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At least 199 records · Page 11Linked to original sources

PubMed1998

The isometric force that induces maximal surface muscle deoxygenation.

To determine the external force that induces maximal deoxygenation of brachioradialis muscle 32 trained male subjects maintained isometric contractions using the elbow flexor muscles up to the limit time (isotonic part of the isometric contraction, IIC) and beyond that time for 120 s (anisotonic part of the isometric contraction). During IIC each subject maintained relative forces of either 25% and 70% maximal voluntary contraction (MVC), 50% and 100% MVC, or 40% and 60% MVC. Muscle oxygenation was assessed using a near infrared spectroscope, and expressed as a percentage of the reference value (deltaO2rest) which was the difference between the minimal oxygenation obtained after 6 min of ischaemia at rest and the maximal reoxygenation following the release of the tourniquet. During IIC at 25% MVC, muscle oxygenation decreased to 17 (SEM 3)% deltaO2rest, then it levelled off [25 (SEM 1)% deltaO2rest]. After the point at which target force could not be maintained, reoxygenation was very weak. During IIC at 40%, 50%, 60%, and 70% MVC, the lowest muscle oxygenation values were obtained after 15-20 s of contraction and corresponded to -18 (SEM 6), -59 (SEM 12) -31 (SEM 6), and -29 (SEM 6)% deltaO2rest, respectively. For the contraction at 100% MVC, the lowest oxygenation [-19 (SEM 9)% deltaO2rest] was obtained while force was decreasing (69% MVC). During the anisotonic part of the isometric contractions, the greatest reoxygenation rate was obtained after 50% MVC IIC (P < 0.001). Our results showed that during isometric elbow flexions between 25% and 100% MVC, there was no linear relationship between external force and muscle oxygenation, and that the maximal deoxygenation of the brachioradialis muscle was obtained at 50% MVC.

Adult↗

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↗

PubMed2003

Rat medial gastrocnemius muscles produce maximal power at a length lower than the isometric optimum length.

The interaction of relative muscle length and force-velocity characteristics was investigated in the fully activated rat medial gastrocnemius muscle in situ. Average maximal isometric force (as a percentage of the of the maximal isometric force at L(o,iso)) at relative lengths measured below isometric optimum (L(o,iso)) was 96% at L(o,iso)-2 mm, 88% at L(o,iso)-4 mm and 58% at L(o,iso)-6 mm. Force-velocity curves were obtained at the four relative muscle lengths. There were no significant differences in maximal shortening velocity (approximately 280 mm x s(-1)) between the different muscle lengths. The highest power output (P<0.05) was found at L(o,iso)-2 mm (mean+/-SEM 435+/-19 mW). Peak power values at L(o,iso) (390+/-10 mW) and L(o,iso)-4 mm (395+/-12 mW) were not significantly different, whereas peak power was lowest (P<0.05) at L(o,iso)-6 mm. There was a significant (P<0.01) shift of approximately 1.5 mm in optimum muscle length for force generation during shortening contractions compared with isometric contractions. Shortening velocity had only a minor influence on optimum muscle length for force generation. It is concluded that fully activated muscles produce their maximal power at a length lower than L(o,iso). The difference in optimum length between isometric and dynamic contractions may be related to length-dependent variations in sarcomere length in series during shortening.

Animals↗

PubMed1992

Isometric and isokinetic measurement of hamstring and quadriceps strength.

This study evaluates quadriceps strength at varying movement velocities in a group of healthy subjects. Specific objectives included (1) investigating the effect of angular velocity on peak torque (PT) (2) investigating the effect of angular velocity on joint angle at peak torque (JAPT) and (3) evaluating whether the use of a relatively new isokinetic testing device yields new insights to previously accepted relationships between angular velocity and PT or JAPT. Twenty healthy subjects were tested for isometric and isokinetic knee extension and flexion strength on a LIDO Active Isokinetic Rehabilitation System at velocities of 0 degrees, 60 degrees, 120 degrees, and 180 degrees/sec. Isometric PT (0 degrees/sec) was found to be highly correlated with isokinetic PT for both extensors and flexors at all velocities (r = .88-.93, p less than .01). PT declined significantly as angular velocity increased for both extension (p less than .01) and flexion (p less than .05). A relationship between isometric and isokinetic JAPT was significant for extension only at 60 degrees and 180 degrees/sec (r = .48-.52, p less than .05). These results indicate that knee isometric and isokinetic PT as measured on the LIDO Active system are highly related for both extension and flexion. PT declined as isokinetic angular velocity increased. Isometric and isokinetic JAPT are significantly related only for extension.

Adult↗

PubMed1988

Comparison of cerebellar intention tremor under isotonic and isometric conditions.

The characteristics of cerebellar intention tremor were investigated by comparing the properties of tremor following movements about the elbow with that following isometric contractions of biceps and triceps brachii. Cerebellar dysfunction was produced by local, reversible cooling of the dentate and interposed nuclei in three Cebus monkeys. Cerebellar nuclear cooling disrupted isometric contractions to the aimed target and produced oscillations in torque. Whereas the cerebellar tremor that followed movements was regular and had a frequency of 3-5 Hz, the oscillations in the isometric task were irregular and were often of a lower frequency. In addition, cycles of tremor following movements were in phase from trial to trial, while the oscillations that occurred following isometric contractions did not show this phase-relationship. It is concluded that a cerebellar tremor can occur in an isometric situation but that movement about a joint is required for development of a rhythmic 3-5-Hz cerebellar intention tremor.

Animals↗

PubMed1992

Modulation of somatosensory evoked potentials during isometric contraction in relation to hand use.

The aim of the study was to assess the effect of the hand used on the modulation of somatosensory evoked potentials (SEPs) during isometric contraction. SEPs to median nerve stimulation were assessed during dynamic (increase and decrease of force) and hold phases of isometric contractions. The experiments were carried out with the right or left hand. The changes in the SEP amplitude were found to vary with the hand used. When isometric contractions were performed by the right hand the amplitudes of the later components diminished during the dynamic phases, except for the component N140-P190 which increased. The amplitude of N55-P100 diminished during both the dynamic and hold phases. When isometric contractions were performed by the left hand the amplitudes of the early components diminished during the dynamic phases. These results show that the changes in SEP during isometric contraction depend on the hand used.

Adult↗

PubMed2006

A three-dimensional computerized isometric strength measurement system.

The three-dimensional Computerized Isometric Strength Measurement System (CISMS) reliably and accurately measures isometric pull and push strengths in work spaces of paraplegic populations while anticipating comparative studies with other populations. The main elements of the system were: an extendable arm, a vertical supporting track, a rotating platform, a force transducer, stability sensors and a computerized data collection interface. The CISMS with minor modification was successfully used to measure isometric push-up and pull-down strengths of paraplegics and isometric push, pull, push-up and pull-down strength in work spaces for seated and standing able-bodied populations. The instrument has satisfied criteria of versatility, safety and comfort, ease of operation, and durability. Results are accurate within 2N for aligned forces. Costing approximately $1,500 (US) including computer, the system is affordable and accurate for aligned isometric strength measurements.

Arm↗

PubMed2005

Short-term isometric exercise reduces systolic blood pressure in hypertensive adults: possible role of reactive oxygen species.

OBJECTIVE: A short-term isometric exercise protocol was tested in ten hypertensive individuals to determine its efficacy as a high blood pressure-reducing intervention. DESIGN: The study was a prospective case study of 10 hypertensive individuals (8 men, 2 woman, mean age = 52 + 5 years) who underwent six weeks of isometric exercise training (three sessions/week). METHODS: Blood pressure, blood lipids and markers of oxidative stress were monitored before, during and following the isometric intervention. Electron spin resonance spectroscopy was used to directly measure radicals in the blood samples. RESULTS: After six weeks, systolic blood pressure decreased an average 13 mm Hg (p < 0.05) from a mean blood pressure of 146 to 133 mm Hg, a level that is below the usual 140 mm Hg hypertension threshold. Blood lipids were unchanged, but markers of oxidative stress were affected, with a dramatic decrease in exercise-induced oxygen centered radicals (-266%), (p < 0.05) and an increased resting whole blood glutathione:oxidized glutathione (+61%) in hypertensive adults following six weeks of isometric exercise. CONCLUSION: Six weeks of isometric exercise training was effective in lowering systolic but not diastolic blood pressure in pre-hypertensive and hypertensive individuals, and enhanced antioxidant protection is a likely underlying mechanism.

Age Distribution↗

PubMed2006

Quadriceps femoris electromyogram during concentric, isometric and eccentric phases of fatiguing dynamic knee extensions.

The objective of this study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during fatiguing knee extensions. Thirty young adults were evaluated for their one-repetition maximum (1RM) during a seated, right-leg, inertial knee extension. All subjects then completed a single set of repeated knee extensions at 50% 1RM, to failure. Subjects performed a knee extension (concentric phase), held the weight with the knee extended for 2s (isometric phase), and lowered the weight in a controlled manner (eccentric phase). Raw EMG of the vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscles were full-wave rectified, integrated and normalized to the 1RM EMG, for each respective phase and repetition. The EMG median frequency (f(med)) was computed during the isometric phase. An increase in QF muscle EMG was observed during the concentric phase across the exercise duration. VL EMG was greater than the VM and RF muscles during the isometric phase, in which no significant changes occurred in any of the muscles across the exercise duration. A significant decrease in EMG across the exercise duration was observed during the eccentric phase, with the VL EMG greater than the VM and RF muscles. A greater decrease in VL and RF muscle f(med) during the isometric phase, than the VM muscle, was observed with no gender differences. The findings demonstrated differential recruitment of the superficial QF muscle, depending on the contraction mode during dynamic knee extension exercise, where VL muscle dominance appears to manifest across the concentric-isometric-eccentric transition.

Adaptation, Physiological↗

PubMed2005

The effects of interelectrode distance on electromyographic amplitude and mean power frequency during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this study was to examine the effects of interelectrode distance (IED) on the absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus isokinetic and isometric torque relationships for the biceps brachii muscle. Ten adults [mean+/-SD age=22.0+/-3.4 years] performed submaximal to maximal, isokinetic and isometric muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), the subjects performed randomly ordered, submaximal step muscle actions in 10% increments from 10% to 90% PT and MVC. Surface EMG signals were recorded simultaneously from bipolar electrode arrangements placed over the biceps brachii muscle with IEDs of 20, 40, and 60mm. Absolute and normalized EMG amplitude (muVrms and %max) increased linearly with torque during the isokinetic and isometric muscle actions (r(2) range=0.988-0.998), but there were no significant changes for absolute or normalized EMG MPF (Hz or %max) from 10% to 100% PT and MVC. In some cases, there were significant (p<0.05) differences among the three IED arrangements for absolute EMG amplitude and MPF values, but not for the normalized values. These findings suggested that for the biceps brachii muscle, IEDs between 20 and 60mm resulted in similar patterns for the EMG amplitude or MPF versus dynamic and isometric torque relationships. Furthermore, unlike the absolute EMG amplitude and MPF values, the normalized EMG data were not influenced by changes in IED between 20 and 60mm. Thus, normalized EMG data can be compared among previous studies that have utilized different IED arrangements.

Adult↗

PubMed1998

The stiffness of skeletal muscle in isometric contraction and rigor: the fraction of myosin heads bound to actin.

Step changes in length (between -3 and +5 nm per half-sarcomere) were imposed on isolated muscle fibers at the plateau of an isometric tetanus (tension T0) and on the same fibers in rigor after permeabilization of the sarcolemma, to determine stiffness of the half-sarcomere in the two conditions. To identify the contribution of actin filaments to the total half-sarcomere compliance (C), measurements were made at sarcomere lengths between 2.00 and 2.15 microm, where the number of myosin cross-bridges in the region of overlap between the myosin filament and the actin filament remains constant, and only the length of the nonoverlapped region of the actin filament changes with sarcomere length. At 2.1 microm sarcomere length, C was 3.9 nm T0(-1) in active isometric contraction and 2.6 nm T0(-1) in rigor. The actin filament compliance, estimated from the slope of the relation between C and sarcomere length, was 2.3 nm microm(-1) T0(-1). Recent x-ray diffraction experiments suggest that the myosin filament compliance is 1.3 nm microm(-1) T0(-1). With these values for filament compliance, the difference in half-sarcomere compliance between isometric contraction and rigor indicates that the fraction of myosin cross-bridges attached to actin in isometric contraction is not larger than 0.43, assuming that cross-bridge elasticity is the same in isometric contraction and rigor.

Actins↗

PubMed1999

Isometric tension development and its calcium sensitivity in skinned myocyte-sized preparations from different regions of the human heart.

OBJECTIVE: In this study we investigated whether differences exist or develop in patients with aortic or mitral valve disease in myofibrillar contractile function and contractile protein composition between subendo- and subepicardial human ventricular tissue. Isometric tension, its calcium sensitivity and contractile protein composition were studied in left ventricular subendo- and subepicardial and in atrial biopsies obtained during open heart surgery from 24 patients with mitral or aortic valve disease. METHODS: Isometric tension was measured in mechanically isolated skinned myocyte-sized preparations at different free calcium concentrations at 15 degrees C. Protein composition was analysed by one-dimensional gel electrophoresis. A comparison was made between the results of subendo- and subepicardial ventricular tissue within each New York Heart Association class and within the different hemodynamically overloaded groups. RESULTS: Maximal isometric tension was significantly lower in atrial than in ventricular preparations. The concentration of calcium required for half-maximal activation was significantly higher in atrial than in ventricular preparations. Within the ventricle no differences were found in contractile protein composition, isometric tension and its calcium sensitivity between subendo- and subepicardial tissue when all patients were treated as one group or when patients were subdivided according to severity of heart disease or hemodynamic overload. CONCLUSIONS: In this group of patients with ventricular volume or pressure overload no regional differences exist or develop during cardiac disease in left ventricular myofibrillar protein composition and force production. Maximal isometric tension and its calcium sensitivity are smaller in atrial than in ventricular preparations.

Analysis of Variance↗

PubMed2003

Stretch-induced, steady-state force enhancement in single skeletal muscle fibers exceeds the isometric force at optimum fiber length.

Stretch-induced force enhancement has been observed in a variety of muscle preparations and on structural levels ranging from single fibers to in vivo human muscles. It is a well-accepted property of skeletal muscle. However, the mechanism causing force enhancement has not been elucidated, although the sarcomere-length non-uniformity theory has received wide support. The purpose of this paper was to re-investigate stretch-induced force enhancement in frog single fibers by testing specific hypotheses arising from the sarcomere-length non-uniformity theory. Single fibers dissected from frog tibialis anterior (TA) and lumbricals (n=12 and 22, respectively) were mounted in an experimental chamber with physiological Ringer's solution (pH=7.5) between a force transducer and a servomotor length controller. The tetantic force-length relationship was determined. Isometric reference forces were determined at optimum length (corresponding to the maximal, active, isometric force), and at the initial and final lengths of the stretch experiments. Stretch experiments were performed on the descending limb of the force-length relationship after maximal tetanic force was reached. Stretches of 2.5-10% (TA) and 5-15% lumbricals of fiber length were performed at 0.1-1.5 fiber lengths/s. The stretch-induced, steady-state, active isometric force was always equal or greater than the purely isometric force at the muscle length from which the stretch was initiated. Moreover, for stretches of 5% fiber length or greater, and initiated near the optimum length of the fiber, the stretch-enhanced active force always exceeded the maximal active isometric force at optimum length. Finally, we observed a stretch-induced enhancement of passive force. We conclude from these results that the sarcomere length non-uniformity theory alone cannot explain the observed force enhancement, and that part of the force enhancement is associated with a passive force that is substantially greater after active compared to passive muscle stretch.

Animals↗

PubMed1984

Isometric detrusor pressure during bladder filling and its dependency on bladder volume and interruption to flow in control subjects.

We characterized the isometric pressures generated by the bladder during voluntary detrusor contraction and interruption of flow. Urodynamic studies were done in 34 healthy female volunteers, with a mean age of 29.6 plus or minus 9.3 years. Control urodynamics were done first to characterize bladder and urethral parameters to filling and voiding. Subsequently, isometric detrusor pressures were elicited during bladder filling at increments of 100 ml. by asking subjects to attempt to void against a urethral obstruction produced by an inflated Foley balloon. Isometric pressures also were obtained by interruption of flow through the lumen of the 22F Foley catheter. The results showed that the maximum isometric pressure increase generated remained relatively constant during bladder filling at 39.6 plus or minus 13.1 cm. water. This pressure is not significantly different from voiding pressures developed with a 10F urethral catheter. On the other hand, isometric pressure increases during voiding showed that the increase in bladder pressure following interruption of flow was volume sensitive. The possibility that this volume dependency may result in errors in the interpretation of bladder contractility is discussed.

Adult↗

PubMed2000

Joint angle-dependence of elbow flexor activation levels during isometric and isokinetic maximum voluntary contractions.

OBJECTIVE: The influences of elbow joint angle and the type of contraction on the activation levels of biceps brachii and brachioradialis during maximum voluntary isometric and isokinetic contractions were investigated. DESIGN: A within-session repeated measures design. BACKGROUND: Activation of synergistic elbow flexor muscles has been reported to be affected disparately by elbow joint angle and contraction type. METHODS: Ten subjects performed concentric isokinetic, eccentric isokinetic, and isometric maximum voluntary contractions of the elbow flexor muscles. For the isokinetic contractions the activation levels of two ranges of motion were compared. For the isometric contractions the activation levels at two joint angles were compared. The activation levels of the biceps brachii and brachioradialis acquired simultaneously using bipolar surface electrodes and a surface electrode array were compared. RESULTS: Results from the electrode array were similar to those acquired using conventional bipolar electrodes. The activation of biceps brachii was significantly affected by joint angle during concentric isokinetic and isometric maximum voluntary contractions. The activation of brachioradialis was significantly affected by joint angle only during eccentric isokinetic maximum voluntary contractions. CONCLUSIONS: The results confirm that joint angle and contraction type contribute to the distinction between the activation of synergistic elbow flexor muscles during isometric and isokinetic contractions. Relevance The results point to the complexity of control of elbow joint synergists and raise questions about the plasticity of this dependency of elbow flexor activation on joint angle. Solutions to these questions are of importance in the areas of upper extremity rehabilitation and modeling the upper extremity neuromechanics.

Adult↗

PubMed2001

A transducer to measure isometric elbow moments.

OBJECTIVE: The purpose of this study was to design and implement a transducer to measure accurately the isometric elbow moments produced by individuals with tetraplegia. DESIGN: The device needed to be insensitive to off-axis moments and proximal joint motions and be capable of being used over a wide range of elbow and shoulder positions in an outpatient clinic setting. BACKGROUND: Measurement of the smaller isometric moments produced by individuals with tetraplegia is especially sensitive to the errors that can be introduced by inaccurate lever arm determination, off-axis loads, and proximal joint motions. Devices traditionally utilized for quantifying isometric strength are difficult to implement for the spinal cord injured population. METHODS: The elbow moment transducer consists of two four-bar parallelogram linkages joined by a lockable pivot. Strain gauges mounted on one beam of the parallelogram produce an output proportional to the elbow moment. RESULTS: Calibration of the device indicates that it accurately quantifies isometric elbow moments over a range that is appropriate for evaluating elbow extension strength in individuals with tetraplegia. CONCLUSIONS: A device was developed and implemented that accurately quantifies isometric elbow moments over a range that is appropriate for evaluating elbow extension strength in individuals with tetraplegia. RELEVANCE: The ability to quantitatively evaluate elbow strength in persons with tetraplegia is useful for understanding and improving the clinical outcomes of rehabilitative interventions that involve the elbow.

Calibration↗

PubMed2002

Neck muscle strength across the sagittal and coronal planes: an isometric study.

OBJECTIVE: To determine the maximal isometric strength of neck muscles across different anatomical positions in the body sagittal and coronal planes. DESIGN: A descriptive study involving maximal isometric strength measurements of the cervical musculature in extension, flexion, and left and right lateral bending. Before measurements, the new device was tested for reliability. BACKGROUND: The literature contains only a few studies on cervical musculature strength assessment. Of these, measurement methods have displayed weak reliability. No studies existed reporting neck strength at various positions in the body coronal plane. METHODS: A new device incorporating local population anthropometry was developed for neck strength assessment on an isokinetic dynamometer. The device's repeatability attributes at measuring the strength of functionally different groups of neck muscles were first evaluated. The isometric apparatus was next used to determine the maximal strength of the cervical musculature of 17 healthy volunteers at different anatomical positions across the body sagittal and coronal motion planes. RESULTS: The results showed good intratester repeatability for intraday and day-to-day studies for all measured parameters. Strength values demonstrated that extensor muscles yielded the greatest torque and that flexor muscles provided the least torque. Linear regression analyses showed strong association of isometric strength with anatomical positions in the two body planes. CONCLUSIONS: Maximal isometric strength of neck muscles in four directions was quantified. Cervical musculature strength levels vary in discernible patterns with head-neck positions and contraction directions. RELEVANCE: Knowledge of normative values of neck muscle strength is essential for designing and implementing appropriate rehabilitation programmes. Results from the present study contribute to such literature.

Adult↗

PubMed2002

Electromyography of trunk muscles in isometric graded axial rotation.

This study was conducted to determine the pattern, magnitude, and phasic inter-relationship of the trunk muscles in maximal isometric and graded isometric axial rotational contractions and compare them with those previously observed from the same subjects in the same experimental session in dynamic conditions. In 50 normal young healthy subjects (27 male and 23 female), after a suitable skin preparation, bipolar silver-silver chloride recessed pregelled surface electrodes were placed on external oblique, internal oblique, rectus abdominis, pectoralis major, latissimus dorsi, erector spinae at T(10) and L(3) levels bilaterally with 2 cm interelectrode distance. EMG signals from grounded subjects were suitably preamplified and amplified by a fully isolated system. These subjects were stabilized in an upright-seated posture in the Axial Rotation Tester (AROT), which was placed in isometric mode for force and rotation output from the AROT. The 14 channels of EMG, the force and the rotation were sampled at 1 kHz. The subjects initially registered their isometric maximal voluntary contraction (MVC) on both sides which was used for reference and then performed their 25%, 50% and 75% of MVC bilaterally in an isometric mode in a random order. The EMG magnitude, the slope of the rise of the EMG, and the phasic interrelationship of muscles were analyzed. The results showed that female sample generated only 65% of torque of their male counterparts. There were no significant differences between the male and the female samples in the EMG variables. Exertions to the left and to the right were not significantly different from each other for the measured variables. However, the magnitude contribution of the muscles and the slope of rise of EMG were significantly different in two directions (p<0.001). The phasic interrelationship of the external obliques, the latissimus dorsi and the erector spinae were different from other muscles (p<0.01). With the increasing grades of contraction the latissimus dorsi and the external obliques increased their magnitude significantly whereas that of the erectores spinae underwent a decrease in proportionate terms (but not in absolute magnitude) suggesting their role as stabilizers but not as rotators.

Adult↗