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PubMed1987

Effect of muscle length on surface EMG wave forms in isometric contractions.

To elucidate the influence of muscle length on surface EMG wave form, comparisons were made of surface EMGs of the biceps and triceps brachii muscles during isometric contractions at different muscle lengths. Muscle lengths were altered by setting the elbow joint angle at several intervals between the limits of extension and flexion. The intensity of the isometric contractions was 25% of maximum voluntary contraction at the individual joint angles. Slowing was obvious in the EMG wave forms of biceps as muscle length increased. The so-called 'Piper rhythm' appeared when the muscle was more than moderately lengthened. The slowing trend with muscle lengthening, though less marked, was also seen in triceps. Zero-cross analysis revealed quasi-linear relationships between muscle length and slowing. Frequency analysis confirmed the development of 'Piper rhythm'. An attempt was made to interpret the slowing associated with muscle lengthening in terms of the propagation of myoelectric signals in muscle fibers. given the effect of muscle length on EMG wave forms, a careful control of joint angle may be required in assessing local making fatigue when using EMG spectral indices.

Adult↗

PubMed1977

Effects of fatiguing isometric and isotonic exercise on resisted and unresisted reaction time components.

Fractionated resisted and unresisted RT for a knee-extension task was assessed on 12 male subjects over a 10-day treatment period. The first 4 days were baseline days and were designed to stabilize all RT values as well as strength measures. For the next 6 days the subjects were alternately administered isometric and isotonic exercise designed to fatigue the quadriceps musculature. Although significant strength decrements were manifested for both isometric exercise (57%) and isotonic exercise (35%), no changes were shown in the unresisted fractionated RT components. Conversely all resisted RT changes were manifested in the muscular component suggesting a peripheral site for neuromuscular fatigue.

Adult↗

PubMed1979

Serial isometric contractions under imposed myotatic stretch conditions in high-strength and low-strength men.

The immediate effects of an imposed myotatic stretch on knee extensor force were studied in 12 high-strength and 12 low-strength men. Under nonfatigued pre-exercise conditions, significant tension increases of 6.5% for the high-strength group and 11.0% for low-strength subjects were observed as a result of the imposed stretch. An exercise treatment involving 28 serial isometic contractions, each of 5 s duration, with an intertrial rest period of 10 s was administered. This fatiguing exercise resulted in significant decrements in strength on the order of 28.0% and 18.5% for the high-strength and low-strength groups, respectively. A Similar treatment which included a 1 s imposed myotatic stretch during each trial resulted in a greater strength decrement for the low-strength group (26.4%) than for the high-strength subjects (15.0%). A neural factor involving the stretch reflex is tentatively suggested as a plausible explanation accounting for the observation that high-strength subjects fatigue faster than low-strength subjects under conditions of isometric exercise, while low-strength subjects fatigue faster than high-strength individuals in isometric exercise which is performed with an imposed stretch.

Adult↗

PubMed1980

Maximal isometric strength and fiber type composition in power and endurance athletes.

The relationship between maximum isometric strength and muscle fiber type composition was examined in seven endurance and eight power trained athletes. Knee extension strength and ankle extension strength was assessed on 10 separate days and muscle biopsies were taken from the vastus lateralis and gastrocnemius muscles. The percent composition of slow twitch (ST) fibers and fast twitch (FT) fibers was determined from the biopsy samples. Correlation between maximal knee extension strength and percent ST fibers of the vastus lateralis was found to be 0.80 (n = 8,p less than 0.05) for the power group and 0.63 (n = 7,N.S.) for the endurance group. Corresponding correlation coefficients for the relationship between ankle extension strength and gastrocnemius percent ST fibers were -0.94 (p less than 0.01) and -0.19 (N.S.), respectively. The results suggest that the relationship to be expected between muscle fiber type composition and maximum isometric strength may well depend upon the muscle group under study as well as the type of athlete in terms of specific training adaptations.

Humans↗

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↗

PubMed1987

Serum creatine kinase activity following repeated bouts of isometric exercise with different muscle groups.

The purpose of this study was to examine the relationship between the muscle mass involved in exercise and post-exercise serum creatine kinase (CK) elevation. Twelve untrained college-aged men completed three isometric exercises: one arm flexion (OAF), two arm flexion (TAF) and one leg knee extension (OLE). These exercises were balanced over subjects and days and separated by two week intervals. Each exercise consisted of 40 maximal isometric concentrations lasting for 10 s with a 20 s rest between contractions. Relative increases in serum CK for OAF, TAF, and OLE were 181 +/- 70% (SD), 222 +/- 69% and 297 +/- 67%, respectively. An ANOVA using a latin square design for analysis of carry over effects showed that these CK increases were not significantly different (p greater than 0.05). However, the increase in serum CK following the first exercise (379 +/- 90%), regardless of what it was (OAF, TAF, or OLE), was significantly greater (p less than 0.05) than those following bouts 2 and 3 (155 +/- 29%; 167 +/- 54%). Regression analysis indicated that post-exercise serum CK elevation was not related to the amount of muscle mass involved in the exercise (r = 0.30, p greater than 0.05) nor to muscle tension developed (r = 0.28, p greater than 0.05). We conclude that post-exercise serum CK elevation is not necessarily related to the muscle mass involved in the exercise. Because each exercise involved the use of different muscle groups, factors outside the exercising muscle may contribute to post-exercise serum enzyme activity.

Adolescent↗

PubMed1983

Skeletal muscle tension, flow, pressure, and EMG during sustained isometric contractions in humans.

In five healthy males sustained isometric torques during elbow flexion, knee extension, and plantar flexion correlated positively with intramuscular tissue pressure (MTP) in the range 0-80% of the maximal voluntary contraction (MVC). During passive compression of the muscle at rest 133-Xenon muscle clearance stopped when MTP reached diastolic arterial pressure (DAP) indicating that the muscle vascular bed was occluded. However, during sustained contraction this relation between DAP, flow and MTP was not seen. In two cases 133-Xenon clearance from M. soleus did not stop in spite of an 80% maximal contraction and MTP stayed below DAP. In other cases MTP would reach as high as 240 mm Hg before clearance was zero. In the deeper parts of the muscles MTP during contraction was increased in relation to the more superficial parts. The means values for the % MVC that would stop MBF varied between 50 and 64% MVC for the investigated muscles. Mean rectified EMG (MEMG) showed a high correlation to MTP during sustained exhaustive contractions: When MEMG was kept constant MTP also remained constant while the exerted force decreased; when force was kept constant both MEMG and MTP increased in parallel. This demonstrated that muscle tissue compliance is decreasing during fatigue. Muscle ischemia occurring during sustained isometric contractions is partly due to the developed MTP, where especially the MTP around the veins in the deeper parts of the muscle can be considered of importance. However, ischemia is also affected by muscle fiber texture and anatomical distorsion of tissues.

Animals↗

PubMed1978

Energy metabolism in different human skeletal muscles during voluntary isometric contractions.

The energy turnover in contracting skeletal muscle was studied by measuring the rate of temperature rise during voluntary, isometric contractions and circulatory arrest in M. soleus, M. sacrospinalis and M. biceps brachii in 14 males, by thermoelements inserted in the muscles. A linear relationship between rate of temperature rise and force intensity given as per cent of maximal voluntary contraction (MVC) was demonstrated in biceps (r = 0.95), but not so clearly confirmed in soleus (r = 0.73). Muscle biopsies were taken from the same muscles and fibre type distribution was determined histochemically by staining for ATPase. The rate of heat production at MVC showed positive correlation to the percentage of fast twitch (FT) fibres in the muscles (r = 0.90). Linear extrapolation indicates that the maximal energy turnover in human FT fibres is approximately six times that of slow twitch (ST) fibres during voluntary isometric contractions.

Adult↗

PubMed1977

Effect of isometric exercise on catecholamines in the coronary circulation.

Arterial and coronary sinus blood levels of catecholamines, adenosine 3', 5'-cyclic monophosphate (c-AMP) and lactate were measured during isometric exercise in fourteen patients. In no patient did lactate production occur. Mean resting total catecholamine levels both arterial (0.53 +/- 0.07 ng/ml; 2.94 +/- 0.38 nmol/l) and coronary sinus (0.4 +/- 0.08 ng/ml; 2.22 +/- 0.44 nmol/l), did not change significantly on exercise. Coronary sinus c-AMP levels fell on exercise from 11.5 +/- 0.8 nmol/l (resting) to 9.9 +/- 0.8 nmol/l (exercise) (P less than 0.01) with an arterial-coronary sinus difference of 1.2 nmol/l (P less than 0.01) on exercise. Our findings suggest that isometric exercise does not normally result in excessive cardiac symphathetic activity.

Adult↗

PubMed1988

Anaerobic chemical changes and mechanical output during isometric tetani of rat skeletal muscle in situ.

The time course was examined of the energy-rich phosphate usage and exerted isometric tetanic force in electrically stimulated rat quadriceps muscle. The maximal rate of energy-rich phosphate usage was calculated from the changes in the intramuscular concentrations of phosphocreatine, lactate, ATP and inosine monophosphate (IMP) and was somewhat higher than those calculated on the basis of exercise in vivo. The IMP concentration increased directly from the onset of the contraction until after about 11 s it remained constant. The increase in the IMP concentration coincided with a decrease in the ATP concentration. The relationship between mechanical output and energy usage was examined in two ways (i) by calculating the ratio time integral of the force (FTI) and the total energy-rich usage (Ptot) and (ii) by calculating the ratio Force (Ft) to the energy flux (dPtot/dt) at a certain time t. Whereas the ratio FTI/Ptot showed a hyperbolic relationship, the ratio Ft/(dPtot/dt) showed a parabolic relationship. From the latter finding and from the results described in the literature it is concluded that the ratio mechanical output/energy-rich phosphate usage depends on the conditions under which exercise is carried out. Recovery under aerobic conditions from a maximal tetanic isometric contraction sustained for 15 s was slow compared to results of experiments in vivo.

Anaerobiosis↗

PubMed1980

Scalp recorded slow potential shifts during isometric ramp and hold contractions in human subjects.

During voluntary isometric slow ramp contractions of the hand up to an internally defined target force level sustained negative potential shifts in the EEG were recorded from 6 human subjects. The potential shifts are composed of (a) a bilateral wide spread negativity with a maximum amplitude at the vertex, and (b) a smaller negative wave contralateral to the responding hand with a maximum over the respective motor area. Both components disappear during a voluntary isometric isotonic contraction when subjects have to maintain a stable force level for some seconds.

Action Potentials↗

PubMed1991

Cardiopulmonary responses to combined rhythmic and isometric exercise in humans.

A rhythmic (R) and an isometric (I) exercise were performed separately and in combination to assess their additive effects on arterial systolic (P(as)) and diastolic (P(ad)) blood pressures, heart rate (fc), and minute ventilation (VI). The isometric effort consisted of a 40% maximal voluntary handgrip contraction (MVC) performed for a duration of 80% of a previously determined 40% MVC fatiguing effort. The R effort consisted of a 13-min cycle effort at 75% maximum oxygen consumption (VO2max). For the combined efforts, I was performed starting simultaneously with or ending simultaneously with R. Data on nine subjects yield statistically significant evidence (P less than 0.05) that the effects of I and R are not additive for the following three cases: (1) P(as) when I and R are ended simultaneously (I alone = 4.9, SEM 0.5 kPa increase; R alone = no significant change from steady state; I + R = 1.2, SEM 0.4 kPa increase), (2) P(ad) when I and R are started simultaneously (I alone = 4.1, SEM 0.4 kPa increase; R alone = 0.7, SEM 0.3 kPa decrease; I + R = 1.9, SEM 0.4 kPa increase), and (3) P(ad) when I and R are ended simultaneously (I alone = 4.1, SEM 0.4 kPa increase; R alone = 0.3, SEM 0.5 kPa decrease; I + R = 0.8, SEM 0.3 kPa increase). For all other variables and cases, there is not sufficient evidence to conclude that the effects of I and R are not additive. We conclude that R and I exercises do not invariably produce strictly additive cardiopulmonary responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1988

Muscle blood flow during isometric activity and its relation to muscle fatigue.

The effect of isometric exercise on blood flow, blood pressure, intramuscular pressure as well as lactate and potassium efflux from exercising muscle was examined. The contractions performed were continuous or intermittent (5 s on, 5 s off) and varied between 5% and 50% maximal voluntary contraction (MVC). A knee-extensor and a hand-grip protocol were used. Evidence is presented that blood flow through the muscle is sufficient during low-level sustained contractions (less than 10% MVC). Despite this muscle fatigue occurs during prolonged contractions. One mechanism for this fatigue may be the disturbance of the potassium homeostasis. Such changes may also play a role in the development of fatigue during intermittent isometric contractions and even more so in the recovery from such exercise. In addition the role of impaired transport of substances within the muscle, due to long-lasting daily oedema formation, is discussed in relation to fatigue in highly repetitive, monotonous jobs.

Adult↗

PubMed1982

Response of blood pressure and plasma norepinephrine to propranolol, metoprolol and clonidine during isometric and dynamic exercise in hypertensive patients.

The effects of metropolol (beta 1-selective), propranolol (nonselective) and clonidine (central alpha-stimulant) on plasma norepinephrine, blood pressure and heart rate were assessed at rest, during isometric work and dynamic exercise in 15 patients with moderate hypertension. Metroprolol resulted in a lower diastolic blood pressure during isometric and dynamic exercise that propranolol, which was paralleled by a lower plasma norepinephrine level during dynamic work; both beta-adrenergic blocking compounds resulted in a lower heart rate in all test situations than that obtained with clonidine; clonidine produced similar control of diastolic blood pressure to that obtained with the beta-adrenergic blocking agents, but did not clearly attenuate the systolic blood pressure response to dynamic exercise. Plasma norepinephrine concentrations tended to be lowest following clonidine, especially during dynamic work. The findings support the hypothesis that the central action of clonidine inhibits peripheral release of norepinephrine, but is insufficient to attenuate cardiac stimulation by physical exercise. The fact that propranolol caused higher plasma norepinephrine concentrations than metoprolol during exercise may explain the difference in the blood pressure responses during exercise.

Adult↗

PubMed1989

The fatigability of two agonistic muscles in human isometric voluntary submaximal contraction: an EMG study. II. Motor unit firing rate and recruitment.

The recruitment and firing rate of biceps brachii (BB) and brachioradialis (BR) motor units (MUs) were studied in the course of fatiguing isometric contractions at 20%-30% of maximal voluntary contraction (MVC). MU recruitment generally occurred throughout the maintained contraction and was similar for BB and BR muscles. Newly recruited MUs started to discharge in the form of bursts, the duration of which increased until a continuous rhythmical firing was achieved. Within each burst, the first interval between two consecutive discharges was usually the shortest. MU threshold was lowered just after the limit time of the maintained contraction. The MU's firing rate either increased or remained stable as a function of the elapsed time. It is concluded that (1) in fatiguing isometric contractions at 20%-30% MVC contractile failure is mainly compensated for by MU recruitment and a lowered MU threshold and (2) differences between in surface changes in the electromyogram of BB and BR muscles cannot easily be explained by related differences in MU firing rate and recruitment.

Adult↗

PubMed1988

Influence of muscle dimensions on economy of isometric exercise in rat medial gastrocnemius muscles in situ.

The effect of muscle dimensions on economy (force-time integral divided by the amount of energy utilized) was investigated in male rats (body mass range 95-490 g), anaesthetized with pentobarbital. The medial gastrocnemius muscle in situ performed 6 maximal isometric contractions of 350 ms duration (1.s-1) at twitch optimum length at 35 degrees C. The areas under the 6 time-force curves were added to obtain force-time integral of the experiment. Differences of concentrations of ATP, phosphocreatine and lactate between experimental and contralateral (resting) muscles were used to calculate high-energy phosphate consumption due to stimulation. Muscle mass and cross-sectional area increased (approximately +400% and +300%, respectively) over the rat body mass range studied. Muscle length and length of the most distal fibre bundle increased by approximately 17 mm and 4 mm, respectively. Force-time integral (N.s) increased proportional to cross-sectional area whereas high-energy phosphate consumption (mumoles) increased proportional to muscle mass. The relative fraction of the total energy consumption utilized for force-independent processes was independent of rat body mass. The economy of the actomyosin system was unaffected during growth, whereas economy of the whole muscle decreased during growth by approximately 30% (p less than 0.001). The effect of muscle dimensions on economy is discussed with respect to human endurance capacity measured by voluntary isometric contractions.

Animals↗

PubMed1990

Phonomyogram and electromyogram relationships with isometric force reinvestigated in man.

Contracting muscle generates sounds which can be recorded easily by means of a microphone. To determine if a phomomyogram (PMG) can be used to monitor muscle force, a comparison was made between simultaneous recordings of PMG and monopolar electromyogram (EMG) from the isometrically contracting biceps brachii muscle and the external flexion force. Locations of the monopolar electrode and microphone were identified in relation to the motor point. Whatever the recording site, PMG amplitude was proportional to EMG amplitude and both showed a quadratic relationship to muscle force. Changes in the PMG spectrum with force were similar to those in EMG, i.e. the mean power frequency increased up to about 30% maximal voluntary contraction and then reached a plateau. Despite a slightly higher variability, PMG was shown to be a valid index of muscular isometric force. At the same force, the amplitude of both PMG and EMG was lower in the prone than in the supine position of the hand. This result indicated a selective recording of biceps brachii muscle activity.

Adult↗

PubMed1995

The relationship between the pressor response to involuntary isometric exercise and the contractile protein profile of the active muscle in man.

The present investigation examined the relationship between the pressor response during electrically evoked isometric ankle plantar flexion and the contractile protein profile of the active muscle in seven young men [mean (SD) age, 26 (6) years] and five older men [70 (4) years]. Muscle biopsy samples were taken from lateral gastrocnemius (LG) and soleus (SOL) of each subject. These were analysed for isomyosin composition using non-denaturing pyrophosphate polyacrylamide gel electrophoresis. The degree of association was examined between the cardiovascular changes and the fast isomyosin content of LG and SOL individually and in combination (SOL/LG). In the total subject group there was no association between the heart rate response or the change in systolic blood pressure (BP) and the fast isomyosin composition. However, the change in diastolic BP was significantly associated with the fast isomyosin composition of SOL/LG (delta diastolicBP = 0.31 + 0.045% FM SOL/LG, r = 0.65, P = 0.029). These findings suggest that the magnitude of the peripheral reflex mediated pressor response to isometric exercise and the fast isomyosin content of the active muscle are related.

Adult↗