVNMS and HVPGS was perceived to be significantly less uncomfortable than LVNMS. No significant differences were found between treatments in the knee extensor muscle subgroup for these dependent variables. Chi-square analysis revealed a subject preference for HVPGS in the whole group and in both subgroups. This study indicates that HVPGS can produce a stronger, less uncomfortable, induced isometric muscle contraction than LVNMS.

Consumer Behavior↗

PubMed2006

Clonidine induced endothelium-dependent tonic contraction in circular muscle of the rat hepatic portal vein.

Clonidine, an alpha2-agonist, has been shown to be useful in the treatment of hepatic portal hypertension in cirrhosis. The mechanism has been attributed to a clonidine-induced decrease in sympathetic activity. While clonidine has been shown to stimulate the alpha2-adrenoceptors of blood vessels, there is limited knowledge of the effects of clonidine on the circular muscle of the hepatic portal vein which regulates its blood flow. To investigate clonidine-induced contraction of the circular muscle of the hepatic portal vein and to clarify the possible role of the endothelium in the contraction, we examined the effects of clonidine on the isometric contraction of endothelium-intact and -removed ring preparations of the rat hepatic portal vein. In endothelium-intact preparations, clonidine caused a concentration-dependent increase in the amplitude of contractions. Inhibition of NO synthesis with Nomega-nitro-L-arginine (L-NNA) elevated the resting tone, and increased the amplitude of the clonidine-induced contractions. Inhibition of cyclooxygenase by diclofenac did not change the amplitude of the clonidine-induced contractions observed both in the presence and absence of L-NNA. Application of a single concentration of clonidine induced a clear increase in amplitude of both twitch and tonic contractions. Twitch and tonic contractions induced by clonidine were inhibited by yohimbine. When the endothelium was damaged by sodium deoxycholate, tonic contractions induced by clonidine were completely suppressed, whereas the increase in twitch contractions was not influenced by chemical damage of the endothelium. Neither SKF-96365, a nonselective cation channel blocker, nor superoxide dismutase, a free radical scavenger, in the presence of catalase, changed the tonic contraction induced by clonidine. These results indicate that stimulation of alpha2-adrenoceptors enhanced twitch contractions and induced tonic contractions in the circular muscle of the rat hepatic portal vein, especially in the absence of NO. The latter, but not the former, occurs through an endothelium-dependent pathway.

Adrenergic alpha-Agonists↗

PubMed1984

Effect of exercise of the lower limbs on the non-exercised biceps brachii muscle.

The electrophysiological pattern of fatigue of the biceps brachii (BB) muscle before and after bicycle leg exercise (LE), was evaluated in 12 healthy volunteers. In healthy people, the onset of isometric contraction is initiated by smaller motor units with briefer durations and higher spike frequencies. With the progression of contraction and muscular fatigue, electromyographic decrease in frequency and an increased spike duration occur. In our study, these findings were presented in subjects at the Final Value (FV) of electromyography before and after LE. In 8 subjects, the Initial Value (IV) of spike duration was longer after LE and a decrease in the frequency of electromyography was found in 9 of the 12 subjects. The changes in these two parameters at the onset of the BB contraction after LE may be due to the existence of an electrophysiological transfer effect. Eight subjects sustained the isometric BB contraction longer after LE than before LE. This may be explained by a diverting activity or other factors. Venous lactic acid was measured in 7 subjects and showed an increase after LE. No definite correlation between electromyographic changes and lactic acid was found.

Action Potentials↗

PubMed1981

EMG and fatigue of human voluntary and stimulated contractions.

During a 60s maximal voluntary isometric contraction (MVC) of the adductor pollicis muscle the loss of force is accompanied by a parallel decline in both the integrated surface electromyogram (EMG) and the single muscle fibre spike counts recorded intramuscularly. This decline is not due to neuromuscular block since the muscle mass action potential (M wave) evoked by single maximal shocks to the nerve is well maintained; nor does the size of the single fibre spike change. It must, therefore, reflect a decline in the firing pattern of the motor neuron pool. The force of a sustained MVC continues to match that from maximal tetanic nerve stimulation; thus, all motor units remain active. Continuous nerve stimulation at the frequency required to match the voluntary force of unfatigued muscle leads to a progressive failure of the M wave, and a more rapid force loss than in an MVC. Both are largely restored by reducing the stimulus frequency. The decline in neural firing rate correlates well with the rate of muscle contractile slowing. It thus optimizes force by maintaining a relatively constant degree of tetanic fusion, while avoiding peripheral failure of electrical propagation.

Action Potentials↗

PubMed1995

The senses of effort and force during fatiguing contractions.

The sensory basis upon which judgments of force are made during fatiguing isometric contractions is reviewed. At issue is whether the perception of force is based on centrally generated sensations arising from the motor command, known as the sense of effort, or from peripheral sensations originating in the muscle, and termed a sense of force. The results from a number of studies indicate during sustained constant-force contractions the perceived magnitude of the force increases, which is consistent with the idea that judgments of force are based on centrally generated signals. It is, however, possible for some subjects to dissociate effort and force and to make accurate judgments of the magnitude of forces during fatigue.

Humans↗

PubMed1989

Effects of external calcium, magnesium, and temperature on spontaneous contractions of pregnant human myometrium.

Spontaneous isometric contractions of small isolated segments from the isthmic region of pregnant human myometrium were recorded to clarify the characteristics of and influences of experimental conditions on contractions. There is a spontaneous periodicity in membrane activity of human myometrium, and contractions evoked with a sustained electric stimulus were affected by spontaneous rhythmic contractions. The frequency of contractions increased markedly, but their duration decreased when temperature of the bathing fluid was increased from 26 degrees C to 39 degrees C. Both excess (7 mM) and low (0.5 mM) calcium levels suppressed the generation of spontaneous contraction. Frequency and half-duration gradually decreased when external magnesium was increased from 0 mM to 3.6 mM. These results indicate that the pattern of contraction depends on the frequency of and intervals between each action potential, and that spontaneous contractility-particularly its frequency-is extremely sensitive to external temperature and ionic conditions.

Calcium↗

PubMed1996

Facilitation of motor evoked potentials in hand extensor muscles of stroke patients: correlation to the level of voluntary contraction.

The influence of ongoing voluntary isometric contractions (ranging from 2.5% to 100% of maximum force production) on motor evoked potentials in the extensor carpi radialis muscle was investigated in 20 healthy subjects and 25 hemiparetic stroke patients using transcranial magnetic stimulation at threshold and at 90% of maximum stimulus intensity. In healthy subjects and in stroke patients, an initial sharp decay in response latencies was observed at low contraction levels. In hemiparetic patients, however, no significant further reduction of response latencies with increasing contraction levels was observed irrespective of whether threshold or 90% stimulus intensities were applied. The continuous decrease in latency in the healthy subjects is supposed to result from an enhanced involvement of rapidly conducting corticospinal neurones that are preferentially damaged in the patient group. In healthy subjects and in hemiparetic patients, however, the increase in response amplitudes runs in parallel with increasing force production, at least with threshold stimulus intensity. Contrary to response latencies, amplitude facilitation appears to be less dependent on the involved corticospinal fibre spectrum but to be predominantly based on temporal and spatial summation effects. The relevance of the latency and amplitude data obtained in healthy subjects and in stroke patients for physiology and localization of facilitatory processes, i.e. whether cortical or spinal, is discussed. For the rehabilitation of stroke patients it is concluded that the effect of slight voluntary contractions is indeed superior to most other facilitatory approaches. The functional relevance is discussed.

Adult↗

PubMed2004

The acute effect of trimetazidine on the high frequency fatigue in the isolated rat diaphragm muscle.

The objective of this study was to determine the acute effect of trimetazidine (TMZ) on the pre-fatigue, fatigue and post-fatigue contractile characteristics and tension-frequency relationships of isolated rat diaphragm muscle. Muscle strips were taken from the ventral-costal aspects of the diaphragm muscle of rats killed by decapitation. The muscle strips were suspended in organ baths containing Krebs solution, with a gas mixture of 95% O2 and 5% CO2 at 37 degrees C and pH 7.35-7.45. After determining the thermoregulation and optimum muscle length the muscles were subjected to direct supramaximal stimulation with 0.05 Hz frequency square pulses for periods of 0.5 msec to obtain control values. After adding 5 x 10(-6) and 5 x 10(-5) M trimetazidine solution to the respective bath media, the contractile parameters of the muscles were recorded. The contractile parameters were also recorded for both the trimetazidine and trimetazidine-free media after application of the high frequency fatigue protocols. Later, the tension-frequency relationship was determined by applying stimulating pulses of 10, 20, 50 and 100 Hz to the muscle strips. Whilst the twitch tension obtained from the 5 x 10(-6) and 5 x 10(-5) M trimetazidine media showed numerical increases compared to that of the controls, these were not statistically significant (p>0.05). The contraction time exhibited a dose dependent increase (p<0.001), whilst the contraction and relaxation rates did not differ significantly. The isometric contraction forces obtained with the different stimulating frequencies showed a significant increase in the tetanic contraction only at 100 Hz (p<0.05). A comparison of the pre- and post-fatigue twitch tensions in the trimetazidine media showed the post- fatigue twitch tensions to be significantly higher than those of the pre-fatigue contraction forces (p<0.05). In the 5 x 10(-6) and 5 x 10(-5) M trimetazidine media the increases in the post-fatigue contraction force were 22 and 30%, respectively. These results demonstrated that in isolated rat diaphragm muscle, TMZ significantly limited the mechanical performance decrease during fatigue. It is our opinion that trimetazidine contributed to the observed fatigue tolerance by eliminating the factors of fatigue, due to preservation of intracellular calcium homeostasis, provision of the ATP energy levels needed by ATPase dependent pumps and especially by keeping the intracellular pH within certain limits.

Animals↗

PubMed1997

Stretch-induced enhancement of mechanical work production in frog single fibers and human muscle.

The relations between the velocity of prestretch and the mechanical energy liberated during the subsequent isovelocity release were studied in contractions of frog single fibers and human muscles. During isometric contractions of frog single fibers, a ramp stretch of varied velocity (amplitude, 0.02 fiber length; velocity, 0.08-1.0 fiber length/s) followed by a release (amplitude, 0.02 fiber length; velocity, 1.0 fiber length/s) was given, and the amount of work liberated during the release was measured. For human muscles, elbow flexions were performed with a prestretch of varied velocity (range, 40 degrees; velocity, 30-180 degrees/s) followed by an isokinetic shortening (velocity, 90 degrees/s). In both frog single fibers and human muscles, the work production increased with both the velocity of stretch and the peak of force attained before the release up to a certain level; thereafter it declined with the further increases of these variables. In human muscles, the enhancement of work production was not associated with a significant increase in integrated electromyogram. This suggests that changes in intrinsic mechanical properties of muscle fibers play an important role in the stretch-induced enhancement of work production.

Adult↗

PubMed2000

Inhibition of motor unit firing during experimental muscle pain in humans.

Electromyographic activity was recorded in the masseter muscle to investigate whether the firing characteristics of single motor units (SMUs) were affected by muscle pain. Capsaicin was injected into the masseter to induce pain. The interspike interval (ISI) and recruitment threshold of SMUs were measured while subjects performed isometric contractions at 5, 7.5, 10, 15, and 20% of maximum voluntary contraction. All subjects were able to maintain a stable isometric force during pain, but the mean ISI was significantly increased without changing the recruitment threshold. In all the experimental conditions, the firing frequency increased with increasing force, and SMUs recruited at low force fired at higher rates for all force levels. These results suggest that acute stimulation of nociceptive muscle afferents inhibits SMU activity without changing recruitment order in the homonymous muscle.

Action Potentials↗

PubMed1984

Vascular smooth muscle energetics.

ATP utilization (delta approximately P) during an isometric contraction has been studied in terms of both measurements of oxygen consumption and lactate production as well as of the tissue nucleotide and metabolite levels. The contribution of breakdown of preformed ATP and phosphocreatine (PCr) pools to delta approximately P during contraction is minor compared to that made by metabolic synthesis of ATP. For tonic vascular smooth muscle (VSM), in fact, no change in ATP or PCr from resting levels can be measured. In contrast to amphibian skeletal muscle, a P:O of 3 can be demonstrated in VSM. In both tonic and phasic VASM, delta approximately P is biphasic with contraction duration, attaining a maximal value before that of isometric force and declining to a steady-state value approximately 60% of the maximal suprabasal rate during the maintenance of constant isometric force. The steady-state rate of ATP utilization per unit force maintained increases with extracellular Ca2+. Both the pre-steady-state temporal dependence and the steady-state dependence on Ca2+ are consistent with the hypothesis that myosin phosphorylation modulates the cross-bridge cycle rates. VSM metabolism, when viewed in terms of ATP synthesis, is primarily oxidative. However, even under fully oxygenated conditions, lactate is the major end product of glucose catabolism. Recent work has shown that aerobic lactate production is specifically coupled to Na-K transport in many, but not all, vascular tissues. Oxidative metabolism, on the other hand, is strongly related to active isometric force. The biochemical basis of this functional compartmentation was investigated at the level of substrate specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

Aerobiosis↗

PubMed2005

The structural basis of the increase in isometric force production with temperature in frog skeletal muscle.

X-ray diffraction patterns were recorded from isolated single fibres of frog skeletal muscle during isometric contraction at temperatures between 0 and 17 degrees C. Isometric force was 43 +/- 2% (mean +/- S.E.M., n = 10) higher at 17 degrees C than 0 degrees C. The intensity of the first actin layer line increased by 57 +/- 18% (n = 5), and the ratio of the intensities of the equatorial 1,1 and 1,0 reflections by 20 +/- 7% (n = 10), signalling radial or azimuthal motions of the myosin head domains. The M3 X-ray reflection from the axial repeat of the heads along the filaments was 27 +/- 4% more intense at 17 degrees C, suggesting that the heads became more perpendicular to the filaments. The ratio of the intensities of the higher and lower angle peaks of the M3 reflection (R(M3)) was 0.93 +/- 0.02 (n = 5) at 0 degrees C and 0.77 +/- 0.02 at 17 degrees C. These peaks are due to interference between the two halves of each myosin filament, and the R(M3) decrease shows that heads move towards the midpoint of the myosin filament at the higher temperature. Calculations based on a crystallographic model of the heads indicated that the observed R(M3) change corresponds to tilting of their light-chain domains by 9 deg, producing an axial displacement of 1.4 nm, which is equal to that required to strain the actin and myosin filaments under the increased force. We conclude that the higher force generated by skeletal muscle at higher temperature can be accounted for by axial tilting of the myosin heads.

Actins↗

PubMed1995

Changes in the electromyographic spectrum power distribution caused by a progressive increase in the force level.

The purpose of the present study was to determine the specific changes occurring in the power spectrum with an increasing force level during isometric contractions. Surface electromyographic signals of the triceps brachii (TB) and the anconeus (AN) of 29 normal subjects were recorded during isometric ramp contractions performed from 0 to 100% of the maximum voluntary contraction (MVC) in a 5-s period. Power spectra were obtained at 10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC. Changes in the shape of these spectra were evaluated visually and with the calculation of several statistical parameters related to the distribution of power along the frequency axis, such as median frequency and mean power frequency, standard deviation, skewness, first and third quartiles and half-power range. For the AN, the behaviour of the spectrum was relatively similar across subjects, presenting a shift toward higher frequencies without any major change in the shape of the spectrum. For the TB, subjects with a thin skinfold thickness presented similar behaviours. In subjects with a thicker skinfold, however, a loss of power in the high frequency region paralleled the increase in the force level. Significant correlations were obtained between the extent of the change in the value of higher order statistical parameters across force and the thickness of the skin. This points out the importance of the skinfold layer when recording with surface electrodes. Furthermore, the use of a combination of several parameters appears to provide a better appreciation of the changes occurring in the spectrum than any single parameter taken alone.

Adult↗

PubMed2004

Changes in PCr/Cr ratio in single characterized muscle fibre fragments after only a few maximal voluntary contractions in humans.

AIM: This methodological study investigated the number of brief maximal voluntary isometric contractions (MVC) needed to show evidence of fibre activation, as indicated by changes in the phosphocreatine to creatine (PCr/Cr) ratio. METHODS: Subjects performed series of four, seven and/or 10 MVC (1 s on, 1 s off) of the m. quadriceps (60 degrees -flexion angle). Biopsy samples of the m. vastus lateralis were taken at rest and immediately post-exercise. Single muscle fibres were dissected from the freeze-dried samples and classified as types I, IIA or IIAX, using mATPase stainings. Fragments of characterized fibres were analysed for PCr and Cr content. Analyses of variance were performed to investigate changes in PCr/Cr per fibre group over time, followed by Bonferroni post-hoc test (P < 0.01). The fifth percentile of resting values of each fibre group was determined. RESULTS: Mean PCr/Cr ratio after four, seven and 10 MVCs were significantly lower for all fibre groups (P < 0.01). The mean decreases were 44, 64 and 76%, respectively. However, only after seven and 10 contractions PCr/Cr ratios of all, but three type I and two type IIAX fibres, individual fibres were below the fifth percentile. CONCLUSION: In very short duration exercise, involving seven brief maximal voluntary contractions, changes in the PCr/Cr ratio indicated activation of different characterized muscle fibre fragments. The results suggest that this approach may be useful for investigating the pattern of fibre type activation in exercise of very short duration.

Adult↗

PubMed1998

Cerebral blood flow velocity during and after sustained isometric skeletal muscle contractions in man.

1. Twenty-seven young subjects used their right hand to perform sustained, isometric contractions at 40% of maximum for 2 min while lying supine. 2. During the last 30 s of exercise, mean arterial blood pressure increased by 38 +/- 4 mmHg (mean +/- S.E.M.) and heart rate by 27 +/- 2 beats/min. 3. Nineteen of the subjects respired eucapnically during exercise, increasing ventilation by 4.1 +/- 0.5 litres/min. Eight subjects hyperventilated (7.1-19.6 litres/min) and decreased end-tidal PCO2 by 8.2 to 15.1 mmHg during the last minute of exercise. 4. In the eucapnic subjects mean flow velocity in the right (i.e. contralateral to the activated cortex) middle cerebral artery increased by 11.4 +/- 1.0 cm/s, a change of 17%, during the contraction. This represents an increase in volume flow to the territory of this vessel, but an increase in global flow to the brain cannot be inferred. 5. In the eight subjects who hyperventilated during exercise, there was no rise of flow velocity in the middle cerebral artery, and in some subjects there was a fall during the first 2 min of recovery. These findings suggest that if subjects hyperventilate during handgrip exercise there could be a fall in volume flow to many regions of the brain during and after the exercise.

Adolescent↗

PubMed1984

Cardiovascular responses to isometric neck muscle contractions: results after dynamic exercise with various headgear loading configurations.

Experiments were conducted to quantify the cardiovascular response (blood pressure and heart rate) elicited by sustained isometric contractions of the neck muscles. The response was secondary to dynamic exercise with various headgear loading combinations. The neck muscles were loaded by the head itself (CON), the standard U.S. Army SPH-4 helmet (HEL), and a combination of the SPH-4 helmet with Night Vision Goggles (H/NVG). During two exercise periods of 5 min and 35 min, each of the five subjects would rotate the head from side-to-side in the CON, HEL, or H/NVG configuration. Immediately thereafter, the subject would position his head in an isometric head dynamometer and exert a sustained right lateral (LAT) neck contraction or forward (FOR) neck contraction at 70% of a maximal voluntary contraction (MVC). During this isometric neck muscle contraction, the subject's endurance time to fatigue was recorded, the blood pressure was manually recorded, and the heart rate was continuously recorded. Characteristic increases in the systolic and diastolic blood pressure and heart rate occurred with sustained isometric neck muscle contractions. There was an average 40% increase in the systolic blood pressure, an average 50% increase in the diastolic blood pressure, and an average 45% increase in the heart rate from resting to the end of a fatiguing 70% MVC (p less than 0.05). These responses appear to be relatively independent of the duration of the exercise period, the loading during the exercise period, and the specific muscle mass involved. The mechanisms for the pressor response and the heart rate response are reviewed.

Adult↗

PubMed2001

Human motor unit recruitment and derecruitment during long lasting intermittent contractions.

Seven healthy subjects were investigated in cyclic ramp-and-hold long lasting isometric contractions. Wire branched electrodes were used for selective recording of single motor unit (MU) potentials from m. biceps brachii. MU behaviour was defined in terms of recruitment/derecruitment thresholds (RT and DT) and the duration of interspike intervals (ISI). A total of 63 MUs was investigated: 40 units were active from the beginning of the task performance and another 23 were recruited later. There were no changes in the recruitment pattern of MUs with fatigue development - a short first ISI followed by a very long second one and an almost constant firing rate after this transient phase. The tendency of RT to gradually decrease dominates the results. Thus, the required constant rate of force increase with fatigue development was maintained mostly by the mechanisms of space coding (i.e., decrease of RT and recruitment of additional MUs). Oppositely, the time behaviour of the DT changes was not uniform and rate coding was an essential mechanism in the adaptation of MU activity to muscle fatigue during relaxation phases. The recruitment pattern and fatigue related behaviour of the additionally recruited MUs were similar to those of MUs active from the first cycle of the motor task performance.

Action Potentials↗