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PubMed1989

Effects of upper limb immobilization on isometric muscle strength, movement time, and triphasic electromyographic characteristics.

The effects of short-term immobilization on isometric muscle strength, movement time, and triphasic integrated electromyographic characteristics were assessed during a simple ballistic elbow flexion and extension movement. The nondominant upper limbs in six subjects (two female, four male) were immobilized in a plaster cast for 14 days. Testing consisted of isometric muscle strength output of elbow flexors and extensors; movement time; and IEMG output of the agonist, antagonist, and second agonist contractions during the ballistic movements. An analysis of variance revealed a decrease in flexor strength of the casted limb (p less than .05). Movement time was not significantly slowed. Antagonist IEMG amplitude was decreased (p less than .01) during flexion, and agonist IEMG amplitude (p less than .05) and antagonist IEMG amplitude (p less than .05) were diminished during extension of the casted limb. In conclusion, short-term upper limb immobilization affects primarily elbow flexion strength and some IEMG characteristics during a ballistic forearm movement, suggesting an influence on the neuromuscular aspects of the affected muscles.

Adult↗

PubMed1993

The effect of frequency of kinetic feedback on learning an isometric force production task in nondisabled subjects.

BACKGROUND AND PURPOSE: The purpose of this study was to investigate the effects of kinetic feedback frequency and concurrent kinetic feedback on the performance and learning of an isometric force production task in young, nondisabled adults. SUBJECTS: Twenty-four nondisabled, right-hand-dominant adults (18 male, 6 female), aged 19 to 33 years (mean = 22.5, SD = 4.1), participated in this study. METHODS: Eight subjects in each of three kinetic feedback groups performed an isometric elbow extension task in an attempt to minimize error between their effort and a force template over a 5-second period. Feedback was provided (1) concurrently with and after each attempt (concurrent feedback), (2) after each attempt (100% feedback), or (3) after every other attempt (50% feedback). Immediate and delayed (48-hour) retention tests were performed without feedback. Separate analyses of variance for repeated measures were used to compare task error among the three feedback groups for acquisition, immediate retention, and delayed retention trials. RESULTS: A significant interaction was found during the acquisition trial blocks, but at each trial block, subjects in the concurrent feedback group exhibited less error than did the subjects in either the 50% or 100% feedback group during the acquisition trials. For the immediate retention test, the 50% and 100% feedback groups exhibited 58% and 39% less error, respectively, than did the concurrent group. For the delayed retention test, the 50% and 100% feedback groups exhibited 52% and 26% less error, respectively, than did the concurrent group. In the immediate and delayed retention tests, subjects in the 50% feedback group displayed less error (31% and 36%, respectively) than did the 100% feedback group. CONCLUSION AND DISCUSSION: When permanent changes in the performance of a motor task are desired, concurrent feedback about task performance may be less desirable than feedback that is provided after the task has been completed. In addition, when feedback was used after the task had been completed, a lower frequency of feedback resulted in more permanent changes in the subjects' ability to complete the task.

Adult↗

PubMedDate not supplied

Increased threshold sural amplitude after upper limb isometric contraction in complete paraplegics.

OBJECTIVE: To determine whether the enhancement of threshold sural sensory nerve action potentials (SNAPs) occurred in patients with spinal cord injury after upper limb isometric contraction. DESIGN: This prospective study, in which ten paraplegic patients with spinal cord injury were recruited from the Harris County community and served as his/her own control, was performed in the electromyography laboratory at Harris County Hospital District Quentin Mease Hospital. The baseline SNAP was established using ten threshold, signal-averaged stimuli to the sural nerve. With the same stimulus strength, the SNAP was recorded while the subjects' arms were pulled apart against a force gauge at 50% and 100% maximum force. Responses were recorded every 4 min until the values returned to baseline. RESULTS: Results showed an increase in the SNAP amplitude after 50% and 100% maximum force. The mean preexercise SNAP amplitude was 4.0 +/- 0.6 (SE) microV. At 50% force, the SNAP amplitude was 7.57 +/- 1.2 microV; at 100% force, it was 7.29 +/- 1.2 microV. The post hoc analysis of the data revealed P = 0.009 and P = 0.01 for 50% and 100% maximum force, respectively. CONCLUSIONS: The threshold SNAP of the sural nerve was enhanced after isometric exercise in paraplegic patients with spinal cord injury.

Action Potentials↗

PubMed1991

Reproducibility of maximal versus submaximal efforts in an isometric lumbar extension task.

The purpose of this study was to investigate the differences in test-retest reliability between maximal and submaximal efforts in an isometric lumbar extension task and to test the hypothesis that submaximal efforts would be less consistent than maximal efforts. Twenty subjects were asked to produce maximum voluntary contractions at seven different positions in a lumbar extension machine. Each subject was tested twice in a maximal-effort condition and twice in a 50% effort condition. The order of the effort conditions was counterbalanced across subjects so that half of the subjects performed the maximal-effort tests first and half performed the submaximal effort first. Results indicated high test-retest correlations at all angles in both conditions. There were no differences in test-retest reliability between effort conditions. Therapist ratings of consistency did not differ between conditions, and therapists could not discriminate between conditions on the basis of effort consistency. Because subjects could reliably reproduce a submaximal effort, it was concluded that consistency interpretations of isometric strength testing may be misleading.

Adult↗

PubMed1994

Measurement of low back mobility, isometric strength and isoinertial performance with isostation B-200 triaxial dynamometer: reproducibility of measurement and development of functional indices.

The reproducibility of measurements with the triaxial lumbar dynamometer Isostation B-200 was measured in 61 volunteers with or without low back problems. Both for the intra- and interrater measurements the reproducibility was good (r = 0.76-0.89) for lateral flexions and almost as good (r = 0.76-0.87) for the sum of all ranges of motion (SROM). The corresponding reproducibilities were excellent (r = 0.82-0.99) for the isometric strength and good (r = 0.60-0.90) for isoinertial flexion-extension repetition test parameters in the main (primary axis) direction. A number of parameters were combined into three functional indices: the reproducibility for "power" and "work" index was good (r = 0.76-0.94) in both intra- and interrater conditions, whereas for "movement" index it was somewhat weaker (r = 0.53-0.87). It is concluded that parameters: lateral flexions, SROM, isometric strength, and functional indices (power and work) are reproducible enough to use in further studies.

Adult↗

PubMed1998

Electromyographic analysis of shoulder joint function of the biceps brachii muscle during isometric contraction.

Surface electromyography was performed for both heads of the biceps brachii in 11 healthy men while the muscles were under 30% maximum isometric shoulder flexion and abduction. Elbow related biceps activity was minimized by using a brace locked in neutral forearm rotation. Electromyographic activity was normalized as a percentage of maximal muscle contraction during 24 shoulder motions. Electromyographic activity was detected in all motions examined, suggesting that the biceps muscle acts as a flexor and an abductor of the shoulder. Both heads of the biceps muscle had higher activities during external rotation than during internal rotation for most motions. Activities of both heads increased with arm elevation, but showed little dependence on elbow position. The long head was still active during internal rotation of the shoulder. These findings also suggest that the biceps muscle is a flexor and an abductor of the shoulder, and that the long head can act as a humeral head stabilizer in superior and anterior directions. Muscle fatigue of the biceps and the deltoid muscle also was determined at 30% of maximum isometric flexion. All muscles had significantly decreased mean power frequency and turns count, and increased amplitude and integrated electromyography. The rate of decrease in mean power frequency was larger for the biceps than for the deltoid muscle, and the rate of increase in amplitude was larger for the long head of the biceps than for the short head or for the deltoid muscle. These findings suggest that the long head of the biceps must increase its mechanical output to keep the arm in elevation to a greater extent than do the short head and the deltoid muscle. This may be one of the causes of tendinitis or rupture of the long head.

Adolescent↗

PubMedDate not supplied

Effect of adrenergic receptor blockade on the responses to isometric handgrip: studies in normal and hypertensive subjects.

Heart rate and blood pressure (BP) responses to a standardized 3 min isometric handgrip (IHG) test were measured in 32 normotensive men and compared with those found in 35 age-matched, drug-free men with established essential hypertension (BP range: 140--170/90--110). IHG testing in the normal subjects induced significant increases in heart rate (mean +/- SE: 7.6 +/- 3.3 beats/min), systolic blood pressure (19.4 +/- 5.3 mm Hg), and diastolic blood pressure (15.6 +/- 4.6 mm Hg). Although beginning from higher resting levels the hypertensive patients had similar degrees of increase in all three parameters. After chronic treatment with propranolol, the heart rate increase with IHG was suppressed in both study groups, but blood pressure responses differed, with a diminished pressor response to IHG seen in normal subjects and augmented pressor effects in the hypertensive group. Intravenous administration of phentolamine and propranolol completely abolished the pressor effects of IHG. These observations suggest that the autonomic control mechanisms mediating the responses to isometric exercise function similarly in drug-free normal and hypertensive patients and that the responses to IHG, mediated largely by endogenous catecholamine release, can be prevented by peripheral sympathetic receptor blockade.

Adrenergic alpha-Antagonists↗

PubMed2000

Submaximal motor unit firing rates after 8 wk of isometric resistance training.

PURPOSE: The purpose of this study was to test the hypothesis that average motor unit firing rates change in parallel with the contractile properties of vastus lateralis following 8 wk of isometric resistance training. METHODS: The firing rates from more than 400 motor units of vastus lateralis were obtained during voluntary isometric contractions of 50% MVC, before and again after training in male subjects (N = 10) and their untrained controls (N = 10). Single motor unit spike trains were recorded with tungsten microelectrodes. RESULTS: Training resulted in a 36% (P < 0.05) increase in MVC. We also found significant increases (P < 0.05) in maximal twitch amplitude (+17%), time to peak tension (+9%) and the maximal instantaneous rate of contraction (+20%) in the trained leg of the experimental group. Neither the maximal integrated EMG nor the rate of increase of integrated EMG was different after training. There were no significant changes in any of these measures from the untrained leg or the control group. Average firing rates were not different after training despite the increase in twitch contractile speed. CONCLUSION: These findings suggest that the control properties of the nervous system are not altered despite sizable changes in the contractile properties of muscle following 8 wk of resistance training.

Action Potentials↗

PubMed1984

Measurement of loads on the lumbar spine under isometric and isokinetic conditions.

Ten male and ten female subjects were tested for their ability to exert maximal force about the lumbo-sacral junction (as is done during lifting) under controlled isometric and isokinetic conditions. The myoelectric activity of ten trunk muscles, intra-abdominal pressure, and torque produced by the back were monitored. There are prominent differences in the manner in which subjects utilize the musculature of the trunk for the production of torque statically and dynamically. A significant lag was identified between the onset of intra-abdominal pressure and torque, and this lag increased with increasing trunk velocity. These differences between isometric and isokinetic exertions suggest that isokinetic trunk testing provides a means of controlled evaluation that is appropriate for manual materials handling situations.

Abdomen↗

PubMed1994

Electromyographic median frequency changes during isometric contraction of the back extensors to fatigue.

STUDY DESIGN: This was a cross-sectional study involving 229 healthy, back pain-free individuals. OBJECTIVES: To examine the relationship between electromyographic manifestations of fatigue and endurance time during isometric contraction of the back extensors to fatigue. SUMMARY OF BACKGROUND DATA: Despite the wide-spread use of electromyography to monitor muscle fatigue, its relationship with endurance time has not been well investigated. METHODS: Using skin-surface electrodes, electromyographic signals were recorded from thoracic (T10) and lumbar (L3) regions of erector spinae during an isometric endurance test, and the rate of change in median frequency of the electromyographic power spectrum (MFGRAD) was calculated. RESULTS: MFGRAD was significantly higher at L3 than at T10. The best predictor of endurance time was given by the greater MFGRAD observed at either region. MFGRAD calculated over a submaximal time period (50% total time or 60 sec) also correlated significantly with endurance time. Women displayed a significantly longer endurance time and lower MFGRAD than men. CONCLUSIONS: Endurance appears to be limited by the most fatigable region of the muscle group. MFGRAD is a suitable technique for monitoring back muscle fatigue, even when it is determined over a submaximal time period. The back extensors of women are less fatigable than those of men when the same task is performed.

Adult↗

PubMed1996

A biomechanical model of the lumbar spine during upright isometric flexion, extension, and lateral bending.

STUDY DESIGN: Task-specific and subject-specific lumbar trunk muscle function, muscle geometry, and vertebral density data were collected from 16 men. A biomechanical model was used to determine muscle strength and the compressive forces acting on the lumbar spine. OBJECTIVES: To develop an anatomic biomechanical model of the low back that could be used to derive task-specific muscle function parameters and to predict compressive forces acting on the low back. Several model-specific constraints were examined, including the notion of bilateral trunk muscle anatomic symmetry, the influence of muscle lines of action, and the use of density-derived vertebral strength for model validation. SUMMARY OF BACKGROUND DATA: Clinical and basic science investigators are currently using a battery of diverse biomechanical techniques to evaluate trunk muscle strength. Noteworthy is the large variability in muscle function parameters reported for different subjects and for different tasks. This information is used to calculate forces and moments acting on the low back, but limited data exist concerning the assessment of subject-specific, multiaxis, isometric trunk muscle functions. METHODS: A trunk dynamometer was used to measure maximum upright, isometric trunk moments in the sagittal (extension, flexion) and coronal (lateral flexion) planes. Task- and subject-specific trunk muscle strength or "gain" was determined from the measured trunk moments and magnetic resonance image-based muscle cross-sectional geometry. Model-predicted compressive forces obtained using muscle force and body force equilibrium equations were compared with density-derived estimates of compressive strength. RESULTS: Individual task-specific muscle gain values differed significantly between subjects and between each of the tasks they performed (extension > flexion > lateral flexion). Significant differences were found between left side and right side muscle areas, and the lines of action of the muscles deviated significantly from the vertical plane. Model-predicted lumbar compressive forces were 38% (lateral flexion) to 73% (extension) lower that the L3 vertebral compressive strength estimated from vertebral density. CONCLUSION: The present study suggests that biomechanical models of the low back should be based on task-specific and subject-specific muscle function and precise geometry. Vertebral strength estimates based upon vertebral density appear to be useful for validation of model force predictions.

Adult↗

PubMed1998

Trunk muscle co-contraction increases during fatiguing, isometric, lateral bend exertions. Possible implications for spine stability.

STUDY DESIGN: Surface electromyographic activities were recorded from bilateral trunk muscles as test participants maintained a 50% maximum, voluntary, isometric, lateral bend contraction to volitional exhaustion. OBJECTIVES: To challenge the trunk with a prolonged lateral bend task and observe the responses of the agonist and antagonist muscles to the resulting fatigue. SUMMARY OF BACKGROUND DATA: Lateral bend exertions of the trunk have been identified as a risk factor for injury to spine tissues. However, little is known about the response of spine muscles to fatigue and the subsequent implications for spine stability, joint loading, and tissue injury. Surface electromyography provides a window on muscle loading and progressive fatigue. METHODS: Eleven male participants performed a set of maximum lateral bend exertions at the start of the trial, then maintained an upright standing posture while resisting 50% of the maximum moment until volitional exhaustion, then performed another set of maximum contractions. All contractions were isometric. Measurements were made, throughout each contraction, of the lateral bend moment and abdominal and trunk extensor electromyographic activities at six bilateral surface electromyography sites. Electromyographic amplitude and mean power frequency were calculated with 500-millisecond segments recorded serially every 800 milliseconds. Mean values were calculated with data from the first and last 5% of the trial durations. Statistics determined if fatigue had a significant effect on maximum moment and the electromyographic statistics for each muscle site. RESULTS: Fatigue resulted in a significant decrease in maximum lateral bend moment and increase in moment variability. Decreases in mean power frequency, indicating fatigue, were observed in three agonists (the trunk extensors) and one antagonist muscle. Unfatigued agonist electromyographic amplitudes correlated well with the mechanical advantage of muscles to generate lateral bend moments. Unfatigued antagonist activities were low except for the thoracic erector spinae. The agonists and antagonists demonstrated average increases of 17% and 8%, respectively, when pooled across muscles. Much of this change was predicted to have been associated with increases in co-contracting muscle forces. CONCLUSIONS: The trunk responded to a prolonged, lateral bend contraction by increasing co-contraction as agonist trunk muscles fatigued. It was proposed that the fatigue compromised neural coordination and that the co-contraction served to maintain spine stability.

Adult↗

PubMed1999

Physiological response to submaximal isometric contractions of the paravertebral muscles.

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

Adult↗

PubMed2002

Fatigue-related changes in torque output and electromyographic parameters of trunk muscles during isometric axial rotation exertion: an investigation in patients with back pain and in healthy subjects.

STUDY DESIGN: A cross-sectional case-control study. OBJECTIVES: To examine the effect of fatigue on torque output as well as electromyographic frequency and amplitude values of trunk muscles during isometric axial rotation exertion in back pain patients and to compare the results with a matched control group. SUMMARY OF BACKGROUND DATA: Back pain patients exhibited different activation strategies in trunk muscles during the axial rotation exertions. Fatigue changes of abdominal and back muscles during axial rotation exertion have not been examined in patients with back pain. METHODS: Twelve back pain patients and 12 matched controls performed isometric fatiguing axial rotation to both sides at 80% maximum voluntary contraction in a standing position. During the fatiguing exertion, electromyographic changes of rectus abdominis, external oblique, internal oblique, latissimus dorsi, iliocostalis lumborum, and multifidus were recorded bilaterally. The primary torque in the transverse plane and the coupling torques in sagittal and coronal planes were also measured. RESULTS: No difference in the endurance capacity was found between back pain and control groups. At the initial period of the exertion, back pain patients demonstrated a statistical trend (P = 0.058) of greater sagittal coupling torque as well as lower activity of rectus abdominis and multifidus and higher activity in external oblique. During the fatigue process similar changes of coupling torque were demonstrated in both sagittal and coronal planes, but a smaller fatigue rate for right external oblique, increase in median frequency for latissimus dorsi, and lesser increase in activity for back muscles were found in the back pain group compared with the control group. CONCLUSIONS: Alterations in electromyographic activation and fatigue rates of abdominal and back muscles demonstrated during the fatigue process provide insights into the muscle dysfunctions in back pain and may help clinicians to devise more rational treatment strategies.

Abdomen↗

PubMed1980

Isometric strength testing as a means of controlling medical incidents on strenuous jobs.

This investigation was performed to determine if isometric strength tests can be used to select workers for strenuous jobs and to reduce occupational injuries which are caused by a mismatch between worker strength and job strength requirements. Twenty jobs in a tire and rubber plant were studied biomechanically to identify critical strength-demanding tasks. Four strength tests were designed to simulate these tasks, and performance criteria were established for passing the tests. New applicants were administered the tests during their preplacement examinations to determine if they possessed sufficient strength to qualify for the jobs. The medical incidence rate of employees who were selected using the strength tests was approximately one-third that of employees selected using traditional medical criteria. It was concluded that isometric strength tests can be used to reduce occupational injuries and should be considered for implementation in industries with strenuous jobs.

Accidents, Occupational↗

PubMed2026

Dose-response effects of isometric wall squats on postexercise hypotension in older women with hypertension.

OBJECTIVES: The isometric wall squat (IWS) is a promising nonpharmacological intervention for blood pressure (BP) management, but its optimal dosage in older adults remains unclear. This study compared postexercise hypotension (PEH) after two IWS doses in medicated older women with systemic arterial hypertension (SAH). METHODS: Thirty older hypertensive women (68.8 ± 10.0 years) were randomized into three groups: control (CG), 8-min IWS (G8-IWS; 4 × 2-min sets), and 12-min IWS (G12-IWS; 6 × 2-min sets). BP and heart rate (HR) were assessed at rest and at 0, 5, 10, 15, 30, 45, and 60 min postexercise and analyzed using linear mixed-effects models with Bonferroni correction ( P  < 0.05). RESULTS: G12-IWS significantly reduced systolic BP and mean arterial pressure (MAP) at 45-60 min and 30-60 min postexercise ( P  < 0.05), while G8-IWS showed no BP changes. G12-IWS decreased HR (30-60 min) and rate-pressure product (RPP; 15-60 min); G8-IWS reduced RPP from 30 to 60 min only. Diastolic BP and pulse pressure remained unchanged in both groups. CONCLUSION: A single G12-IWS session induced clinically meaningful PEH (~8-10 mmHg), exceeding reported with isometric handgrip exercise (~4-6 mmHg), with greater RPP suppression than G8-IWS. This equipment-free protocol represents a relevant nonpharmacological strategy for BP management in older women with SAH.

Humans↗

PubMed1994

Comparison of isometric and load moving length-tension models of two bicompartmental muscles.

The length-tension relations of two bicompartmental muscles, including parallel and pennated fibers, were experimentally determined for isometric and load moving contractions. Comparison of the isometric and constant mass load length-tension curves obtained from the same preparations demonstrated significant reductions in force, a shift of the optimal length and increased elongation range for shortening contractions under constant mass load. Pennated muscle demonstrated a larger reduction in force and greater shift in the optimal length relative to the changes in a muscle with parallel fibers. A bicompartmental model was fitted to the experimental data to provide quantitative insight to the changes described above, and for use in mathematical models of other bicompartmental muscles, and for design of optimal electrical stimulation system for restoration of function in spinal cord injury patients.

Animals↗

PubMed1997

Relating agonist-antagonist electromyograms to joint torque during isometric, quasi-isotonic, nonfatiguing contractions.

This paper describes an experimental study which relates simultaneous elbow flexor-extensor electromyogram (EMG) amplitude to joint torque. Investigation was limited to the case of isometric, quasi-isotonic (slowly force-varying), nonfatiguing contractions. For each of the flexor and extensor muscle groups, the model relationship between muscle group torque contribution and EMG amplitude was constrained to be a sum of basis functions which had a linear dependence on a set of fit parameters. With these constraints, the problem of identifying the EMG-to-torque relationship was reduced to a linear least squares problem. Surface EMG's from elbow flexors and extensors, and joint torque were simultaneously recorded for nonfatiguing, quasi-isotonic, isometric contractions spanning 0-50% maximum voluntary contraction. Single-/multiple-channel unwhitened/whitened/adaptively whitened EMG amplitude processors were used to identify an EMG-to-torque relation, and then estimate joint torque based on this relation. Each unwhitened multiple-channel EMG-to-torque estimator had a standard error (SE) approximately 70% of its respective single-channel estimator. The adaptively whitened multiple-channel joint torque estimator had an SE approximately 90% of the unwhitened multiple-channel estimator, providing an estimation error approximately 3% of the combined flexion/extension torque range. The experimental studies demonstrated that higher fidelity EMG amplitude processing led to improved joint torque estimation.

Adult↗