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PubMed1980

Muscle fatigue in relation to EMG during repeated and maintained maximal isometric contractions.

The decrease of muscular force output during two different maximal isometric voluntary contractions of knee extension were studied for 6 male subjects in relation to EMG. EMGs were recorded by bipolar surface electrodes from the m. vastus lateralis and were integrated by a modified Miller's circuit. In repeated contractions at one-second intervals, only slight changes were observed in the integrated EMG despite a marked decrease in force output to about 45% of the initial value. In counterparts of the maintained contraction for 2 min, both force output and integrated EMG decreased with time, and tended to plateau at a level corresponding to 20% and 45% of the initial value, respectively. From the viewpoint of blood supply, it was concluded that in the repeated contractions the decrease in force output was mainly due to the local factors in the muscle, especially in the fast twitch fibres, while in the maintained contraction in decrease in force output was caused not only by the failures in local factors but also central fatigue.

Adult↗
PubMed1972

Effect of temperature on muscle energy metabolism and endurance during successive isometric contractions, sustained to fatigue, of the quadriceps muscle in man.

1. The effects of altered tissue temperature on muscle metabolism during successive isometric contractions, sustained to fatigue, have been studied in the quadriceps muscle of man by combining biochemical analyses of metabolites in needle biopsy samples with measurements of endurance time with a force of 2/3 maximum voluntary contraction. Fatigue and recovery were observed repeatedly in a series of seven contractions at intervals of 20 sec, following immersion of the test leg in water at 12, 26 or 44 degrees C for 45 min. Muscle temperatures corresponding to these water temperatures were 22.5, 32.6 and 38.6 degrees C respectively.2. Increased levels of several glycolytic intermediates at rest in the heated muscle suggested an increased rate of glycolysis. ATP and phosphoryl creatine were lower at the end of the first contraction and the calculated rate of ATP utilization (including the contribution from anaerobic glycolysis) was highest in the heated nuscle.3. Significantly shorter endurance times were found for the heated muscle. These could not be attributed to depletion of local energy resources in muscle. Fatigue may be due to a reduction in the rate of regeneration of ATP from anaerobic glycolysis below that needed to maintain the contraction force. Lower values for the ratio of fructose 1,6-diphosphate: fructose 6-phosphate at the end of contractions, particularly at the highest temperature, are compatible with the hypothesis that there is partial inhibition of the rate controlling enzyme phosphofructokinase, possibly due to the accumulation of hydrogen ions in muscle.

Adenosine Diphosphate↗
PubMed1973

The orderly recruitment of human motor units during voluntary isometric contractions.

1. The contractile properties of human motor units from the first dorsal interosseus muscle of the hand were studied during voluntary isometric contractions using recently developed techniques.2. The twitch tensions produced by motor units varied widely from about 0.1-10 g. The twitch tension of a motor unit varied nearly linearly as a function of the level of voluntary force at which it was recruited over the entire range of forces studied (0-2 kg).3. The number of additional motor units recruited during a given increment in force declined sharply at high levels of voluntary force. This suggests that even though the high threshold units generate more tension, the contribution of recruitment to increases in voluntary force declines at higher force levels.4. Contraction times for these motor units varied from 30 to 100 msec. Over 80% had contraction times less than 70 msec, and might be classed as fast twitch motor units. The larger motor units, which were recruited at higher threshold forces, tended to have shorter contraction times than the smaller units.

Hand↗
PubMed1999

Intra-ocular pressure changes during maximal isometric contraction: does this reflect intra-cranial pressure or retinal venous pressure?

Recent publications have suggested that intra-ocular pressure (IOP) may be an indirect assessment of intra-cranial pressure (ICP). Both IOP and ICP have similar physiologic pressure ranges and similar responses to changes in intra-abdominal, intra-thoracic and aortic pressure. Previous studies have demonstrated the relationships between retinal arterial pressure and aortic pressure, intra-ocular pressure and retinal venous pressure, intra-cranial pressure and retinal venous pressure. Power athletes routinely utilize the Valsalva maneuver during weightlifting. In fact there are reports of stroke, cerebral hemorrhage, subarachnoid hemorrhage, conjunctival, foveal and retinal hemorrhage, retinal detachment, hiatal hernia and pneumothorax associated with weightlifting. These events are thought to occur secondary to the extreme pressure elevations that occur in the intra-abdominal, intra-thoracic, intra-cranial, intra-ocular and vascular compartments. To date no human studies have examined the IOP changes that may occur with heavy resistance exercise. Therefore, we recruited power athletes (n = 11), who had participated in prior studies, from the local metropolitan area. The athletes had blood pressure status, drug screening and medical histories performed during previous investigations. Intra-ocular pressure was measured by noncontact tonometry at rest and during maximal isometric contraction. All subjects resting IOP were within normal ranges (mean 13 +/- 2.8 mmHg). Intra-ocular pressures were significantly (p < 0.0001) elevated in each subject during maximal contraction (mean 28 +/- 9.3 mmHg). One subject's IOP reached 46 mmHg during maximal contraction. Linear regression analysis demonstrated a significant linear relationship (r = 0.62, p < 0.0001) in the net change of IOP from rest to maximal contraction for each subject. This study demonstrates that IOP elevates to pathophysiologic levels during resistance exercise. The findings of conjunctival hemorrhages in two subjects further supports IOP being reflective of retinal venous pressure. The enormous pressures generated by power athletes during weightlifting leads to elevations in ICP which obstruct venous outflow leading to hemorrhage and elevations in IOP. The question remains as to whether these intermittent bursts of elevated IOP can lead to long-term pathological sequelae.

Adult↗
PubMed1999

Physiological response to submaximal isometric contractions of the paravertebral muscles.

STUDY DESIGN: Brief (30-second) isometric trunk extensions at 5%, 20%, 40%, 60%, and 80% of maximal voluntary contraction (MVC) and 3 minutes of prolonged trunk extension (20% MVC) in erect position were studied in nine healthy male subjects. OBJECTIVES: To investigate the intercorrelation between intramuscular pressure and tissue oxygenation of the paravertebral muscles during submaximal isometric contractions and further, to evaluate paravertebral electromyogram and intramuscular pressure as indicators of force development. SUMMARY OF BACKGROUND DATA: Local physiologic responses to muscle contraction are incompletely understood. METHODS: Relative oxygenation was monitored with noninvasive near-infrared spectroscopy, intramuscular pressure was measured with a transducer-tipped catheter, and surface electromyogram was monitored at three recording sites. RESULTS: The root mean square amplitudes of the paravertebral electromyogram (L4, left and right; T12, right) and intramuscular pressure measured in the lumbar multifidus muscle at L4 increased with greater force development in a curvilinear manner. A significant decrease in the oxygenation of the lumbar paravertebral muscle in response to muscle contraction was found at an initial contraction level of 20% MVC. This corresponded to a paravertebral intramuscular pressure of 30-40 mm Hg. However, during prolonged trunk extension, no further decrease in tissue oxygenation was found compared with the tissue oxygenation level at the end of the brief contractions, indicating that homeostatic adjustments (mean blood pressure and heart rate) over time were sufficient to maintain paravertebral muscle oxygen levels. CONCLUSION: At a threshold intramuscular pressure of 30-40 mm Hg during muscle contraction, oxygenation in the paravertebral muscles is significantly reduced. The effect of further increase in intramuscular pressure on tissue oxygenation over time may be compensated for by an increase in blood pressure and heart rate. Surface electromyogram amplitudes and intramuscular pressure can be used as indicators of paravertebral muscle force.

Adult↗
PubMed1975

Cardiac muscle models for both isotonic and isometric contractions.

The choice of a suitable muscle model consistent with the mechanical behaviour of Rat papillary muscle at rest or during isotonic or isometric contraction has been considered. Three different preload levels within the ascending limb of the Frank-Starling curve have been used in five papillary muscles. Series elastic (SE) and parallel elastic (PE) length-tension relationships have been evaluated according to five mechanical muscle models using data from systolic and diastolic quick-release manoeuvres. For each preload level and muscle model, the time course of the force-velocity relationship of the contractile element (CE) has been calculated using a PDP 8/L digital computer. Several mechanical and biological characteristics are considered which exclude four of the five selected muscle models. The Maxwell model with a damper seems to be the most suitable mechanical model for Rat papillary muscle. According to the selected muscle model it seems that VCEmax is dependent on muscle length variation; the reliability of this parameter as an index of myocardial contractility is discussed.

Animals↗
PubMed1973

The contractile properties of human motor units during voluntary isometric contractions.

1. The electrical activity of single motor units has been recorded from the first dorsal interosseus muscle of normal human subjects during voluntary, isometric contractions, together with the force generated by the muscle.2. By averaging the force correlated with the impulses from a single motor unit, the contraction time and twitch tension generated by that motor unit could be measured. When the rate of discharge was limited, either voluntarily or by automatic selection of intervals for analysis, the time for the tension to decline to half its maximum value (half-relaxation time) could also be measured for some motor units.3. Under our experimental conditions the trains of impulses from different motor units in most subjects were generated quite independently as tested by (a) measuring the correlation between activity in single units and that in the whole muscle as recorded by the surface electromyogram (e.m.g.), (b) measuring the cross-correlations between pairs of single units and (c) comparing the tension generated by stimulating single motor units with the average tension correlated in time with voluntary activity of single units in the same location.4. In one normal subject evidence of synchronization between separate motor units was obtained. Cross-correlation studies suggested that the cause of the synchronization was the presence of substantial common excitation received by the various motor units in the muscle.5. The frequency response for the contractions of single motor units was well fitted by that for a linear, second-order system with nearly critical damping. However, when stimulation of a few motor units was superimposed on a voluntary contraction, underdamped (oscillatory) responses were seen which were probably of reflex origin.6. The significance of these results in relation to the normal postural tremor in hand muscles is discussed.

Electric Stimulation↗
PubMed1977

The orderly recruitment of motor units of the masseter and temporal muscles during voluntary isometric contraction in man.

1. The contractile properties of the motor units of the masseter and temporal muscles of human subjects were studied during voluntary isometric contractions, using a method previously employed to examine units of a small hand muscle. 2. Over the range of forces studied (0-6 kg), the units of both muscles were recruited in an orderly fashion, with a nearly linear relationship between the voluntary force at which units were recruited and their measured twitch tensions. 3. The range of contraction times (25-90 msec) was similar to that observed for the hand muscle. In some subjects it seemed that small units, recruited at low forces, exhibited shorter contraction times.

Action Potentials↗
PubMed1985

The pressor response to voluntary and electrically evoked isometric contractions in man.

Blood pressure and heart rate changes during sustained isometric exercise were studied in 11 healthy male volunteers. The responses were measured during voluntary and involuntary contractions of the biceps brachii at 30% of maximal voluntary contraction (MVC), and the triceps surae at 30% and 50% MVC. Involuntary contractions were evoked by percutaneous electrical stimulation of the muscle. Measurements of the time to peak tension of maximal twitch showed the biceps brachii (67.0 +/- 7.9 ms) muscle to be rapidly contracting, and the triceps surae (118.0 +/- 10.5 ms) to be slow contracting. The systolic and diastolic blood pressures increased linearly throughout the contractions, and systolic blood pressure increased more rapidly than diastolic. There was no significant difference in response to stimulated or voluntary contractions, nor was there any significant difference between the responses to contractions of the calf or arm muscles at the same relative tension. In contrast the heart rate rose to a higher level (P less than 0.01) in the biceps brachii than the triceps surae at given % MVC, and during voluntary compared with the electrically evoked contractions in the two muscle groups. It was concluded that the arterial blood pressure response to isometric contractions, unlike heart rate, is primarily due to a reflex arising within the active muscles (cf. Hultman and Sjöholm 1982) which is associated with relative tension but independent of contraction time and muscle mass.

Adult↗
PubMed1975

Activity of single motor units from human forearm muscles during voluntary isometric contractions.

1. Microelectrode recordings from single motor units of the first dorsal interosseus and the extensor indicis muscles of normal human subjects were studied during voluntary, isometric contractions. The conduction velocity of the nerve fiber innervating the muscle unit was used as an estimator of the size of the motoneuron. 2. During slowly increasing contractions, the units were recruited at force levels which were closely correlated to conduction velocity. The units associated with low conduction velocity were recruited first, those with high conduction velocity, last. 3. If small, stepwise force increments were used instead of slowly, continuously increasing contractions, the units were first activated during the steps and became inactive during the subsequent plateaus. If higher steady-force levels were reached, the activity was maintained also during the plateaus. This steady force, where a unit remained continuously active independent of the rate of rise of tension, represents its tonic threshold. 4. The tonic threshold is positively correlated with conduction velocity, as is the threshold force of recruitment. As a consequence, high-threshold units have a large force range below tonic threshold where they can only be transiently activated, whereas low-threshold units have a large physiological force range above tonic threshold where they operate tonically. The phasic or tonic appearance of discharge pattern reflects quantitative differences in tonic threshold between units of different size. All units examined could be activated phasically (below) and tonically (above tonic threshold). No evidence was found indicating the existence of two qualitatively different classes of units corresponding to a tonic and phasic type, although both muscles investigated consist of about equal numbers of type I and type II muscle fibers. 5. The change in firing rate per unit force was inversely related to conduction velocity: the slower conducting units showed larger changes in firing rate per unit force than faster conducting units. This corresponds to the larger excitability of the smaller units indicated by their earlier recruitment. 6. The data of this study are consistent with the hypothesis that the functional characteristics of human motoneurons are determined by the graded excitability of motoneurons according to size.

Adult↗
PubMed1998

Differences in stretch reflex responses of elbow flexor muscles during shortening, lengthening and isometric contractions.

Stretch reflexes were evoked in elbow flexor muscles undergoing three different muscle contractions, i.e. isotonic shortening (SHO) and lengthening (LEN), and isometric (ISO) contractions. The intermuscle relationships for the magnitude of the stretch reflex component in the eletromyographic (EMG) activities of two main elbow flexor muscles, i.e. the biceps brachii (BB) and the brachioradialis (BRD), were compared among the three types of contractions. The subjects were requested to move their forearms sinusoidally (0.1 Hz) against a constant pre-load between elbow joint angles of 10 degrees (0 degrees = full extension) and 80 degrees during SHO and LEN, and to keep an angle of 45 degrees during the ISO. The perturbations were applied at the elbow angle of 45 degrees in pseudo-random order. The EMG signals were rectified and averaged over a period of 100 ms before and 400 ms after the onset of the perturbation 40-50 times. From the ensemble averaged EMG waveform, the background activity (BGA), short (20-50 ms) and long latency (M2, 50-80, M3, 80-100 ms) reflex and voluntary activity (100-150 ms) components were measured. The results showed that both BGA and reflex EMG activity of the two elbow flexor muscles were markedly decreased during the lengthening contraction compared to the SHO and ISO contractions. Furthermore, the changes of reflex EMG components in the BRD muscle were more pronounced than those in the BB muscle, i.e. the ratios of M2 and M3 magnitudes between BRD and BB (BRD:BB) were significantly reduced during the LEN contractions. These results would suggest that the gain of long latency stretch reflex EMG activities in synergistic muscles might be modulated independently according to the model of muscle contraction.

Adult↗
PubMed1991

Abnormal most-rapid isometric contractions in patients with Parkinson's disease.

Fast isometric elbow flexor muscle contractions of specified amplitude in six normal subjects were compared with those of 11 patients with Parkinson's disease. Despite treatment, all patients exhibited deficits in this motor task. Three patients were able to produce rapid force pulses with normal contraction times, but the variability of their force responses was increased in comparison with the highly stereotyped responses produced by normal subjects. The other eight patients had prolonged contraction times and segmentation of the force profiles. The integrated area of the first agonist EMG burst and the rate of development of force (dF/dt) were less at any target level than what was needed to produce a fast response. The area of the EMG burst, however, did increase with target amplitude, and the relative increase of dF/dt, with target amplitude, was normal. It is concluded that the motor program subserving fast muscle contraction is preserved in Parkinson's disease, but its execution is characterised by improper scaling of motor output.

Adult↗
PubMed1996

Metabolic heat production during fatigue from voluntary repetitive isometric contractions in humans.

The effect of repetitive isometric knee extensions on the energy cost of contraction was examined. The rate of temperature rise (dT/dt) was determined in test contractions at 30 and 50% of maximal voluntary contraction (MVC) force before and during 30% MVC repetitive isometric exercise (RIE) to exhaustion and regularly in a 30-min postexercise recovery period (n = 9). Pulmonary O2 uptake and muscle temperature (Tmus) were determined at regular intervals. During the 30% MVC test contractions, dT/dt was 5.6 +/- 0.6 mK/s in unfatigued muscle, increasing linearly by 68% during exercise. In the 50% MVC test contractions, dT/dt rose by 84% from 9.8 +/- 1.1 mK/s. dT/dt determined during test contractions at both force levels did not decrease significantly throughout the 30-min postexercise recovery period. The rise in dT/dt was paralleled by 76% increased in O2 uptake. In contrast, Tmus rose initially and then leveled off. The present data indicate that RIE induced a gradual rise in the rate of energy turnover associated with isometric force production. Neither increased Tmus nor recruitment of less economic type II fibers can fully explain the increased energy cost. We suggest that energetic changes may occur at the cellular level and argue that this may be associated with the changes in muscle mechanics occurring during fatigue from submaximal voluntary RIE.

Adult↗
PubMed1978

Isometric contraction and relaxation times of right and left ventricles in normal subjects and in patients with right ventricular overloading measured with bidirectional echocardiography.

With the use of bidirectional echocardiography, the isometric contraction (ICT) and relaxation times (IRT) of both ventricles were measured in 14 normal subjects (N), 6 cases with right ventricular (RV) diastolic overloading (DO), and 5 cases with RV systolic overloading (SO). The RVDO group consisted of patients with atrial septal defect of ostium secundum type who had large left-to-right shunting, and the RVSO group those with pulmonary hypertension of various origins. The mean ICT and IRT in N were 28.5 +/- 4.8 and 43.8 +/- 1.7 msec for RV, and 43.3 +/- 5.6 and 60.9 +/- 9.0 msec for left ventricle (LV), respectively. The RVDO group showed no significant change in the mean ICT and IRT of RV (29.7 +/- 4.6 and 54.3 +/- 11.8 msec, respectively), but significantly greater means of ICT and IRT of LV (58.5 +/- 9.5 and 83.6 +/- 14.1 msec, respectively. In the RVSO group, the mean ICT and IRT were 51.0 +/- 4.1 and 86.8 +/- 8.2 msec for RV, and 72.4 +/- 12.2 and 116.0 +/- 20.4 msec for LV, respectively. These values were all significantly greater than the means for both N and RVDO groups, except that the mean ICT of LV was insignificantly different between the RVDO and RVSO groups. It was noted that the intervals of LV tended to increase with the increasing intervals of RV, suggesting the changes in LV function secondarily due to RV overloading. It was concluded that the measurement of ICT and IRT of both ventricles is of clinical value for evaluation of overall cardiac function in the patients with RV overloading.

Adolescent↗
PubMed1986

Modification of relaxation of isometrically contracting rabbit papillary muscle by calcium, isoprenaline, ouabain and amiloride.

The influence of isoprenaline, ouabain, amiloride, as well as enhanced extracellular Ca on the time course of relaxation of the isometrically contracting papillary muscle was investigated. Under the experimental conditions used the relaxation rate shows different phases, which are modified characteristically. It is concluded that this reflects the different involvement of the Ca transport mechanisms for the relaxation.

Amiloride↗
PubMed1996

Motor unit control properties in constant-force isometric contractions.

1. The purpose of this study was 1) to characterize the decrease observed in mean firing rates of motor units in the first 8-15 s of isometric constant-force contractions and 2) to investigate possible mechanisms that could account for the ability to maintain force output in the presence of decreasing motor unit firing rates. 2. The decrease in mean firing rates was characterized by investigating myoelectric signals detected with a specialized quadrifilar needle electrode from the first dorsal interosseus (FDI) and the tibialis anterior (TA) muscles of 19 healthy subjects during a total of 85 constant-force isometric contractions at 30, 50, or 80% of maximal effort. The firing times of motor units were obtained from the myoelectric signals with the use of computer algorithms to decompose the signal into the constituent motor unit action potentials. Time-varying mean firing rates and recruitment thresholds were also calculated. 3. Motor units detected from the TA muscle were found to have a continual decrease in their mean firing rates in 36 of 44 trials performed during isometric ankle dorsiflexion at force values ranging from 30 to 80% of maximal effort and a duration of 8-15 s. Likewise, motor units detected in the FDI muscle displayed a decrease in firing rate in 32 of 41 trials performed during constant-force isometric index finger abduction for contractions ranging from 30 to 80% of maximal effort. In 14 contractions (16% of total), firing rates were essentially constant, whereas in 3 contractions (4%), firing rates appeared to increase. 4. Motor units with the higher recruitment thresholds and lower firing rates tended to display the greater decreases in firing rate over the constant-force interval, whereas motor units with lower recruitment thresholds and higher firing rates had lesser rates of decrease. Furthermore, increasing contraction levels tended to intensify the decrease in the motor unit firing rates. 5. Three possible mechanisms were considered as factors responsible for the maintaining of force output while motor units decreased their firing rates: motor unit recruitment, agonist/antagonist interaction, and twitch potentiation. Of these, motor unit recruitment was discarded first because none was observed during the 8-15 s duration of any of the 85 contractions. Furthermore, contractions outside the physiological range of motor unit recruitment (at 80% of maximal effort) revealed the same decreasing trend in firing rates, ruling out recruitment as the means of sustaining force output. 6. The role of agonist or antagonist muscle interaction was investigated with the use of the muscles controlling the wrist joint. Myoelectric signals were recorded with quadrifilar needle electrodes from the wrist extensor muscles while myoelectric activity in the wrist flexor muscles was concurrently monitored with surface electrodes during constant-force isometric wrist extension at 50% of maximal effort. Firing rates of the motor units in the wrist extensor muscles simultaneously decreased while the flexor muscles were determined to be inactive. 7. All the findings of this study regarding the behavior of the firing rates could be well explained by the reported characteristics of twitch potentiation that have been previously documented in animals and humans. 8. The results of this study, combined with the results of other investigators, provide the following scenario to explain how a constant-force isometric contraction is sustained. As the contraction progresses, the twitch force of the muscle fibers undergoes a potentiation followed by a decrease. Simultaneously, the "late adaptation" property of the motoneuron decreases the firing rate of the motor unit. Findings of this study suggest that voluntary reduction in firing rates also cannot be ruled out as a means to augment the adaptation in motoneurons. (ABSTRACT TRUNCATED)

Adult↗