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PubMed1992

Measurement of isometric force in children with and without Duchenne's muscular dystrophy.

The purpose of this study was to examine the consistency of measurements of maximal isometric force production in children using an electronic strain gauge. A standardized protocol of force testing from the Tufts Quantitative Neuromuscular Examination was adapted to obtain isometric force measurements in seven muscle groups bilaterally. Ten children with Duchenne's muscular dystrophy (DMD) and 10 age-matched children without DMD were recruited for the study. A generalizability framework was used to examine variations in measurements attributable to group differences, test occasions within the same day, and extremity side. Intertester reliability estimates were also completed on a subsample of children with and without DMD. A three-way analysis of variance between groups, sides, and test occasions demonstrated a significant group main effect for all muscle groups. In general, significant main effects were not present for side or test occasion. Test-retest intraclass correlation coefficients (ICCs) for all muscle groups ranged from .88 to .99 for the children with DMD and from .85 to .98 for the children without DMD. Intertester ICCs were slightly lower, ranging from .74 to .97 for the children with DMD and from .71 to .98 for the children without DMD. The results indicate that the electronic strain-gauge protocol produces reproducible force measurements in seven selected muscle groups in children with and without DMD. Brussock CM, Haley SM, Munsat TL, Bernhardt DB. Measurement of isometric force in children with and without Duchenne's muscular dystrophy.

Adolescent↗

PubMed1998

Effects of dehydration on isometric muscular strength and endurance.

To examine the effects of rapid dehydration on isometric muscular strength and endurance, seven men were tested at baseline (control) and after a dehydration (dHST) and a euhydration (eHST) heat stress trial. The dHST consisted of intermittent sauna exposure until 4% of body mass was lost, whereas the eHST consisted of intermittent sauna exposure (same duration as dHST) with water replacement. Peak torque was determined for the knee extensors and elbow flexors during three isometric maximal voluntary contractions. Time to fatigue was determined by holding a maximal voluntary contraction until torque dropped below 50% peak torque for 5 s. Strength and endurance were assessed 3.5 h after the HSTs (no food or water intake). Body mass was decreased 3.8+/-0.4% post dHST and 0.4+/-0.3% post eHST. Plasma volume was decreased 7.5+/-4.6% and 5.7+/-4.4%, 60 and 120 min post dHST, respectively. A small (1.6 mEq x L[-1]) but significant increase was found for serum Na+ concentration 60 min post dHST but had returned to predehydration level 120 min post dHST. Serum K+ and myoglobin concentrations were not affected by HSTs. Peak torque was not different (P > 0.05) among control, dHST, and eHST for the knee extensors (Mean (Nm)+/-SD, 285+/-79, 311+/-113, and 297+/-79) and elbow flexors (79+/-12, 83+/-15, and 80+/-12). Time to fatigue was not different (P > 0.05) among control, dHST and eHST for the knee extensors (Mean (s)+/-SD. 42.4+/-11.5, 45.3+/-7.6, and 41.8+/-6.0) and elbow flexors (48.2+/-8.9, 44.0+/-9.4, and 46.0+/-6.4). These results provide evidence that isometric strength and endurance are unaffected 3.5 h after dehydration of approximately 4% body mass.

Adult↗

PubMed2000

Muscle forces and spinal loads at C4/5 level during isometric voluntary efforts.

PURPOSE: The goal of this study was to determine neck muscle forces and spinal loads that result from isometric muscle contractions. METHODS: Electromyographic (EMG) activity of the neck musculature and a three-dimensional biomechanical model of the neck were used. The model was EMG-based and estimated muscle forces and spinal loads at the C4/5 level. EMG signals were collected from eight sites at the C4/5 level of the neck using Ag-AgCl surface electrodes from 10 adult male subjects. The subjects performed isometric contractions gradually developing to maximum efforts in flexion, extension, left lateral bending, and right lateral bending. RESULTS: During maximum voluntary contraction (MVC) trials most muscles generated high levels of EMG signal during cervical rotation. The posterior surface of the neck (trapezius) was the only electrode site at which maximum activity EMG consistently occurred by the same method (rotation) in all subjects. Variations in the EMG patterns were observed in different experiments that produced overall neck moments of equal magnitudes. With these data the model computed variations in load distribution among the agonist muscles. Consistent also with EMG distributions, the model also computed co-contractions of antagonist muscles. The average (+/- SD) magnitudes of peak moments were 28.3 (+/- 3.3) Nm in extension, 17.7 (+/- 3.1) Nm in flexion, 16.9 (+/- 2.8) Nm in left lateral bending, and 17.0 (+/- 2.9) Nm in right lateral bending. The model predicted C4/5 joint compressive forces during peak moments were 1372 (+/- 140) N in extension, 1654 (+/- 308) N in flexion, 956 (+/- 169) N in left lateral bending, and 1065 (+/- 207) N in right lateral bending. CONCLUSIONS: Results suggest that higher C4/5 joint loads than previously reported are possible during maximum isometric muscle contractions.

Adult↗

PubMed1983

Comparison of cardiovascular risk in maximal isometric and dynamic exercise.

With the hypothesis that isometric exercise testing offers important clinical information yet is neglected because of overemphasis on its risk, we compared it with dynamic testing for variables indicative of myocardial ischemia. In a medical office, 287 patients were each subjected once to both a one-minute maximal handgrip stress test and maximal treadmill test while monitored noninvasively for cardiovascular responses. Systolic blood pressures usually increased more with the treadmill test, diastolic more with the handgrip. The treadmill tests caused greater mean heart rate and rate-pressure products, as well as more ST segment displacements, arrhythmias, and symptoms and signs. However, there were some unpredictable individual differences in the isometric blood pressure responses. Isometric testing is easy and useful and, if properly monitored, causes less risk to the patient than dynamic testing.

Adult↗

PubMed1995

Isometric and dynamic performances of the trunk and associated factors.

STUDY DESIGN: Static and dynamic performances of the trunk were measured on an isoinertial device and data were collected regarding the working conditions and the anthropometric and psychosocial characteristics of the workers. OBJECTIVES: To describe the difference in static and dynamic performances of the trunk between workers with and without low back pain and to investigate the anthropometric, personal, and occupational factors associated with these performances. SUMMARY OF BACKGROUND DATA: The isometric torque and the parameters of displacement, velocity, and torque during dynamic tests were chosen to describe the isoinertial performances of the trunk. Dynamic characteristics in particular were investigated because it has been suggested in the literature that these discriminate best between workers with and without low back pain. METHODS: The protocol included a 52-item questionnaire, a clinical examination with a series of anthropometric measurements, and the isometric and dynamic tests (against 25% and 50% of the isometric values) on the isostation. RESULTS: The variables significantly associated with the trunk performances were weight, height, muscularity, sports, and leisure activities as positive factors, and linearity, health problems, and vehicle driving as negative factors. When these covariates were taken into consideration, the significant differences between subjects with and without low back pain were related only to the velocities during dynamic tests, which were reduced by 5.9%. CONCLUSIONS: The performances of the trunk are associated mainly with the anthropometric characteristics of the subjects but also with factors such as lumbar status, health, working conditions, and leisure activities.

Adult↗

PubMed1985

Isometric occupational exercise and the incidence of hypertension.

The effect on blood pressure of regular exposure to isometric exercise was studied by measuring the incidence of hypertension among 4,273 men classified according to occupational isometric activity. Incidence of hypertension was lower among men in jobs with moderate or heavy isometric activity. This difference persisted after statistical adjustment for age, social class, obesity, and use of alcohol.

Adult↗

PubMed2005

Ability of very young children to produce reliable isometric force measurements.

PURPOSE: This study was designed to examine the reliability of measuring isometric muscle force in very young children with a hand-held dynamometer (HHD). METHODS: Forty-five children aged two, three, or four years old and with typical development, were divided into three groups of 15 subjects each. Group 1 included two-year-old children (X age = 32 +/- 2.4 months), group 2 contained three-year-old children (X age = 43 +/- 3.8 months), and group 3 included four-year-old children (X age = 54 +/- 3.7 months). Isometric force for shoulder flexion, elbow flexion, elbow extension, knee flexion, and knee extension were measured with a HHD on two occasions separated by five to nine days. The frequency of challenging child behaviors and extra test trails also were recorded. RESULTS: Intraclass correlation coefficients (ICCs) (2, 1) for the combined group (n = 45) ranged from 0.90 to 0.95. The ICCs (2, 1) for the two-year-old children ranged from 0.76 to 0.91, for the three-year-old group from 0.85 to 0.92, and for the four-year-old children from 0.54 to 0.94. The frequency of challenging behaviors did not appear to be related to the level of reliability. CONCLUSION: Using a HHD and standardized procedures, isometric force of children who are typically developing and as young as two years of age can be measured with fair to excellent reliability.

Child Behavior↗

PubMed2006

Advancements in power wheelchair joystick technology: Effects of isometric joysticks and signal conditioning on driving performance.

OBJECTIVE: An estimated 125,000 Americans with movement disorders that preclude independent mobility in a power wheelchair could benefit from improved control devices. We developed variable gain algorithm (VGA) software for our isometric joystick (IJ) that allows it to emulate a commercially available motion-sensing joystick (MSJ) in performance but retain the unique customizable features of an isometric control. Force sensing algorithm (FSA) software allowed the IJ to function as a simple isometric device. DESIGN: Using a Fitts' Law paradigm, we compared driving performance with floor targets using a standard MSJ and an IJ with both FSA and VGA software in 11 electric power wheelchair users with a variety of impairments. Outcome measures were reaction time (RT), movement time (MT), and driving accuracy (DA). RESULTS: The IJ with FSA had a significantly shorter RT than the MSJ (P < 0.0020). The IJ with FSA had a significantly longer MT than the MSJ to far targets (P < 0.0159). No differences were found between the IJ with VGA and the MSJ with respect to RT or MT. No differences in DA were found among any of the joysticks. CONCLUSIONS: VGA software allowed the IJ to function similarly to the MSJ with respect to RT, MT, and DA.

Adult↗

PubMed1998

A simple model of force generation by skeletal muscle during dynamic isometric contractions.

The force that an isometric skeletal muscle will produce in response to time-varying stimulation ("dynamic isometric" force) is important both for understanding muscle function and for designing neuroprostheses. This paper reports a model for predicting the force produced by an isometric skeletal muscle at rest length in response to a wide range of stimulation patterns. The model consists of two linear, first-order systems separated by a static nonlinearity. The rate constant of the second first-order system varies with force level. The model was validated using three cat soleus and three cat plantaris muscles. The following whole-nerve stimulation trains were used: single pulses (twitches), 2-4 pulses, constant rates, triangularly modulated interpulse intervals, and randomly modulated interpulse intervals. The model reproduced most responses accurately. The model shows that a force-dependent rate constant is essential for model validity, and could be used in the control of neuroprostheses.

Analog-Digital Conversion↗

PubMed2000

Evaluation of an isometric and a position joystick in a target acquisition task for individuals with cerebral palsy.

Individuals with cerebral palsy (CP) with upper limb disability have difficulty operating standard computer input devices such as, a mouse and a keyboard. The present study evaluated the performance of unimpaired individuals and those with CP in the use of a zero-order spring-centered position joystick and a zero-order isometric joystick when interacting with the computer. We hypothesize that the isometric device due to its stiff resistance would provide better control on cursor movement than the position joystick. The subjects acquired differently sized targets at different distances by positioning a cursor on the target. Performance with the position joystick was superior to that of the isometric joystick. The time to acquire the target was directly proportional to the cursor-target distance A, and indirectly proportional to the target size W. Subjects chose to move more slowly toward the smaller and closer targets and they increased speed for larger and more distant targets. The phase-plane diagram that plots values of the velocity profile of the cursor over its displacement revealed the presence of one large amplitude movement that accounts for the peak velocity of the cursor, and several submovements. Fitts' index of difficulty, log(e)(2W/A) was found to be a good predictor of the movement time in a cursor positioning task for both, unimpaired individuals and those with CP.

Adult↗

PubMed1983

Left ventricular response to isometric exercise in aortic valve diseases and its value in the optimal timing of aortic valve replacement.

In order to evaluate the left ventricular response to isometric exercise in different types of aortic valve disease, isometric exercise tests were performed during cardiac catheterization in 14 patients with pure aortic stenosis, 20 with combined aortic stenosis and regurgitation, and 18 with pure aortic regurgitation. Patients with angina pectoris in whom coronary angiography had not been performed were excluded. Thirty-seven patients were recatheterized 12 months after aortic valve replacement, and the ventricular response to exercise was re-evaluated. Preoperatively, the ejection fraction did not change significantly during exercise in patients with aortic stenosis, tended to decrease in patients with combined valve lesion, and decreased significantly in patients with aortic regurgitation (p less than 0.001). In the three patients whose ejection fraction during preoperative exercise decreased to below 0.40, it remained below 0.50 after successful aortic valve replacement. It appears possible to reveal left ventricular dysfunction in many patients with aortic regurgitation and in some with combined aortic valve disease by means of isometric exercise. The severely depressed ventricular dysfunction during exercise does not appear to correct totally after surgery.

Adult↗

PubMed1979

The effects of alpha- and beta-adrenergic receptor blockers on the pressure responses to isometric exercise in hypertensive patients.

1. The cardiovascular responses to handgrip exercise have been studied in ten patients with uncomplicated essential hypertension in a randomized crossover study of propranolol and prazosin. 2. Isometric handgrip exercise was performed with a calibrated strain gauge dynamometer at 30% of maximum voluntary contraction for 3 min. 3. Blood pressure and heart rate were measured in the supine position at rest and in the last 10 s of the exercise period. 4. These exercise studies were undertaken at the end of a run-in period and at the end of 1 month's optimal therapy with the two drugs. 5. The active treatment periods were separated by a 2 weeks placebo washout period. 6. Both drugs lowered the supine and standing systolic and diastolic pressures and there was no difference between these drugs in their effect on these variables. 7. Propranolol lowered the resting heart rate and neither drug suppressed the pressor response to isometric exercise. 8. The degree of pressure rise was similar with both drugs but propranolol suppressed isometric exercise-induced tachycardia.

Adrenergic alpha-Antagonists↗

PubMed1984

Cumulative electromyography of the human masseter muscle during fatiguing isometric contractions.

Six adult males performed maximum voluntary tooth clenching (MVC) for 10, 20, 30, 40 s and, after 15 min, for 40, 30, 20, 10 s. During the isometric exercises the electrical currents of the masseter muscle were sampled by integrated and cumulative surface electromyography. Subjective masseter fatigue was present after 30 and 40 s of MVC clenching, accompanied by changes in myoelectrical activity. Strength testing of the masseter muscle, before and after endurance testing, showed that the strength increased by a significant 16% following two endurance tests. This observation was explained by a post-tetanic potentiation and/or a differentiated use of motor units in the fatigued muscle. It is concluded that brief MVC isometric activity, or strength testing, is not a reliable measure of fatigue in the masseter muscle when cumulative electromyography is used. Prolonged MVC isometric activity, or progressive endurance testing, monitors reliably the onset and progression of masseter fatigue.

Adult↗

PubMed1985

Isometric endurance of the human masseter muscle during consecutive bouts of tooth clenching.

Three human subjects performed teeth clenching at maximum voluntary contraction strength (MVC) of the mandibular elevator muscles. At intervals of 10s, ten successive bouts of MVC clenching were exercised until total exhaustion of the contracting muscles (isometric endurance). Isometric endurance time decreased exponentially from 59 to 12s. The curve of the decrease showed a monotone power function (y = x-1). Electromyographic recordings from masseter muscles suggested that fatigable motor units, possibly fast glycolytic units, dropped out during the initial stages of maintained MVC isometric activity. The silent period of the monosynaptic jaw jerk reflex was increased by about 35% following exhaustion of the masseter muscle.

Adult↗

PubMed1991

Activity index and isometric contraction velocity of human jaw muscles.

To determine the relative contribution of the masseter and anterior temporalis muscles to global isometric bite force, activity indices were constructed on the basis of the peak mean voltage (microV) and the integrated voltage (microV.s) of bipolar surface electromyograms obtained during brief maximum voluntary teeth clenching (MVC). The index that was based on integrated myoelectrical activity showed that the masseter muscle contributed the major part of the isometric MVC force. The index that was based on instantaneous peak myoelectrical activity also showed that activity in the masseter muscle predominated over that in the anterior temporalis muscle. In addition, the latter index showed a negative linear association with the initial (0-50% MVC) isometric contraction velocities of the masseter and anterior temporalis muscles. Both indices appear to be promising clinical diagnostic tools.

Action Potentials↗

PubMedDate not supplied

Relationships between isometric and isotonic mechanical parameters and cross-bridge kinetics.

1. Isometric and isotonic measures have been widely used both to characterize muscular contraction in striated muscle and to compare the mechanical properties of striated muscle types for a wide variety of species. The interrelationship of these measures and their interpretation, in terms of underlying cross-bridge mechanisms, requires clarification. 2. We demonstrate that a strain-dependent three-state model can simulate these isometric and isotonic mechanics and offer a unified explanation for these mechanics in terms of cross-bridge kinetics. 3. Our exploration reveals the partial view each individual measure of mechanics provides and emphasizes the complementary insights the various isometric and isotonic measures give of the underlying cross-bridge kinetics.

Adrenergic beta-Agonists↗

PubMed1997

ATPase kinetics on activation of rabbit and frog permeabilized isometric muscle fibres: a real time phosphate assay.

1. The rate of appearance of inorganic phosphate (Pi) and hence the ATPase activity of rabbit psoas muscle in single permeabilized muscle fibres initially in rigor was measured following laser flash photolysis of the P3-1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP) in the presence and absence of Ca2+. Pi appearance was monitored from the fluorescence signal of a Pi-sensitive probe, MDCC-PBP, a coumarin-labelled A197C mutant of the phosphate-binding protein from Escherichia coli. Fibres were immersed in oil to optimize the fluorescence signal and to obviate diffusion problems. The ATPase activity was also measured under similar conditions from the rate of NADH disappearance using an NADH-linked coupled enzyme assay. 2. On photolysis of NPE-caged ATP in the presence of Ca2+ at 20 degrees C, the fluorescence increase of MDCC-PBP was non-linear with time. ATPase activity was 41 s-1 in the first turnover based on a myosin subfragment 1 concentration of 150 microM. This was calculated from a linear regression of the fluorescence signal reporting 20-150 microM of Pi release. Tension was at 67% of its isometric level by the time 150 microM Pi was released. ATPase activities were 36 and 31 s-1 for Pi released in the ranges of 150-300 microM and 300-450 microM, respectively. The ATPase activity had a Q10 value of 2.9 based on measurements at 5, 12 and 20 degrees C. 3. An NADH-linked assay showed the ATPase activity had a lower limit of 12.7 s-1 at 20 degrees C. The response to photolytic release of ADP showed that the rate of NADH disappearance was partially limited by the flux through the coupled reactions. Simulations indicated that the linked assay data were consistent with an initial ATPase activity of 40 s-1. 4. On photolysis of NPE-caged ATP in the absence of Ca2+ the ATPase activity was 0.11 s-1 at 20 degrees C with no discernible rapid transient phase of Pi release during the first turnover of the ATPase. 5. To avoid the rigor state, the ATPase rate in the presence of Ca2+ was also measured on activation from the relaxed state by photolytic release of Ca2+ from a caged Ca2+ compound, nitrophenyl-EGTA. At 5 degrees C the ATPase rate was 5.8 and 4.0 s-1 in the first and second turnovers, respectively. These rates are comparable to those when NPE-caged ATP was used. 6. The influence of ADP and Pi on the ATPase activities was measured using the MDCC-PBP and NADH-linked assays, respectively. ADP (0.5 mM) decreased the initial ATPase rate by 23%. Pi (10 mM) had no significant effect. Inhibition by ADP, formed during ATP hydrolysis, contributed to the decrease of ATPase activity with time. 7. The MDCC-PBP assay and NPE-caged ATP were used to measure the ATPase rate in single permeabilized muscle fibres of the semitendinosus muscle of the frog. At 5 degrees C in the presence of Ca2+ the ATPase activity was biphasic being 15.0 s-1 during the first turnover (based on 180 microM myosin subfragment 1). Tension was 74% of its isometric level by the time 180 microM Pi was released. During the third turnover the ATPase rate decreased to about 20% of that during the first turnover. 8. ATPase activity in isometric rabbit muscle fibres during the first few turnovers is about an order of magnitude greater than that when a steady state is reached. Possible reasons and the consequences for understanding the mechanism of muscular contraction are discussed.

Actin Cytoskeleton↗

PubMed1982

Left ventricular response to graded isometric exercise in patients with coronary heart disease.

The effects of graded isometric exercise on left ventricular performance were characterized in 11 male patients (53 +/- 2 years) with coronary artery disease (CAD) and in 12 normal subjects (11 male and one female 35 +/- 5 years). The echocardiographic indices of left ventricular function at rest were similar in both groups. Heart rate and blood pressure increased significantly in both groups in response to 40 and 60% of handgrip maximal voluntary contraction (MVC). Left ventricular end-diastolic dimension increased significantly (from 50 +/- 1 to 56 +/- 1 mm; P less than 0.01) with 60% of MVC in CAD group but not in the healthy subjects. The patients with CAD also exhibited significant (P less than 0.01) increases in end-systolic dimension (from 34 +/- 1 to 40 +/- 2 with 40% and to 44 +/- 1 mm with 60% MVC). End-diastolic and end-systolic dimensions did not change during isometric exercise in the healthy subjects. In the CAD group mVCF decreased significantly (from 1.08 +/- 0.06 to 0.86 +/- 0.06 with 40% and to 0.74 +/- 0.04 d . s-1 with 60% MVC; P less than 0.01). At comparable mean blood pressures, mVCF was significantly lower in the CAD group than in normal subjects. These results demonstrate that progressive deterioration of left ventricular function during increasing levels of isometric exercise in patients with CAD can be detected with echocardiography.

Adult↗