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PubMed1992

Facilitation of responses to motor cortex stimulation: effects of isometric voluntary contraction.

In 7 normal subjects we compared the facilitatory effect of isometric contraction of the tibialis anterior on the size of electromyographic responses evoked in this muscle by electric stimuli applied over the cervical column and by electric and magnetic percutaneous stimulation of the motor cortex. No significant difference was found between the degrees of facilitation of the responses to any of the stimuli. Using collision techniques, we also showed that the pyramidal fibers activated by spinal and cortical stimuli are the same. Facilitation induced by isometric contraction (20% maximum) was of similar or greater magnitude than that found with constant vibration of the tendon of the target muscle. In cases where vibration and contraction had equal facilitatory effects, there was no further facilitation of the responses when both conditions were applied together. These findings indicate that the facilitatory effect of isometric contraction of the target muscle essentially originates at a spinal level rather than in the motor cortex.

Adult↗
PubMed1985

Effects of prior instruction and anaesthesia on long-latency responses to stretch in the long flexor of the human thumb.

Long-latency (40-80 ms) electromyographic (e.m.g.) responses of the contracting flexor pollicis longus to stretches applied at the thumb-tip, were studied in normal human subjects. Stretches were applied during four classes of contraction: (i) isometric 'hold', in which the subject held a steady isometric contraction; (ii) isometric tracking, in which the subject tracked a steadily rising force target; (iii) isotonic tracking, in which the subject flexed against a constant torque to track a position target; (iv) weight-lifting, in which the subject lifted a weight hung at one end of a lever by pressing the thumb-tip on the other end of the lever. The effects on the responses of prior instructions to 'resist' or to 'let go', and of local anaesthesia of the thumb, were studied. The ability to modify the size of the long-latency e.m.g. response in accordance with prior instruction was variable. All subjects tested could do so during isometric holding contractions, but many could not do so during the other forms of contraction. Local anaesthesia of the thumb significantly reduced the long-latency e.m.g. response in only some subjects, and abolished it in none. The reduction was most reliably seen for isometric force tracking contractions. During thumb anaesthesia in different subjects, there was a significant correlation between the proportional increase in apparent heaviness of an object lifted by thumb flexion and the proportional reduction in the size of the long-latency e.m.g. response to muscle stretch.

Adult↗
PubMed2003

Long-term spinal cord injury increases susceptibility to isometric contraction-induced muscle injury.

Complete spinal cord injury (SCI) results in inactivation and unloading of affected skeletal muscles. Unloading causes an increased susceptibility of muscle to contraction-induced injury. This study used magnetic resonance imaging (MRI) to test the hypothesis that isometric contractions would evoke greater muscle damage to the quadriceps femoris muscle (mQF) of SCI subjects than that of able-bodied (AB) controls. MR images were taken of the mQF prior to, immediately post, and 3 days post electromyostimulation (EMS). EMS consisted of five sets of ten isometric contractions (2 s on/6 s off, 1 min between sets) followed by another three sets of ten isometric contractions (1 s on/1 s off, 30 s between sets). Average muscle cross-sectional area (CSA) and the relative areas of stimulated and injured muscle were obtained from MR images by quantifying the number of pixels with an elevated T2 signal. SCI subjects had significantly greater relative area [90 (2)% versus 66 (4)%, P<0.05; mean (SE)] but a lesser absolute area [16 (3) cm(2) versus 44 (6) cm(2), P<0.05] of mQF stimulated than AB controls. During EMS, peak torque was reduced by 66% and 37% for SCI and control subjects, respectively. Three days post EMS, there was a greater relative area of stimulated mQF injured for the SCI subjects [25 (6)% versus 2 (1)%, P<0.05]. Peak torque remained decreased by 22% on day 3 in the SCI group only. These results indicate that affected muscle years after SCI is more susceptible to contraction-induced muscle damage, as determined by MRI, compared to AB controls. They also support the contention that electrically elicited isometric contractions are sufficient to cause muscle damage after a prolonged period of inactivity.

Adult↗
PubMed2006

Effects of visual stimulation on cortico-spinal coherence during isometric hand contraction in humans.

The effects of visual stimuli on cortico-spinal synchronization were investigated by measuring the coherence between electroencephalogram (EEG) and electromyogram (EMG) during isometric contraction of the first dorsal interosseous muscle of the right hand. Because a spinal motoneuron and the corresponding muscle fibers form a motor unit with one-to-one correspondence of their action potentials, the EMG indirectly measures the activity of the corresponding spinal neuronal group. The tasks were isometric contraction (Control condition); and isometric contraction with concurrent ignoring of visual stimuli (VS condition). By comparing the Control and VS conditions, the following results were obtained. The coherence increased significantly in magnitude, but was unchanged in frequency range (beta band) and scalp location; the EEG and EMG spectral power in the beta band were unchanged in amplitude; and the alpha and gamma bands of EEG spectral power were significantly increased and decreased, respectively. These findings suggest that the cortico-muscular coherence reflects the cognitive effort needed to maintain isometric muscle contraction. When visual stimuli need to be ignored, the cognitive effort and cortico-spinal coherence are enhanced.

Adult↗
PubMed2003

Actomyosin energy turnover declines while force remains constant during isometric muscle contraction.

Energy turnover was measured during isometric contractions of intact and Triton-permeabilized white fibres from dogfish (Scyliorhinus canicula) at 12 degrees C. Heat + work from actomyosin in intact fibres was determined from the dependence of heat + work output on filament overlap. Inorganic phosphate (Pi) release by permeabilized fibres was recorded using the fluorescent protein MDCC-PBP, N-(2-[1-maleimidyl]ethyl)-7-diethylamino-coumarin-3 carboxamide phosphate binding protein. The steady-state ADP release rate was measured using a linked enzyme assay. The rates decreased five-fold during contraction in both intact and permeabilized fibres. In intact fibres the rate of heat + work output by actomyosin decreased from 134 +/-s.e.m. 28 microW mg(-1) (n = 17) at 0.055 s to 42% of this value at 0.25 s, and to 20% at 3.5 s. The force remained constant between 0.25 and 3.5 s. Similarly in permeabilized fibres the Pi release rate decreased from 5.00 +/- 0.39 mmol l(-1) s(-1) at 0.055 s to 39% of this value at 0.25 s and to 19% at 0.5 s. The steady-state ADP release rate at 15 s was 21% of the Pi rate at 0.055 s. Using a single set of rate constants, the time courses of force, heat + work and Pi release were described by an actomyosin model that took account of the transition from the initial state (rest or rigor) to the contracting state, shortening and the consequent work against series elasticity, and reaction heats. The model suggests that increasing Pi concentration slows the cycle in intact fibres, and that changes in ATP and ADP slow the cycle in permeabilized fibres.

Actomyosin↗
PubMed1994

Somatosensory evoked potentials modification related to isometric voluntary contraction.

Somatosensory evoked potentials were elicited by applying an electrical stimulus to the median nerve while the subjects performed a handgrip isometric contraction. Somatosensory evoked potentials (SEPs) were recorded from scalp positions C3 + 2 and C3-2. SEPs during the increase phase and decrease phase of force, and during the hold phase (period of maintenance of steady force) of isometric contraction were compared with potentials during rest. SEPs during the three phases of isometric contraction were also compared. Changes in SEP latencies were not observed. The increase or decrease phase of force and the hold phase caused suppression of the relatively late N55-P100 component when compared to rest. There was no difference in the amplitude of SEPs during the increase or decrease phase. During the hold phase the early N20-P30 component was increased when compared to the other two phases. The selective changes of SEPs during the three phases of isometric-anisotonic contraction suggested task dependence of the gating process.

Adult↗
PubMed1990

[Cardiac effects of exhausting isometric muscular contraction in trained and endurance athletes].

The effect of exhausting isometric contraction (60% of the maximal voluntary contraction) on left ventricular function has been investigated using echocardiography (M and B mode) in 4 groups, each of 8 subjects (27 +/- 5 years; mean +/- SD): weight lifters, 2 years of training; amateur cyclists, 4 years of training; sedentary controls, and hypertensive subjects, 1st WHO class. Heart rate, arterial pressure, ventricular diameters (diastolic and systolic), wall thickness, wall stress and the double product (heart rate x arterial pressure) were determined at rest, at exhaustion and after 30, 90 and 180 s of recovery. Maximal voluntary contraction was 48 +/- 9 kg in weight lifters; 32 +/- 5 kg in amateur cyclists; 32 +/- 4 kg in sedentary control and 36 +/- 7 kg in 1st WHO class. Exhaustion time ranged from 80 s to 120 s. Our results showed that: 1) in all subjects at exhaustion, arterial pressure and heart rate increased significantly and the index of ventricular function was decreased but not significantly; 2) after 180 s of recovery both arterial pressure and heart rate returned to the pre-exercise value; 3) wall stress was significantly lower in weight lifters than in sedentary controls; 4) the double product (index of myocardiac oxygen consumption) resulted lower in amateur cyclists that in sedentary controls. The hypertrophy of the heart could justify the slight increase in wall stress observed in weight lifters during isometric exercise. However, the low index of myocardiac oxygen consumption in amateur cyclists probably reflects the changes in cardiovascular function due to the endurance training. In hypertensive subjects, the isometric contraction causes significant and substantial increase of the wall stress and the double product.

Adult↗
PubMed1989

Respiratory responses to sustained isometric muscle contractions in man: the effect of muscle mass.

1. Respiratory responses to sustained isometric contractions of a small mass of muscle (the finger flexors) during handgripping, and of a larger mass of muscle (the quadriceps) during extension of the leg at the knee, have been studied in man. 2. For both masses of muscle the increases of ventilation and of oxygen consumption were greater for contractions at 40% maximum voluntary contraction (MVC) than for contractions at 20% MVC. 3. The increase of ventilation was not related to the mass of muscle involved. 4. At 20% MVC oxygen consumption during contraction of the quadriceps was greater than that during handgripping. At 40% MVC the oxygen consumptions were similar. The oxygen debts following both handgrip and knee extensor contractions at 20% MVC were negligible. Following 40% MVC contractions of the quadriceps a significant oxygen debt was recorded but no debt was apparent following 40% MVC contractions of the finger flexors. 5. The increases of ventilation during isometric exercise were generally inappropriately high for the increases of gas exchange. This led to reductions of the end-tidal carbon dioxide pressure (PET,CO2), especially towards the end of exercise. 6. Following 40% MVC handgripping hyperventilation continued despite the reduced alveolar PCO2. By contrast, following 40% MVC knee extension PET,CO2 transiently rose above the resting level, but did not stimulate ventilation. 7. It appears that following fatiguing isometric contractions hyperventilation continues and appears to be independent of alveolar PCO2. It is suggested that stimuli which increase ventilation during exercise may continue to act during the early phase of recovery.

Adult↗
PubMed2006

Myocardial contractility in chickens (Gallus gallus): analysis of systolic time intervals.

The avian cardiovascular system is of special interest because avian hearts are relatively larger than mammalian hearts, and activation of ventricular myocardium in birds has a "flash" pattern. Systolic time intervals and indices of myocardial contractility were examined in anaesthetized open-chest chickens by polycardiography, including synchronous recordings of electrocardiogram, phonocardiogram, and apex cardiogram. The asynchronous contraction time, isometric contraction time, pre-ejection period and ejection time were 26 +/- 3 (Mean +/- SD), 21 +/- 9, 47 +/- 12, and 83 +/- 23 ms, respectively, for heart rates of 260 +/- 57 bpm. The myocardial tension index, isometric contraction index and the pre-ejection period/ejection time ratio were 0.39 +/- 0.11, 0.42 +/- 0.10, and 0.54 +/- 0.14, respectively. A "flash" pattern of ventricular myocardial depolarization causes more rapid excitation and as a consequence shorter asynchronous contraction time of relatively larger chicken hearts compared with rabbit hearts. Inverse relation (P < 0.05) of the asynchronous contraction time to the heart rate in chickens is probably associated with the specific activation pattern of avian ventricles. Establishment of the values of systolic time intervals will facilitate a better understanding of cardiac function in birds. The obtained results are discussed in comparison with the rabbit. The indices calculated from the systolic time intervals show disadvantageous contractile function of chicken heart compared to rabbit heart.

Animals↗
PubMed1991

Changes in perceived finger force produced by muscular contractions under isometric and anisometric conditions.

We compared matching of finger forces under isometric conditions with matching of forces produced against a spring load (anisometric conditions) in twenty normal subjects. The instruction was to generate the same force in both hands holding a grip between thumb and index finger in each hand. Visual feedback indicating the target force and the actual force applied were presented for one (reference) hand only. The forces produced in each hand were measured continuously during matching trials. A special device provided the opportunity to change from isometric to anisometric force production. Matching was required under symmetric conditions, in which force was generated in both hands either isometrically or anisometrically, as well as under asymmetric conditions in which isometric force has to be matched to anisometric force or the reverse. Under symmetric conditions matching error was consistently smaller in anisometric than in isometric force production. However, the striking feature was a severe mismatch between hands when forces had to be produced differently. For most subjects, a force generated against the spring load in the reference hand was greatly overestimated by the matching hand working isometrically. For the reverse condition consistent underestimations were observed. This effect cannot be attributed to left/right differences or a simple confusion of subjects in the asymmetric tasks. Some of the factors confounded with the conditions of force production were ruled out as an explanation by additional experimental controls. The mismatch neither depends on signals related to different finger positions associated with target forces nor is it alleviated when differently produced forces are matched sequentially.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1999

Pharmacologic evaluation of isometric contraction-relaxation coupling indexes in rabbit ventricular muscle.

Investigations of the coupling between contraction and relaxation (contraction-relaxation [CRC] process) in isometric conditions are essential in determining whether pharmacologic interventions or cardiac diseases specifically modify isometric relaxation (intrinsic lusitropic effect) or change it in proportion with the accompanying changes in contractility (or inotropy). For this purpose, the CRC process is quantified by various indexes, derived from differentiation and/or curve fitting the whole or relaxation phase of the isometric twitch, one of the most used being tau, the time constant of the final iso(volu)metric phase of relaxation. Nevertheless, the possible redundancy and validity of such indexes have not been thoroughly investigated. Accordingly, we performed a pharmacologic evaluation of such indexes in isolated rabbit ventricular muscles isometrically contracting in vitro, using modifiers of either intracellular Ca(2)+ handling (nifedipine, ryanodine, 2,5-di-tert-butyl-benzohydroquinone, all negative inotropic compounds, and BAY K 8644, a positive inotropic drug), or myofibrillar Ca(2)+ sensitivity (CGP 48506, a Ca(2)+ sensitizer, and butanedione monoxime, a Ca(2)+ desensitizer, respectively positive and negative inotropic compounds). The isometric twitch in control conditions and in the presence of increasing concentration of each compound was analyzed to determine the classically used CRC and/or lusitropic indexes, derived either from single parameters such as the maximal rate or contraction and relaxation (+dT(max) and -dT(max), respectively), or from curve fitting of the whole, or part, of the twitch. As the rate of isometric relaxation is dependent on myofilament properties, we expected that compounds modifying myofibrillar Ca(2)+ sensitivity in an opposite direction (CGP 48506 vs butanedione monoxime) would be the only drugs exerting an intrinsic lusitropic and opposite effect on a validated CRC index. Results showed that (1) none of the tested compounds affected the slope of the linear relationship between peak twitch tension and dT(max), a previously assumed CRC index, sensitive only to myofibrillar Ca(2)+ sensitivity modifiers; (2) the lusitropic parameter B, derived from mathematical curve fitting of the whole isometric twitch, and the ratio +dT(max)/dT(max), exhibited similar drug- and dose-dependency, but no opposite sensitivity to CGP 48506 and BDM for either index; and (3) negative inotropic compounds dose-dependently slowed relaxation (and conversely for positive inotropes), whether the latter was quantified by the rate constant beta, derived from double exponential curve fitting of the whole relaxation phase, or by the time constants tau(L) and tau(E), derived from the curve fitting (logistic and monoexponential, respectively) of the final phase of relaxation. Nevertheless, the pharmacologicly induced changes in beta were statistically significant at lower concentrations and exhibited less individual variability, compared with the time constants. We demonstrate that intrinsic lusitropic changes can be quantified by the value of the slope of the relationship relating beta to peak isometric tension: the slope value was unchanged by Ca(2)+ handling modifiers, decreased by CGP 48506, and reversed by BDM (indicating number, negative, and positive intrinsic lusitropic effects respectively). Based on these data, we propose that the linear relationship between beta and peak isometric tension could be used a new method to assess whether pharmacologic interventions or cardiac diseases exert intrinsic effects on isometric relaxation.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1977

Isometric cardiac contraction. a possible cause of the disorganized myocardial pattern of idiopathic hypertrophic subaortic stenosis.

The bizarre septal architecture of idiopathic hypertrophic subaortic stenosis (IHSS) may be secondary to a small systolic cavity with late systolic isometric contraction. We examined ventricular muscle for IHSS-type muscle-fiber disarray in infant hearts in which isometric contraction would occur during development-namely, pulmonary or aortic-valve atresia with intact ventricular septum and normal atrio-ventricular valves. Fifteen patients with aortic atresia and 10 with pulmonary atresia were compared to 25 normal controls of matched age and heart weight. Aortic atresia showed disorganization of muscle-fiber alignment of left ventricle, particularly septum, and intramural coronary-artery changes virtually identical to IHSS. Pulmonic atresia had similar right ventricular disarray and vessel changes, again most marked in the septum. Thus, cardiac muscle-cell disorientation similar to IHSS occurs in infant ventricles with outflow-tract obstruction. This IHSS-type myocardial-fiber disarray may result from altered wall stresses related to isometric systolic contraction.

Adult↗
PubMed1989

Microsecond rotational motion of spin-labeled myosin heads during isometric muscle contraction. Saturation transfer electron paramagnetic resonance.

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to detect the microsecond rotational motions of spin-labeled myosin heads in bundles of skinned muscle fibers, under conditions of rigor, relaxation, and isometric contraction. Experiments were performed on fiber bundles perfused continuously with an ATP-regenerating system. Conditions were identical to those we have used in previous studies of myosin head orientation, except that the fibers were perpendicular to the magnetic field, making the spectra primarily sensitive to rotational motion rather than to the orientational distribution. In rigor, the high intensity of the ST-EPR signal indicates the absence of microsecond rotational motion, showing that heads are all rigidly bound to actin. However, in both relaxation and contraction, considerable microsecond rotational motion is observed, implying that the previously reported orientational disorder under these conditions is dynamic, not static, on the microsecond time scale. The behavior in relaxation is essentially the same as that observed when myosin heads are detached from actin in the absence of ATP (Barnett and Thomas, 1984), corresponding to an effective rotational correlation time of approximately 10 microseconds. Slightly less mobility is observed during contraction. One possible interpretation is that in contraction all heads have the same mobility, corresponding to a correlation time of approximately 25 microseconds. Alternatively, more than one motional population may be present. For example, assuming that the spectrum in contraction is a linear combination of those in relaxation (mobile) and rigor (immobile), we obtained a good fit with a mole fraction of 78-88% of the heads in the mobile state. These results are consistent with previous STEPR studies on contracting myofibrils(Thomas et al., 1980). Thus most myosin heads undergo microsecond rotational motions most of the time during isometric contraction, at least in the probed region of the myosin head.These motions could arise primarily from the free rotations of heads detached from actin. However, if most of these heads are attached to actin during contraction, as suggested by stiffness measurements, this result provides support for the hypothesis that sub-millisecond rotational motions of actin-attached myosin heads play an important role in force generation.

Animals↗
PubMed1990

Myosin heads have a broad orientational distribution during isometric muscle contraction: time-resolved EPR studies using caged ATP.

To study the orientation of spin-labeled myosin heads in the first few seconds after the production of saturating ATP, we have used a laser flash to photolyze caged ATP during EPR data acquisition. Rabbit psoas muscle fibers were labeled with maleimide spin label, modifying 60% of the myosin heads without impairing muscle fiber biochemical and physiological activity (ATPase and force). The muscle bundles were incubated for 30 min with 5 mM caged ATP prior to the UV flash. The flash, from an excimer laser, liberated 2-3 mM ATP, generating maximum force in the presence of Ca2+ and relaxing fully in the absence of Ca2+. Control experiments, using fibers decorated with labeled myosin subfragment, showed that the flash liberates sufficient ATP to saturate myosin active sites in all regions of the muscle bundles. To increase the time resolution, and to minimize the time of the contraction, we followed in time the intensity at a single spectral position (P2), which is associated with the high degree of orientational order in rigor. ATP liberation produced a rapid decrease of P2 with liberation of ATP, indicating a large decrease in orientational order in both relaxation and contraction. This transient was absent when caged AMP was used, ruling out nonspecific effects of the UV flash and subsequent photochemistry. The steady-state level of P2 during contraction was almost as low as that reached in relaxation, although the duration of the steady state was much more brief in contraction. Upon depletion of ATP in contraction, the P2 intensity reverted to the original rigor level, accompanied by development of rigor tension. The steady-state results obtained in the brief contractions induced by caged ATP are quantitatively consistent with those obtained in longer contractions by continuously perfusing fibers with ATP. In isometric contraction, most (88% +/- 4%) of the heads are in a population characterized by a high degree of axial disorder, comparable to that observed for all heads in relaxation. Since the stiffness of these fibers in contraction is 80% of the stiffness in rigor, it is likely that most of the heads in this highly disoriented population are attached to actin in contraction and that most actin-attached heads in contraction are in this disoriented population.

Adenosine Triphosphate↗
PubMed2000

Significance of peripheral afferent input to the alpha-motoneurone pool for enhancement of tremor during an isometric fatiguing contraction.

The objective of this study was to investigate the contribution of peripheral afferent input to the enhancement of isometric tremor during a sustained submaximal isometric contraction. It was hypothesised that during muscle fatigue, when excitatory drive is high, peripheral afferent input may augment oscillations in the stretch reflex arc and result in bursting motor-unit activity and increased tremor. Nine healthy subjects maintained isometric plantar flexions at 30% of their maximum voluntary contraction until the limit of endurance, under three test conditions. Two paradigms were used to reduce afferent input to the triceps surae alpha-motoneurone pool: (1) continued vibration of the Achilles tendon, and (2) ischaemic partial block of the tibial nerve. These were compared to a control experiment, in which there was no intervention. By recording H-reflexes from the gastrocnemius and soleus muscles, it was possible to assess the effectiveness of reducing the afferent input. When H-reflex suppression had stabilised, the fatiguing contraction was commenced and tremor was computed from the continuously recorded torque signal. Superimposed maximum twitches were elicited as indirect measures of excitatory drive. The increase in tremor root mean square throughout the fatiguing contraction was significantly less for both the vibration and ischaemic conditions. Furthermore, tremor mean power frequency decreased significantly with endurance time in the control experiment, while no significant change was seen in the other two experimental conditions. It is concluded that the enhancement of isometric tremor seen during a fatiguing submaximal isometric contraction is facilitated by peripheral afferent input to the alpha-motoneurone pool.

Adult↗
PubMed1992

[Control of isometric muscle contraction in muscle hypotonia of central origin: EMG mapping analysis].

Electromyographic and mechanographic investigations in patients with muscular hypotonia, which is, for instance, a side-effect after stereotactic treatment of tremor syndromes, permit the presumption that in this sensomotor open-loop situation the decreased muscular resistance to stretching during isometric contraction (initial stiffness) is caused by changes of muscular innervation pattern. Probably, the innervation pattern during tonic activity is changed by a shift of a more tonic motoneurone behaviour to motoneurone activities with predominantly phasic characteristics. In 17 controls and 4 patients with muscular hypotonia caused by stereotactic lesions of VIM area (treatment of tremor syndromes) the EMG of right and left side forearm flexors (especially the activity of the M. biceps brachii) was investigated by a sophisticated, topographically oriented 16-channel-surface-EMG-technique ("EMG-Mapping") during slight isometric contraction. EMG-Maps of forearm flexors (especially of M. biceps brachii) in patients with centrally evoked muscular hypotonia demonstrate that in these open-loop conditions the motor control is changed. For this the reason could be a shift of the activated motor units from a predominantly static to a more phasic functional behaviour. The latest results on muscular activation processes in cats support this presumption.

Adult↗
PubMed1993

Electromyographic turns analysis of sustained contraction in human masseter muscles at various isometric force levels.

The jaw-closing muscles differ from peripheral limb muscles in that the maximum attainable force does not decline following a sustained isometric contraction. Also, the averaged electromyographic (EMG)/force ratio in these muscles does not change with sustained contraction, whereas it increases in fatiguing limb muscles. The present study analysed EMG records from masseters in healthy male subjects. No statistically significant difference was seen between average rectified EMG signals at the beginning or the end of a sustained isometric contraction at 25-100% of maximum voluntary contraction (MVC). However, when the number of turns, or reversals of direction, was taken at various percentage MVC levels, a significant decrease was seen after 60s or at the end of the contraction. The turns/force ratio decreased monotonically with percentage MVC, but the ratio was not significantly different at the end of a contraction from that at the beginning. This result confirms an earlier suggestion that neuromuscular fatigue does not accompany sustained contractions of these muscles.

Adult↗