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PubMed1981

Interactions between fatiguing and nonfatiguing isometric contractions.

Isometric contractions of the handgrip muscles were exerted by eight male subjects (age range 19-24 yr) to determine the relationship between fatiguing and nonfatiguing isometric contractions. In a first series of experiments, subjects exerted fatiguing isometric contractions at tensions of 25, 40, or 70% of the maximum strength of the subjects (MVC) following a contraction at a nonfatiguing tension of 5 or 10% MVC for 3, 7, or 20 min. In a second series of experiments, subjects exerted pairs of fatiguing isometric contractions at the same tensions; however, during the 3-, 7-, or 20-min interval between the contractions the subjects either rested or exerted a contraction at 5 or 10% MVC. Nonfatiguing isometric contractions exerted prior to fatiguing contractions had little influence on the endurance for the fatiguing static effort. In contrast, when contractions at nonfatiguing tensions were exerted in the recovery interval following an isometric contraction at a fatiguing tension, the recovery of endurance was dramatically reduced.

Adult↗

PubMed1990

Increased fatigue of isovelocity vs. isometric contractions of canine diaphragm.

A comparison of fatigue as a loss of force with repeated contractions over time was performed in canine respiratory muscle by isometric (nonshortening) and isovelocity (shortening) contractions. In situ diaphragm muscle strips were attached to a linear ergometer and electrically stimulated (30 or 40 Hz) via the left phrenic nerve to produce either isometric (n = 12) or isovelocity (n = 12) contractions (1.5 s) from optimal muscle length (Lo = 8.8 cm). Similar velocities of shortening between isovelocity experiments [0.19 +/- 0.02 (SD) Lo/S] were produced by maximizing the mean power output (Wmax = 210 +/- 27 mW/cm2) that could be developed over 1.5 s when displacement was approximately 0.30 Lo. Initial peak isometric tension was 1.98 kg/cm2, whereas initial peak isovelocity tension was 1.84 kg/mc2 (P less than 0.01) or 93% of initial isometric tension. Fatigue trials of 5 min were conducted on muscles contracting at a constant duty cycle (0.43). At the end of the trials, peak isovelocity tension had fallen to 50% of initial isometric tension (P less than 0.01), whereas peak isometric tension had only fallen by 27%. These results indicate that muscle shortening during force production has a significant influence on diaphragm muscle fatigue. We conclude that the effects of shortening on fatigue must be considered in models of respiratory muscle function, because these muscles typically shorten during breathing.

Animals↗

PubMed2005

Fatigue and recovery of power and isometric torque following isotonic knee extensions.

The purpose of this study was to assess fatigue and recovery of isotonic power and isometric contractile properties after a series of maximal isotonic contractions. Using a Biodex dynamometer, 13 men [26 yr (SD 3)] performed isotonic [50% of isometric maximal voluntary contraction (MVC) every 1.2 s through 75 degrees range of motion] single-limb knee extensions at the fastest velocity they could achieve until velocity was reduced by 35%. Time to task failure was 38 s, and, compared with baseline, power declined by approximately 42% [741.0 (SD 106.0) vs. 426.5 W (SD 60.3) at task failure], and MVC declined by approximately 26% [267.3 (SD 42.5) vs. 198.4 N.m (SD 45.7) at task failure]. Power recovered by 5 min, whereas MVC did not recover, and at 10 min was only approximately 85% of baseline. Isometric MVC motor unit activation was approximately 95% at rest and was unchanged at task failure (approximately 96%), but a small amount of failure was apparent between 1.5 and 10 min of recovery (approximately 87 to approximately 91%). Half relaxation time measured from a 50-Hz isometric tetanus was significantly prolonged by approximately 33% immediately after task failure but recovered by 1.5 min. A decline in the 10- to 50-Hz ratio of the evoked isometric contractions was observed at 5 and 10 min of recovery, which suggests excitation-contraction coupling impairment. Changes in velocity and half relaxation time during the protocol were strongly and negatively correlated (r = -0.85). Thus mainly peripheral mechanisms were implicated in the substantial depression but relatively fast recovery of isotonic power. Furthermore, isometric muscle contractile properties were related to some, but not all, changes in isotonic function.

Adult↗

PubMed1990

Organizing principles for single-joint movements. IV. Implications for isometric contractions.

1. Normal human subjects made isometric pulse and step contractions about the elbow to visually defined target torques of different amplitudes and at different rates. We measured joint torque and electromyograms (EMG) from two agonist and two antagonist muscles. 2. When the task specification requires that the subject explicitly alter the rate at which torque is increased, the rates of rise of the agonist and antagonist EMG bursts covary with the rate of rise of the torque. For pulses of torque the duration of motoneuron excitation varies with the duration of the task-defined contractile event. 3. When a subject is asked to generate torques of different amplitudes without specifying a time interval, torque amplitude is positively correlated with how long, and therefore how high, the EMG rose. Subjects usually proportionately covary the strength of the agonist and antagonist contractions but are not constrained to do so. Some subjects use a strategy of varying the antagonist inversely with the agonist contraction. 4. We extend the organizing principles for the control of movement about a single joint to the control of isometric torque. These rules state that control of torque about a single joint is exercised by one of two strategies: the speed-sensitive strategy modulates the rate at which contraction rises by varying the intensity of motoneuron-pool excitation. The speed-insensitive strategy varies the duration over which contraction rises but does not change the rate. These two respective patterns of torque emerge from pulse-height and pulse-width modulation of motoneuron-pool excitation. 5. The rules defining speed-sensitive and speed-insensitive strategies for movements are broadened for isometric contractions because of the wider range of torque patterns that we observe under these conditions. We propose a step-excitation component for prolonged isometric step contractions and slowly rising ramp patterns of excitation for contractions that develop over several hundreds of milliseconds. 6. The choice of strategies is based on task-specific torque requirements. The same two strategies that control torque to produce movement apply to the control of isometric torque. Unlike movements, however, isometric tasks are more often controlled by a blending of the two patterns. Possible reasons for this are discussed.

Electromyography↗

PubMed1996

Motor unit control properties in constant-force isometric contractions.

1. The purpose of this study was 1) to characterize the decrease observed in mean firing rates of motor units in the first 8-15 s of isometric constant-force contractions and 2) to investigate possible mechanisms that could account for the ability to maintain force output in the presence of decreasing motor unit firing rates. 2. The decrease in mean firing rates was characterized by investigating myoelectric signals detected with a specialized quadrifilar needle electrode from the first dorsal interosseus (FDI) and the tibialis anterior (TA) muscles of 19 healthy subjects during a total of 85 constant-force isometric contractions at 30, 50, or 80% of maximal effort. The firing times of motor units were obtained from the myoelectric signals with the use of computer algorithms to decompose the signal into the constituent motor unit action potentials. Time-varying mean firing rates and recruitment thresholds were also calculated. 3. Motor units detected from the TA muscle were found to have a continual decrease in their mean firing rates in 36 of 44 trials performed during isometric ankle dorsiflexion at force values ranging from 30 to 80% of maximal effort and a duration of 8-15 s. Likewise, motor units detected in the FDI muscle displayed a decrease in firing rate in 32 of 41 trials performed during constant-force isometric index finger abduction for contractions ranging from 30 to 80% of maximal effort. In 14 contractions (16% of total), firing rates were essentially constant, whereas in 3 contractions (4%), firing rates appeared to increase. 4. Motor units with the higher recruitment thresholds and lower firing rates tended to display the greater decreases in firing rate over the constant-force interval, whereas motor units with lower recruitment thresholds and higher firing rates had lesser rates of decrease. Furthermore, increasing contraction levels tended to intensify the decrease in the motor unit firing rates. 5. Three possible mechanisms were considered as factors responsible for the maintaining of force output while motor units decreased their firing rates: motor unit recruitment, agonist/antagonist interaction, and twitch potentiation. Of these, motor unit recruitment was discarded first because none was observed during the 8-15 s duration of any of the 85 contractions. Furthermore, contractions outside the physiological range of motor unit recruitment (at 80% of maximal effort) revealed the same decreasing trend in firing rates, ruling out recruitment as the means of sustaining force output. 6. The role of agonist or antagonist muscle interaction was investigated with the use of the muscles controlling the wrist joint. Myoelectric signals were recorded with quadrifilar needle electrodes from the wrist extensor muscles while myoelectric activity in the wrist flexor muscles was concurrently monitored with surface electrodes during constant-force isometric wrist extension at 50% of maximal effort. Firing rates of the motor units in the wrist extensor muscles simultaneously decreased while the flexor muscles were determined to be inactive. 7. All the findings of this study regarding the behavior of the firing rates could be well explained by the reported characteristics of twitch potentiation that have been previously documented in animals and humans. 8. The results of this study, combined with the results of other investigators, provide the following scenario to explain how a constant-force isometric contraction is sustained. As the contraction progresses, the twitch force of the muscle fibers undergoes a potentiation followed by a decrease. Simultaneously, the "late adaptation" property of the motoneuron decreases the firing rate of the motor unit. Findings of this study suggest that voluntary reduction in firing rates also cannot be ruled out as a means to augment the adaptation in motoneurons. (ABSTRACT TRUNCATED)

Adult↗

PubMed2002

Maximal isometric muscle strength of the cervical spine in healthy volunteers.

OBJECTIVE: To describe the maximal isometric neck muscle strength in healthy Chinese volunteers, in six different directions, as measured by a Multi Cervical Rehabilitation Unit. DESIGN: A standardized cross-sectional observational study. SETTING: A university rehabilitation unit. SUBJECTS: Ninety-one healthy volunteers aged 20-84. METHODS: During the measurement the subject was instructed to do three consecutive steady contractions as hard as possible, with a 10-second rest in between each contraction and a 2-minute rest between different directions. The peak isometric strength for each of the six directions (flexion, extension, lateral flexions, protraction and retraction) was calculated. RESULTS: No significant difference was found in muscle strength between different age groups. Isometric muscle strength in the direction of right lateral flexion was significantly greater than that to the left in men (p = 0.030), but no difference was found in women (p = 0.297). Isometric strength in all directions in men was 1.2-1.7 times that in women (all p < 0.028). Correlations between physical measurements (height and weight) and strength values were all insignificant in both genders. CONCLUSION: Men have approximately 20-70% greater isometric neck muscle strength than women. Both men and women can maintain high levels of cervical muscle strength in six different directions up to their seventh decade. There is no significant correlation between physical measurements and isometric neck muscle strength.

Adult↗

PubMed1997

Two years of growth hormone (GH) treatment increase isometric and isokinetic muscle strength in GH-deficient adults.

GH deficiency in adults is associated with reduced muscle mass and muscle strength. The objective of this trial was to follow the effect of 2 yr of GH treatment in GH-deficient adults on muscle performance in relation to a reference population. Knee extensor and flexor strengths for isometric and isokinetic concentric muscle strength were measured using a Kin-Com dynamometer. Hand-grip strength was measured in both hands. The fatigue index was calculated as the percent reduction in peak torque at 50 repeated isokinetic knee extensions. Superimposed, single twitch electrical stimulation was performed. The GH-deficient subjects had lower isometric knee extensor, knee flexor, and hand-grip strength than the reference population. Two years of GH treatment increased and normalized the mean isometric knee extensor and flexor strengths. The concentric knee flexor and extensor strength at an angular velocity of pi rad/s increased, as did the concentric knee flexor strength at an angular velocity of pi/3 rad/s. The increase in muscle strength was more marked in younger patients and in patients with lower initial muscle strength than predicted. Quadriceps endurance decreased, whereas the effect of superimposing single twitches on isometric contraction and hand-grip strength was unaffected by the GH treatment. Two years of GH therapy in GH-deficient adults increased and normalized isokinetic and isometric muscle strength studied in proximal muscle groups. Hand-grip strength and the degree of lack of maximal motor unit activation on voluntary isometric knee extensor force did not change. The dynamic local muscle fatigue index decreased.

Adult↗

PubMed2005

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions.

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.

Adult↗

PubMed1993

Reliability of dynamic and isometric testing of shoulder external and internal rotators.

Advances in isokinetic technology allow the physical therapist to assess concentric, eccentric, and isometric muscle performance at the shoulder in various positions. Research is limited, however, on the reliability of isokinetic measurements. The purposes of this study were: 1) to determine the test-retest reliability of concentric, eccentric, and isometric muscle performance measurements of shoulder external and internal rotation in the scapular plane test position and 2) to compare this reliability between the involved and uninvolved limb of subjects with a history of unilateral shoulder pathology. Fourteen males and 10 females (17-58 years) were tested on two occasions at 1-week intervals with the Biodex isokinetic dynamometer. Peak torque, total work, and average power were recorded for concentric tests at angular velocities of 60 and 120 degrees/sec and for eccentric tests at 60 degrees/sec. Maximum average isometric torque was recorded in two positions. Means, standard deviations, and mean differences between sessions with 95% confidence intervals were calculated. Intraclass correlation coefficients (ICCs) were used to determine test-retest reliability. Isometric tests were generally most reliable (ICC = .81-.93), followed by concentric (ICC = .60-.95) and eccentric tests (ICC = .44-.92). Isokinetic and isometric reliability were usually higher for involved than uninvolved shoulders. The implication of these findings is that there appears to be greater variability with eccentric than concentric or isometric testing of shoulder rotation. Factors that possibly contributed to variability are discussed. Clinicians should recognize potential sources of testing error when obtaining isokinetic measurements for use in clinical decision-making. Further refinement of isokinetic testing protocols at the shoulder is recommended.

Adolescent↗

PubMed1996

Detection of submaximal effort in isometric and isokinetic knee extension tests.

The use of torque variability and slope measures to detect submaximal efforts has been studied in isometric tests, but not fully investigated in isokinetic tests. The purpose of this study was to investigate differences between maximal and submaximal efforts in isometric and isokinetic knee extension using torque variability and slope measures obtained from 32 volunteers (age = 25.2 +/- 4.7 years). The coefficient of variation of average torque, coefficient of variation of peak torque, and slope to peak torque were obtained from maximal and submaximal torque curves during isometric and two isokinetic tests (60 degrees/sec and 180 degrees/sec). Significant differences between effort levels (maximal and submaximal) were shown for all variables in isometric and isokinetic tests. An optimized cut-off value to determine submaximal efforts without false positives was determined for each variable. The coefficient of variation of average torque detected 75% of submaximal efforts at 180 degrees/sec and slope to peak torque detected 63% of submaximal efforts at 60 degrees/sec. For both speeds, combining the coefficient of variation of average torque with slope improved the submaximal detection rate to 84%. No variable provided a satisfactory detection rate for the isometric test. Therefore, submaximal isokinetic knee extensions are detectable with torque variability and slope measures, but submaximal isometric exercise is not detectable.

Adult↗

PubMed1999

Relation between intrinsic viscoelasticity and activation level of the human finger muscle during voluntary isometric contraction.

The purpose of the present study was to isolate the length perturbation-evoked force attributed to intrinsic muscle viscoelasticity, and to investigate the relation between muscle viscoelasticity and the level of muscle activation during isometric contraction in five healthy male subjects. A small length perturbation (stretching or contraction) was applied to the flexor pollicis longus muscle while the subject maintained constant isometric force; the time courses of the length perturbation was found to be almost identical in all the experiments. The force (Fv) induced by the muscle viscoelasticity was calculated using the equation Fv = F - Fc - Fp over an interval of 35 ms after the onset of perturbation, where F is the measured force, Fc is the tonic isometric force before the onset of perturbation and Fp is the force at rest obtained from the same length perturbation. The force response attributed to the stretch reflex is not included during this interval. These experiments were repeated at varying levels of isometric force. An almost linear relationship was obtained between the muscle viscoelasticity-induced force and the tonic isometric force during both the stretching and contraction of the muscle, i.e. the intrinsic muscle viscoelasticity varied almost linearly with the level of isometric contraction.

Adult↗

PubMedDate not supplied

[Reliability and validity of the analysis of hand grip and pinch force in isometric and isokinetic conditions].

Strength measurement of the hand grip is at the core of most protocols of functional assessment of the upper limb and in rehabilitation plays a major role in the analysis of treatment efficacy and patients' occupational ability. The aims of this study were to: a) verify the repeatability of strength measurements made during performance of the hand grip and three types of pinch, carried out under isometric and isokinetic conditions; b) compare maximal isometric strength with the corresponding isokinetic value for each of the manoeuvres studied; c) investigate the correlations between the strength expressed in the different manoeuvres, under both isometric and isokinetic conditions. We studied 14 voluntary subjects over three sessions conducted at 48-hr intervals, employing a computerized isokinetic dynamometer Lido WorkSet equipped with device N(o) 21 for the study of pinch (lateral pinch, pulp pinch, chuck pinch) and device N(o) 52 for the grip study. Isometric contractions resulted stronger than isokinetic ones, and the hand grip was found to be the manoeuvre able to produce most strength. The repeatability of each strength measurement test over the three days was high (Intraclass Correlation Coefficients: 0.89-0.93). Correlations between the isometric and isokinetic performance for each of the manoeuvres examined were always high (Pearson's r coefficients: 0.89-0.95) as were those between the different manoeuvres, whether performed in isometric or isokinetic modality (r: 0.60-0.94).

Adult↗

PubMed2001

The endurance training effect on the oxygenation status of an isometrically contracted forearm muscle.

The aim of the study was to find the influence of training on the oxygenation status of a forearm muscle during submaximal isometric contraction. Six subjects performed a test of isometric contraction of a forearm muscle by continuous pressing of a handgrip dynamometer with a force of 15 kp, until exhaustion. The continuous measurements of oxygen saturation (%) (OXY), concentration of oxygenated haemoglobin (microM) (OXYHb), deoxygenated haemoglobin (microM) (DEOXYHb) and total haemoglobin (microM) (TOTHb) in blood, which passed through the muscle, were made by the near infrared spectroscope Oxymeter 96208 (ISS, USA). This test was repeated before and after the four weeks training period, which consisted of 5 continuous isometric contractions of 30 s at the beginning and of 1:15 min during the last week. Training did not significantly influence maximal isometric force (55.5 +/- 10 kp before, 60.2 +/- 9 kp after). On the other hand, the duration of isometric contraction increased from 271 +/- 117 s to 388 +/- 152 s (P < 0.05). Subjects were substantially different in duration of muscle contraction and also in oxygenation status during the contractions. Therefore, the relationship between these parameters and the training effect was calculated. The training effect was represented as the difference between the duration of muscle contraction before and after the training period., This parameter significantly correlated with relative oxygen saturation (r = -0.88; P < 0.05) and with relative concentration of deoxygenated haemoglobin (r = 0.87; P < 0.05). These results demonstrated the importance of individual training adaptations of forearm muscles. The increase of the duration of submaximal isometric contraction, as a training effect, can be dependent on larger muscle deoxygenation.

Adult↗

PubMed2001

Physical characteristics as predictors of quadriceps muscle isometric strength: a pilot study.

This one-group experimental study was carried out to investigate the relationship between isometric strength of quadriceps femoris muscle group and physical characteristics of subjects namely: age; weight; and height. Prediction equations were also derived for quadriceps isometric strength from these physical characteristics. Fifty volunteer, right-legged healthy normal male subjects participated in the study. They were aged between 19 and 27 years. The subjects had no previous history of neuromuscular and skeletal injuries to the lower limbs. Their ages, height and weight were measured in years, centimeters and kilograms, respectively. Quadriceps isometric strength was measured using an adapted cable tensiometer (ACT) and recorded in kilogramforce (kgf). Pearson's product correlation co-efficient (r) was used to study the relationship between quadriceps strength and each of age, height and weight. Linear and multiple regression analyses were also carried out. The result showed a high and positive Pearson's moment correlation coefficient (r) between quadriceps isometric strength and each of weight and height. A positive but low correlation (r) was also found between age and quadriceps isometric strength. Prediction equations were also derived from the linear and regression analyses. The study concluded that there was linear relationship between the physical characteristics and quadriceps isometric strength. It was recommended that the prediction equation be employed to estimate quadriceps strength while setting muscle strengthening goals in the clinics during medical rehabilitation for patients within the age range used in this study.

Adult↗

PubMed1989

[Characteristics of functional tests with isometric loads used in the diagnosis of ischemic heart disease].

Cardiovascular response to isometric stress (IS) applied to the upper and the lower extremities, and general isometric stress amounting to 30% of the maximum productive force (MPF) were compared in 50 normal subjects and 60 coronary patients. Hemodynamic values were significantly higher at 30% MPF general isometric stress tolerance threshold, as compared to those seen in the last minute of the maintenance of IS of the same force by the upper extremities. Hemodynamic responses to IS, applied to the lower extremities, and to general 30% MPF isometric stress were basically similar. The general 30% MPF isometric stress was more sensitive with respect to the diagnosis of coronary disease, while the test's specificity and predictive value were fairly high. The sensitivity of the lower-extremity IS test was somewhat lower, as compared to that of the general isometric stress test, yet it is much higher, as compared to the upper-extremity IS test, and can therefore be used in screening programs.

Arm↗

PubMed1987

[Value of the stop-test in the diagnosis of detrusor hyperactivity. Concept of the isometric zone. Preliminary study of 30 cases].

The authors propose a new pathophysiological and diagnostic approach to detrusor hyperactivity by the study of the isometric pressure obtained during the stop-test. This test consists of the sudden interruption of micturition by voluntary contraction of the striated urethral sphincter, resulting in sudden contraction of the bladder against the closed sphincter. This isometric contraction is equal to 20 to 30 cm of water. The increase in the pressure (delta P) between the basal micturition pressure and the absolute value of the isometric pressure (P iso) reflects the contractile force of the detrusor. The progressive return to the basal micturition pressure reflects the relaxation of the detrusor, which is the sum of visco-elastic properties and the mechanisms of cortico-sub-cortical neurological inhibition. Calculation of the isometric area (delta P X T/2), where T represents the time taken to return to the initial pressure, allows cases of central neurological hyperactivity (isometric area always greater than 100) to be distinguished from cases of detrusor hyperactivity of mechanical origin (isometric area less than 80). These data have been confirmed in a series of 30 patients.

Adolescent↗

PubMed1979

Response of hypertensive adolescents to dynamic and isometric exercise stress.

Isometric handgrip and dynamic exercise stress tests were performed on 109 hypertensive and 74 normotensive subjects 14 to 17 years old. The hypertensive subjects had resting systolic or diastolic pressures persistently above the 95th percentile on four consecutive examinations. Blood pressures and ECGs were recorded during isometric handgrip (25% maximum effort for four minutes) and bicycle ergometry until the subject was exhausted. The hypertensive subjects increased systolic pressure by an average 16 mm Hg with isometric exercise and 53 mm Hg with dynamic exercise. Control subjects had similar pressure changes, averaging 18 and 54 mm Hg, respectively. During isometric handgrip stress, diastolic pressures increased 12 mm Hg in hypertensive subjects and 18 mm Hg in control subjects. Only two hypertensive adolescents developed systolic pressures exceeding 200 mm Hg during dynamic exercise stress, and none developed systolic pressures above 200 mm Hg during isometric exercise stress. None of the normotensive or hypertensive subjects developed cardiac arrhythmias and the prevalence of ST segment depression during maximal stress was less than 2% in both groups. Therefore, in adolescents with mild to moderate hypertension the risk of developing significant ECG or hemodynamic abnormalities during mild isometric or heavy dynamic exercise is small. We believe the decision to restrict physical activity of an adolescent with elevated pressures should be based on the development of abnormal ST segment depression, cardiac arrhythmias, or excessive blood pressures at the time of exercise stress testing.

Adolescent↗

PubMed1981

Quantification through the surface EMG of muscle fatigue and recovery during successive isometric contractions.

The surface electromyogram (EMG) was analyzed from above the handgrip muscles of the forearm in six male volunteer graduate and medical students during brief and fatiguing isometric contractions in order to assess the influence of successive isometric contractions on the amplitude and frequency components of the surface EMG. Isometric contractions were sustained at three tensions; 25, 40 and 70% of each individual's maximum voluntary strength (MVC). Two successive isometric contractions were conducted in each series of experiments. The interval between these contractions was set at 2, 4, 6, 8, 10, or 12 min. In the first series of experiments, the second contraction in the series consisted of a fatiguing isometric contraction conducted at the same tension as the first contraction. In the second series of experiments, the second contraction was an MVC. From the data, a computer program was written to predict the degree of fatigue and recovery following fatiguing isometric contractions. The program was based on an analysis of the frequency components of the EMG and proved to be quite useful for showing fatigue and recovery in this muscle group.

Adult↗