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PubMed1989

Human strength: measurements of maximum isometric forces in industry.

Within the framework of an on-going research project, the maximum isometric forces of staff working in production were measured for nine test conditions representative for manual material handling (n=3600), The technique for the measurement of maximum human forces depending on body posture, the test set-up, and the procedures used for data analysis plus the preliminary group results (n=1245) are described. The results show a marked influence of the initial grip height on the maximum forces achievable for the lifting of a stacking case with both hands. Under comparable grip height and type of grip, small differences between the lifting forces with one or with two hands exist. In comparison to males, females clearly have lower maximum isometric forces than is generally supposed.

Ergonomics↗

PubMedDate not supplied

An exponential model of isometric muscular fatigue as a function of age and muscle groups.

Despite the recommendations that an important design criterion is not to exceed 15% of an operator's maximum voluntary contraction (MVC) with any muscle that must be used for a long period of time, many tasks involve the exertion of much higher forces coupled with long-term contractions. Many studies have investigated the force-time relationship of isometric muscle contractions to determine the endurance time of a given relative force. To date, however, direct studies of muscle performance throughout fatiguing tasks have not been conducted to the same degree. This research was concerned with studying the effects of different muscle groups (biceps vs quadriceps) of subjects with different age groups (20-29 vs 50-59 years of age) on long-term muscular isometric contractions at different levels of %MVC (20%, 40%, 60%, 80%, and 100% MVC), and modelling the functional data to describe the time course of strength decrement. The data revealed that the time course of strength decrement was best modelled by the function: [formula: see text] An experiment, using 20 subjects with each subject performing 10 conditions (two muscle groups x five levels of %MVC), showed that this function accounted for over 95% of the variance of strength decrement. All parameter estimates were statistically significant.

Adult↗

PubMed1994

The influence of triazolam and flunitrazepam on isokinetic and isometric muscle performance.

The influences of two benzodiazepines (triazolam 0.25 mg and flunitrazepam 2 mg) on isokinetic and isometric muscle performance and on cardiovascular parameters were examined after a standard period of sleep. A randomized and double-blind test procedure was used (n = 15). Triazolam had no significant (p < 0.05) influence on either of the test conditions. Administration of flunitrazepam significantly lowered values for maximal isometric force and for the cardiovascular parameters. It was concluded that triazolam does not influence performance. On the other hand, flunitrazepam does influence a number of strength characteristics and cardiovascular parameters in effort situations.

Adult↗

PubMed2002

The effect of exercise-induced muscle damage on isometric and dynamic knee extensor strength and vertical jump performance.

In this study, we assessed the effect of exercise-induced muscle damage on knee extensor muscle strength during isometric, concentric and eccentric actions at 1.57 rad x s(-1) and vertical jump performance under conditions of squat jump, countermovement jump and drop jump. The eight participants (5 males, 3 females) were aged 29.5+/-7.1 years (mean +/- s). These variables, together with plasma creatine kinase (CK), were measured before, 1 h after and 1, 2, 3, 4 and 7 days after a bout of muscle damaging exercise: 100 barbell squats (10 sets x 10 repetitions at 70% body mass load). Strength was reduced for 4 days (P< 0.05) but no significant differences (P> 0.05) were apparent in the magnitude or rate of recovery of strength between isometric, concentric and eccentric muscle actions. The overall decline in vertical jump performance was dependent on jump method: squat jump performance was affected to a greater extent than countermovement (91.6+/-1.1% vs 95.2+/-1.3% of pre-exercise values, P< 0.05) and drop jump (95.2+/-1.4%, P< 0.05) performance. Creatine kinase was elevated (P < 0.05) above baseline 1 h after exercise, peaked on day 1 and remained significantly elevated on days 2 and 3. Strength loss after exercise-induced muscle damage was independent of the muscle action being performed. However, the impairment of muscle function was attenuated when the stretch-shortening cycle was used in vertical jumping performance.

Adult↗

PubMed2004

Independence of reaction time and response force control during isometric leg extension.

In this study, we examined the relative control of reaction time and force in responses of the lower limb. Fourteen female participants (age 21.2 +/- 1.0 years, height 1.62 +/- 0.05 m, body mass 54.1 +/- 6.1 kg; mean +/- s) were instructed to exert their maximal isometric one-leg extension force as quickly as possible in response to an auditory stimulus presented after one of 13 foreperiod durations, ranging from 0.5 to 10.0 s. In the 'irregular condition' each foreperiod was presented in random order, while in the 'regular condition' each foreperiod was repeated consecutively. A significant interactive effect of foreperiod duration and regularity on reaction time was observed (P < 0.001 in two-way ANOVA with repeated measures). In the irregular condition the shorter foreperiod induced a longer reaction time, while in the regular condition the shorter foreperiod induced a shorter reaction time. Peak amplitude of isometric force was affected only by the regularity of foreperiod and there was a significant variation of changes in peak force across participants; nine participants were shown to significantly increase peak force for the regular condition (P < 0.001), three to decrease it (P < 0.05) and two showed no difference. These results indicate the independence of reaction time and response force control in the lower limb motor system. Variation of changes in peak force across participants may be due to the different attention to the bipolar nature of the task requirements such as maximal force and maximal speed.

Adult↗

PubMed2006

Effects of dynamic resistance training on fascicle length and isometric strength.

The aims of this study were to assess changes in muscle architecture, isometric and dynamic strength of the leg extensor muscles, resulting from dynamic resistance training, and the relationships between strength and muscle architecture variables. The participants (n = 30) were randomly assigned to one of two groups. The training group (n = 16; age 21.8 +/- 2.3 years, body mass 74.8 +/- 9.2 kg, height 1.75 +/- 0.08 m) performed dynamic resistance training for 13 weeks. The control group (n = 14; age 19.9 +/- 1.5 years, body mass 74.0 +/- 8.5 kg, height 1.76 +/- 0.05 m) did not perform any resistance training. Maximal dynamic and isometric strength were tested in both groups, before and after the training period. The members of the training group used the free-weight squat lift (90 degrees ) as their training exercise. The concentric phase of the squat was performed explosively. Skeletal muscle architecture of the vastus lateralis was visualized using ultrasonography. At the end of the study, significant increases in vastus lateralis muscle thickness (+6.9%, P < 0.001), fascicle length (+10.3%, P < 0.05), one-repetition maximum (+8.2%, P < 0.05), rate of force development (+23.8%, P < 0.05) and average force produced in the first 500 ms (+11.7%, P < 0.05) were seen only in the training group. Adaptations to the muscle architecture in the training group limited the loss of fibre force, and improved the capacity for developing higher velocities of contraction. The architectural changes in the training group were similar to those seen in studies where high-speed training was performed. In conclusion, dynamic resistance training with light loads leads to increases in muscle thickness and fascicle length, which might be related to a more efficient transmission of fibre force to the tendon.

Adult↗

PubMed2000

Isometric pull and push strengths of paraplegics in the workspace: 1. Strength measurement profiles.

The isometric strength profiles of male and female paraplegics were determined for pull and push strengths in the normal, maximum, and extreme working reach envelopes. A computerized isometric strength measurement system was designed and constructed for the purpose. The strongest pull location was at extreme reach vertically above the shoulder and the strength values for males and females were 473 and 318 newtons (N), respectively. The strongest push location was at maximum reach, at vertical (phi) angle of 45 degrees and at horizontal (theta) angle of 45 degrees for males and at 0 degrees for women and the strength values were 235 and 172 N, respectively. The nature of the strength profiles was found to be similar for both the sexes. The pull and push strengths of the female were 77 and 68% that of the male, respectively.

Adult↗

PubMed1956

Electron microscopical study of skeletal muscle during isotonic (afterload) and isometric contraction.

Bundles of the curarized semitendinosus muscle of the frog were fixed during isotonic (afterload) and isometric contraction and the length of the A and I bands investigated by electron microscopy. The sarcomere length, during afterload contraction initiated at 25 per cent stretch, varied depending on the afterload applied between 3.0 and 1.2 micro, i.e. the shortening amounted to 5 to 50 per cent. The shortening involved both the A and I bands. Between a sarcomere length of 3.0 to 1.7 micro (shortening 5 to 35 per cent) the A bands remained practically constant at about 1.5 micro (6 to 8 per cent shortening); the length of the I bands decreased from 1.4 to 0.3 micro (80 per cent shortening). Below a sarcomere length of 1.7 to 1.2 micro the A bands shortened from 1.5 to 1.0 micro (from 6 to 8 to 25 per cent). At sarcomere lengths 1.6 to 1.2 micro the I band was replaced by a contraction band. During isometric contraction the A bands shortened by about 8 to 10 per cent; the I bands were correspondingly elongated.

Animals↗

PubMed2002

Finite element model of intramuscular pressure during isometric contraction of skeletal muscle.

The measurement of in vivo intramuscular pressure (IMP) has recently become practical and IMP appears well correlated with muscle tension. A numerical model of skeletal muscle was developed to examine the mechanisms producing IMP. Unipennate muscle is modelled as a two-dimensional material continuum that is incompressible and nonlinearly anisotropic. The finite element technique is used to calculate IMP and muscle stress during passive stretch and during isometric contraction. A novel element models the contractile portion of muscle, incorporating sarcomere length-force and velocity-force relations. A range of unipennate muscle geometries can be modelled. The model was configured to simulate the rabbit tibialis anterior muscle over a range of lengths. Simulated IMP and stress results were validated against animal experimentation data. The simulation agreed well with the experimental data over the range of 0.8-1.1 of the optimal length. Severe pressure gradients were produced near the musculo-tendinous junctions while IMP was more uniform in the central muscle belly. IMP and muscle stress in relaxed (unstimulated) muscle increased nonlinearly with muscle length. IMP and stress in isometrically contracting muscle showed a local maximum at optimal length and were reduced at shorter lengths. At muscle lengths longer than optimal, stress and IMP increased predominately due to tension in the passive elastic structures.

Animals↗

PubMed1977

Effect of isometric exercises on body potassium and dietary protein requirements of young men.

A diet supplying 0.5 or 1 g egg and milk protein/kg per day and adequate energy for maintenance of body weight was given to seven healthy men aged 18 to 21. After 2 weeks for adaptation to the diet they participated in 75 min of daily isometric exercises for 4 to 6 weeks. Two men on each protein intake continued to exercise for 4 to 5 additional weeks after undergoing changes in their daily protein intake from 0.5 to 1 g/kg or vice versa. A progressive decrease in total body potassium became significant by the fourth week of exercises in all men who began on the 0.5 g protein diet. Two men on that diet who remained as nonexercising controls lost no body K. These findings indicated that 0.5 g of egg and milk protein/kg per day was insufficient for men performing isometric exercises without prior training, whereas 1 g/kg per day appeared to be adequate. Two other nonexercising men on the 0.5 g diet who lost weight showed a marked negative nitrogen balance and loss of total body potassium which improved after their dietary energy intake increased. The changes in nitrogen balance and body weight observed in this investigation support the concept that protein requirements vary with energy intakes. Conversely, they suggest that energy requirements are influenced by the level of protein intake.

Adult↗

PubMed2004

Effect of three anaesthetic techniques on isometric skeletal muscle strength.

BACKGROUND: Our aim was to quantify human involuntary isometric skeletal muscle strength during anaesthesia with propofol, sevoflurane, or spinal anaesthesia using bupivacaine. METHODS: Thirty-three healthy patients undergoing anaesthesia for elective lower limb surgery were investigated. Twenty-two patients received a general anaesthetic with either propofol (n=12) or sevoflurane (n=10); for the remaining 11 patients spinal anaesthesia with bupivacaine was used. We used a non-invasive muscle force assessment system before and during anaesthesia to determine the contractile properties of the ankle dorsiflexor muscles after peroneal nerve stimulation (single, double, triple, and quadruple stimulation). We measured peak torques; contraction times; peak rates of torque development and decay; times to peak torque development and decay; half-relaxation times; torque latencies. RESULTS: Males elicited greater peak torques than females, medians 6.3 vs 4.4 Nm, respectively (P=0.0002, Mann-Whitney rank-sum test). During sevoflurane and propofol anaesthesia, muscle strength did not differ from pre-anaesthetic values. During spinal anaesthesia, torques were diminished for single-pulse stimulation from 3.5 to 2.0 Nm (P=0.002, Wilcoxon signed rank test), and for double-pulse from 7.6 to 5.6 Nm (P=0.02). Peak rates of torque development decreased for single-pulse stimulation from 113 to 53 Nm s(-1) and for double pulse from 195 to 105 Nm s(-1). Torque latencies were increased during spinal anaesthesia. CONCLUSIONS: At clinically relevant concentrations, propofol and sevoflurane did not influence involuntary isometric skeletal muscle strength in adults, whereas spinal anaesthesia reduced strength by about 20%. Muscle strength assessment using a device such as described here provided reliable results and should be considered for use in other scientific investigations to identify potential effects of anaesthetic agents.

Adolescent↗

PubMed1995

Abnormal muscle coactivation patterns during isometric torque generation at the elbow and shoulder in hemiparetic subjects.

To study abnormal spatial patterns of muscle activation in hemiparetic stroke, we compared EMG activity in paretic and contralateral elbow and shoulder muscles of 10 hemiparetic subjects during 1.5-s voluntary isometric contractions, against five to eight different loads. Isometric forces were generated in eight directions, referenced to a plane orthogonal to the long axis of the forearm, and were recorded by a three degrees of freedom load cell, mounted at the wrist. Surface and intramuscular EMGs of six elbow and six shoulder muscles were recorded from both impaired and contralateral upper extremities of each subject. The spatial characteristics of EMG activation of individual muscles were summarized using two measures. The first, called the 'net resultant EMG vector' is a new measure which calculated the vector sum of EMG magnitudes for each of the eight directions, and the second, index of EMG focus, is a measure of the range of EMG activation recorded for each load level. Use of these measures permitted us to describe spatial EMG characteristics quantitatively, which has not been done previously. We observed consistent and statistically significant shifts in the resultant EMG vector directions in the impaired limb, especially in shoulder and other proximal muscles. Significant increases in the angular range of EMG activity were also identified and were most evident at the elbow. Correlation analysis techniques were used to assess the degree of coactivation of different muscle pairs. There were consistent EMG coactivation patterns observed across all subjects (both normal and hemiparetic). However, in spasticparetic limbs, additional novel coactivational relationships were also recorded, especially between elbow flexors/shoulder abductors and elbow extensors/shoulder adductors. These novel coactivation patterns represent a reduction in the number of possible muscle combinations, or in the number of possible 'synergies' in the paretic limb of the stroke subject. This reduction in number of 'synergies' could result from a loss of descending command options; from an increased reliance on residual, descending brainstem pathways (such as the reticulospinal and vestibulospinal projections); from changes in spinal interneuronal excitability; or from a combination of several of these factors. The relative merits of these hypotheses are addressed.

Adult↗

PubMed1994

Effect of heart rate on myocardial relaxation in isometric twitches.

OBJECTIVE: Increased heart rate increases the amount of calcium that triggers myofilament interaction and thereby the load of calcium to be sequestered to cause relaxation, while decreasing the time for calcium sequestration. By studying the effect of heart rate on myocardial relaxation the hypothesis was therefore tested that increased heart rate impairs myocardial relaxation. METHODS: In situ blood-perfused right ventricular papillary muscles of seven isolated Suga-Sagawa cross circulated canine hearts were studied with a servo system that clamped muscle length to produce isometric force, F(t), at one isometric length in each muscle at paced heart rates within the range 100-150. Each curve of F(t) was fitted, by Marquardt's algorithm, with the relation F(t) = C(t/A)B(e)1-(t/A)B. It was previously shown that the parameter B reflects changes in relaxation, whereas the parameters A and C reflect chronotropic and inotropic states. Changes in B reflect changes in the time course of relaxation that are not simply due to changes in total twitch duration guaranteed to be produced by altered heart rate. Linear regression analysis was performed on data from each muscle to determine whether each of the parameters A, B, and C changed significantly with heart rate. RESULTS: The mean coefficient of determination, a measure of goodness of fit of the model to observed data, was 0.996(SD 0.001). With increase in heart rate, A and C increased significantly in six of the seven muscles. On the other hand B did not show significant change with heart rate in six of the seven muscles; in the seventh muscle, B increased significantly with heart rate. Similar results were obtained when other indices of lusitropy were used. CONCLUSIONS: Increase in heart rate does not impair myocardial lusitropy as measured by the parameter B which reflects changes in the time course of relaxation independent of change in total time course of mechanical activity.

Animals↗

PubMed2005

Reliability of a single-session isokinetic and isometric strength measurement protocol in older men.

BACKGROUND: The purposes of the current study were (a) to determine the test-retest reliability of a single-session isokinetic and isometric strength testing protocol in older healthy men, and (b) to compare the outcomes of the reliability measures derived from averaged torque scores with those derived from a single peak torque score. METHODS: In 19 men (mean age, 72 +/- 5 years), both lower limbs were assessed independently on 2 separate test days using the Biodex System 3 dynamometer. After completing a 5-minute warm-up, each man performed three submaximal knee extensions followed by five maximal contractions at 90 degrees /s (CON), 0 degrees /s (ISO), and -90 degrees /s (ECC). Average (best 3 of 5) and peak CON, ISO, and ECC torque, and CON work and CON power were determined. Peak CON work and CON power were recorded from the highest peak torque concentric contraction (HPTCC). RESULTS: Intraclass correlation coefficients ranging from 0.84 to 0.94 were found to have good reliability. The typical error as a coefficient of variation ranged from 8% to 10% for averaged measures and from 8% to 17% for peak torque and HPTCC. The ratio limits of agreement for average and peak CON, ISO, and ECC torque ranged from 23% to 33% and from 40% to 54% for average CON and HPTCC work and power. CONCLUSIONS: The test-retest reliability of a single-session isokinetic and isometric strength testing protocol in this group of older healthy men displayed good relative reliability (intraclass correlation coefficient > 0.84); however, because the typical error as a coefficient of variation and ratio limits of agreement (absolute reliability) were large, single-session testing is not recommended.

Aged↗

PubMed1983

Value of isometric exercise testing in the optimal timing of aortic valve replacement in aortic regurgitation.

To evaluate the possible irreversibility of isometric exercise-induced left ventricular dysfunction in aortic regurgitation, we performed handgrip exercise tests during cardiac catheterization on 17 patients with chronic aortic regurgitation (AR-group), both preoperatively and one year after successful aortic valve replacement. Nine normal subjects served as a control group. Preoperatively, the ejection fraction decreased from 59 +/- 7% to 53 +/- 8% (P less than 0.001) in the AR-group while it remained unchanged in the control group during exercise. A positive correlation existed between the changes in the ejection fraction during pre- and postoperative exercise tests (r = 0.85, P less than 0.001), which revealed that in patients with severely depressed ejection fraction during the preoperative exercise test, the ventricular response to exercise was not totally corrected after surgery. The regression of left ventricular mass was also smaller in the patients with most depressed ventricular response to exercise preoperatively. As the isometric exercise-induced left ventricular dysfunction appears to be partly irreversible, we conclude that the valve replacement should perhaps be performed before stress-induced ventricular dysfunction has occurred. Non-invasive monitoring of ventricular response to exercise might be helpful in optimizing the timing of valve replacement in aortic regurgitation.

Adult↗

PubMed1978

The effect of deep heat on isometric strength.

The effect of a 20-minute treatment of short-wave diathermy on isometric strength of the quadriceps muscle group was studied in 12 subjects over a 2-hour period. The heat treatment caused an initial decrease in isometric strength followed by an increase 30 minutes after the heat was terminated. Strength remained significantly above the control level for two hours following heat. The findings of this study suggest that the relationship between the application of short wave diathermy and the optimal time which should elapse between the heat treatment and therapeutic exercise should be examined.

Adult↗

PubMed1981

Electromyographic activity recorded from an unexercised muscle during maximal isometric exercise of the contralateral agonists and antagonists.

The purpose of this study was to determine if integrated electromyographic activity recorded from an unexercised muscle during contralateral exercise was dependent upon the contralateral muscle (agonist or antagonist) being exercised and upon the position of the unexercised limb. Twenty normal subjects participated in the study. Electromyographic activity was recorded, using surface electrodes, from the right rectus femoris and the right vastus lateralis muscles during four experimental conditions. Statistical analysis revealed that 1) integrated electromyographic activity recorded from the rectus femoris muscle during contralateral, isometric exercise was not significantly influenced by the position of the unexercised knee or the exercised contralateral muscle and 2) integrated electromyographic activity recorded from the vastus lateralis muscle during contralateral, isometric exercise was significantly greater when the unexercised knee was initially positioned in 10 degrees of flexion and the contralateral agonists were exercised. Integrated electromyographic activity recorded during contralateral exercise appeared to be related to the associated movement of the unexercised limb.

Adult↗

PubMed1987

Differences in elbow flexion torque measured concentrically, eccentrically, and isometrically.

The purposes of this study were to describe torque-velocity relationships during concentric, eccentric, and isometric testing of elbow flexor muscles and to determine test-retest reliability of a specific isokinetic testing protocol. Thirty healthy women were tested using an isokinetic dynamometer at velocities of 0 degrees, 30 degrees, 120 degrees, and 210 degrees/sec. Reliability was assessed in 20 of the subjects by retesting concentric and eccentric torques at 30 degrees and 120 degrees/sec after a 30-minute rest interval. The highest torque from three maximal efforts was considered to be peak torque. Mean eccentric peak torque was greater than either isometric or concentric peak torque; as isokinetic test velocity increased, the differences between concentric and eccentric torques became larger. Intraclass correlation coefficients ranged from .72 to .83, indicating lack of consistency between test and retest torques. Consecutive concentric-eccentric testing and lack of mechanical stabilization might have contributed to test-retest variability and to the relatively small differences between concentric and eccentric torques. Continued study of reliability of isokinetic testing protocols and further investigation of concentric-eccentric torque-velocity relationships are advocated.

Adult↗