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PubMed2003

Comparison of maximal voluntary isometric contraction and hand-held dynamometry in measuring muscle strength of patients with progressive lower motor neuron syndrome.

Context. Maximal voluntary isometric contraction, a method quantitatively assessing muscle strength, has proven to be reliable, accurate and sensitive in amyotrophic lateral sclerosis. Hand-held dynamometry is less expensive and more quickly applicable than maximal voluntary isometric contraction. Objective. To investigate if hand-held dynamometry is as reliable and valid as maximal voluntary isometric contraction in measuring muscle strength in patients with an adult-onset, non-hereditary progressive lower motor neuron syndrome. Design. Two testers performed maximal voluntary isometric contraction and hand-held dynamometry measurements in six muscle groups bilaterally in patients with progressive lower motor neuron syndrome to assess reliability and validity of both the methods. Setting. Outpatient units of an academic medical center. Patients. A consecutive sample of 19 patients with non-hereditary progressive lower motor neuron syndrome (median disease duration 32.5 months, range 10-84) was tested. Outcome measures. Comparison between maximal voluntary strength contractions as measured by hand-held dynamometry and maximal voluntary isometric contraction. Results. Low intra- and interrater variation in all muscle groups were found, intraclass correlation coefficients vary between 0.86 and 0.99 for both methods. Both methods correlated well in all muscle groups with Pearson's correlation coefficients ranged between 0.78 and 0.98. Scatter plots indicated a trend to under-estimate muscle strength above 250 N by hand-held dynamometry as compared with maximal voluntary isometric contraction. Conclusions. For longitudinal evaluation of muscle strength in patients with progressive lower motor neuron syndrome (i.e. between 0 and 250 N), muscle strength can be accurate quantified with both hand-held dynamometry and maximal voluntary isometric contraction. Hand-held dynamometry has the advantage of being cheap and quickly applicable. However, our results indicate that hand-held dynamometry is less sensitive than maximal voluntary isometric contraction in detecting subnormal muscle strength in strong muscle groups (i.e. >250 N), due to limited strength of the tester.

Adult↗
PubMed1993

Vascular and electromyographic responses evoked in forearm muscle by isometric contraction of the contralateral forearm.

There is controversy over whether isometric contraction of the forearm evokes vasoconstriction or vasodilatation in the muscles of the contralateral forearm. In the present study we have investigated in normal man, the effects of isometric contraction of one arm at 75, 50 and 25% maximum voluntary contraction (MVC) on arterial pressure, heart rate, blood flow and vascular resistance of the contralateral forearm and on electromyographic (EMG) activity recorded from that same arm with sensitive, surface electrodes. When EMG activity was not being recorded from the 'resting' arm, isometric contraction of the contralateral arm for 2 min evoked increases in arterial pressure and heart rate whose magnitudes were graded with % MVC and an increase in forearm blood flow and a decrease in forearm vascular resistance at 75, 50 and 25% MVC, indicating vasodilatation. Further experiments in which EMG activity was recorded from the 'resting' arm demonstrated that the decrease in forearm vascular resistance evoked by 75% MVC was associated with a substantial increase in EMG activity of the extensor and flexor muscles of that arm. By contrast, when forearm contraction was performed at 75% MVC whilst subjects viewed the EMG activity in the 'resting' arm on an oscilloscope and kept EMG activity minimal, vascular resistance increased in that arm, indicating vasoconstriction. Further, when subjects performed contraction at 25% MVC whilst showing minimal EMG activity in the contralateral arm, vascular resistance in that same arm increased (from 78 +/- 16 to 124 +/- 29 mmHg/ml/min/100 ml tissue). These results are discussed in relation to those of previous studies. We propose, that in normal man, isometric contraction of the forearm evokes primary vasoconstriction in the muscles of the contralateral forearm, but that this response may be overcome by muscle vasodilatation occurring secondary to unintended muscle contraction or as part of the alerting response to acute stress.

Adult↗
PubMed1993

Regulation of glycogen synthase in human muscle during isometric contraction and recovery.

Six subjects performed isometric contraction (66% maximal force) to fatigue with the knee extensor muscles. Biopsies were taken from the quadriceps femoris muscle at rest, at fatigue and 1 min after termination of contraction. In three of the subjects recovery from contraction occurred in the presence of an intact circulation (non-occluded, NON) to the thigh, whereas in the other three the circulation during recovery was occluded (OCC). Glycogen synthase fractional activity (GSF) decreased in all subjects from (mean +/- SE) 0.53 +/- 0.06 at rest to 0.37 +/- 0.04 at fatigue (P < 0.001). In the OCC group GSF returned to the pre-exercise value within 1 min after termination of contraction (0.59 +/- 0.07 at rest vs. 0.57 +/- 0.04 at 1 min post-exercise), whereas in the NON group GSF increased to a higher extent (0.48 +/- 0.09 at rest vs. 0.70 +/- 0.06 at 1 min post-exercise). The increase in GSF during the 1-min recovery was almost three-fold higher in the NON group (0.15 +/- 0.02 vs. 0.38 +/- 0.03). Cyclic AMP-dependent protein kinase (cAMP-PK) (assayed at 0/100 microM and 0.2/100 microM cAMP) did not change at fatigue or during recovery in either group. Glycogen synthase phosphatase (GSP) increased at fatigue by approximately 30% (P < 0.05 vs. rest). It is concluded that isometric contraction mediated inactivation of GS (i.e. phosphorylation of GS) is due to activation of a protein kinase(s) but not cAMP-PK. The rapid activation of GS in the NON group demonstrates that a humoral factor(s), possibly insulin and/or oxygen, is responsible for this phenomenon.

Adolescent↗
PubMed1986

Physiological modifications of local haemodynamic conditions during bilateral isometric contractions.

Nine subjects (five women and four men) simultaneously performed two isometric contractions sustained until exhaustion at different relative forces: 40% of maximum voluntary contraction (MVC) for the right elbow flexors; 50% MVC for the left elbow flexors. Contraction of the left elbow flexors commenced at 50% of the limit time (maximum maintenance time) of isometric contraction of the right elbow flexors. Increase in heart rate during concomitant contraction of the left elbow flexors led to an increase in blood flow to the right elbow flexors. Under these conditions, the limit time of isometric contraction of the right elbow flexors was prolonged with respect to the limit time obtained for an isolated isometric contraction at the same relative tension. The difference was more significant in the female (+40%, P less than 0.05) than in the male subjects (+20%, P greater than 0.05).

Adult↗
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↗
PubMed1989

Differences in the activation of m. biceps brachii in the control of slow isotonic movements and isometric contractions.

We have compared muscle activation in the control of slow isotonic movements and isometric contractions. Specific attention has been given to the contribution of the two force-grading mechanisms, the recruitment of motor units and the modulation of firing frequency in motor units that have already been recruited. The recruitment order of the m. biceps motor units under study was the same during isometric contractions and slow isotonic movements. However, the recruitment thresholds of the m. biceps units were considerably lower for both isotonic flexion and extension movements, even at velocities as low as 2 deg/s, than for isometric contractions. Furthermore, firing frequency at recruitment was found to depend on the motor task: at recruitment the motoneurone starts firing with a higher firing frequency during isotonic flexion movements and a lower firing frequency during isotonic extension movements than during isometric contractions. Two main conclusions can be drawn from these results. First of all, the concept of one single activation parameter (total synaptic drive?) cannot account for the motor-unit behaviour observed during our experiments: the relative contribution of the two force-grading mechanisms is different for different tasks. Secondly, the distribution of activity among flexor motoneurone pools is different for isometric contractions and isotonic movements.

Humans↗
PubMed1994

Timing of anticipatory responses following an isometric contraction.

The effects on timing of anticipation following an isometric contraction were investigated. Using a Bassin timer, 35 subjects performed 10 trials for each of 4 forewarning periods at 3 stimulus speeds resulting in 120 trials for the control condition. An additional 120 trials were performed by all subjects following an isometric contraction for each trial. Over all trials, average responses indicated early anticipation of the timing target. The shortest forewarning period and slowest speed of stimulus produced the earliest responses and the longest forewarning period and fastest speed of stimulus produced responses closest to temporal coincidence. Subjects responded faster after the isometric contraction. Significant differences were observed for the main effects of forewarning period and speed of the stimulus. A significant interaction resulted for condition by forewarning periods. The postcontraction condition produced a significantly earlier average response than the control condition for all but the shortest forewarning period. The findings supported the concept that the time needed to respond during postcontraction effects is reduced. With prior contraction, anticipatory timing gradually diminished over time following the contraction.

Adolescent↗
PubMed1989

Comparison of 133Xe clearance and laser Doppler flowmetry in assessment of blood flow changes in human masseter muscle induced by isometric contraction.

Changes induced by 30 s of isometric contraction at 10% and 50% of the maximum voluntary contraction were assessed in 10 healthy subjects; there was a median increase of 1.8 and 0.3 times in 133Xe clearance and 5.3 and 2.7 times in laser Doppler flowmetry. In the first minute after 10% maximum voluntary contraction, 133Xe clearance increased 12.1 times in relation to the initial resting level, and flowmetry values decreased to become 3.5 times higher than the basal flow; corresponding values after 50% maximum voluntary contraction were 24.3 times and 3.7 times. From the second minute after contraction, there was a rapid decrease in 133Xe clearance and a slow decrease in flowmetry values. Five minutes after biting at 10% maximum voluntary contraction, both methods showed a total net increase of about 15 times; after biting at 50% maximum voluntary contraction the corresponding values were 38 and 23 times. Thus laser Doppler flowmetry can be used to assess changes in blood flow in the masseter; it registers a greater increase in flow during isometric contraction than does 133Xe clearance. The significant difference between methods immediately after either chosen level of contraction disappears when the total net post-contraction hyperaemia is assessed over a period of 2 and 5 min, respectively.

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

Old adults exhibit greater motor output variability than young adults only during rapid discrete isometric contractions.

The purpose of this study was to examine the ability of young and elderly individuals to control submaximum levels of force (5-90%) during continuous and rapid discrete isometric contractions of the quadriceps femoris. Participants were 24 young (25.3 +/- 2.8 years) and 24 elderly individuals (73.3 +/- 5.5 years) that were healthy and active. The strength of elderly participants was approximately 40% lower than young participants. The standard deviation and coefficient of variation (CV) of force were greater during discrete contractions than during continuous isometric contractions. During continuous isometric contractions, young and elderly participants exhibited similar CVs of force. During discrete contractions, however, elderly participants exhibited greater CVs for peak force and impulse and greater standard deviations and CVs for temporal characteristics than young participants. Results suggest that the control of force in active elderly people declines only during rapid discrete contractions and that this decline may be associated with declines in temporal characteristics of the force production.

Adult↗
PubMed1997

Motor unit firing behavior in human arm flexor muscles during sinusoidal isometric contractions and movements.

Simultaneous recordings of action potentials (APs) of multiple single motor units (MUs) were obtained in brachialis and biceps (caput breve) muscles during sinusoidally modulated isometric contractions of elbow flexor muscles and during sinusoidal flexion/extension movements in the elbow against a preload in the extension direction. The results show that MUs typically fire in one short burst for each sinusoidal cycle. The mean phase lead of the bursts of APs relative to a sinusoidally modulated isometric torque in the elbow joint or relative to sinusoidal movements in the elbow increases gradually with frequency. The increase of the mean phase lead during isometric contractions was very similar for all MUs and could be explained well by modeling the force production of MUs with a second-order linear low-pass system. For sinusoidal flexion/extension movements each MU reveals a specific, reproducible phase lead as a function of frequency. However, there is a large variability in phase behavior between MUs. Also, the modulation of the firing rate for sinusoidal isometric contractions versus sinusoidal movements appeared to be different for various MUs. In simultaneous recordings some MUs clearly revealed a larger firing rate in each burst for movements relative to isometric contractions, whereas other MUs revealed a smaller firing rate. This suggests that some MUs are preferentially activated during movements whereas others are preferably activated during isometric contractions. The results demonstrate task-dependent changes in the relative activation of MUs within a single muscle for sinusoidal isometric contractions and movements.

Action Potentials↗
PubMed1987

Muscle exercise after anterior cruciate ligament reconstruction. Biomechanics of the simultaneous isometric contraction method of the quadriceps and the hamstrings.

Biomechanical analysis of two-dimensional models composed from roentgenographic pictures and electromyographic analysis on simultaneous isometric contraction exercises of the quadriceps and hamstrings were conducted in 20 healthy adult males. During simultaneous isometric contraction at 5 degrees knee flexion, an anterior drawer force equivalent to 15% of the tension of the quadriceps was exerted to the tibia, and decreased with increased angle of flexion. The mean angle at which this force became zero was 7.4 degrees, with a standard deviation of 5.0 degrees. When the angle increased further, a posterior drawer force to the tibia occurred and gradually increased. Each tension of the quadriceps or hamstrings during maximum simultaneous isometric contraction of the quadriceps and hamstrings was estimated as 30%-60% of that during separate isometric contractions of each muscle. In the early stage of the rehabilitation after the anterior cruciate ligament reconstruction, the simultaneous isometric contraction of the quadriceps and the hamstrings is useful as one of the muscle exercise methods because it can be performed safely with the knee position near the full extension and can generate sufficient muscle force to be an effective exercise.

Adult↗
PubMed1985

Muscle weakness following sustained and rhythmic isometric contractions in man.

The effects of sustained and rhythmically performed isometric contractions on electrically evoked twitch and tetanic force generation of the triceps surae have been investigated in 4 healthy male subjects. The isometric contractions were performed separately and on different occasions at 30%, 60% and 100% of the force of maximal voluntary contraction (MVC). The area under the maximal voluntary contraction (MVC) force/time curve during the rhythmic and sustained contractions was the same for each experiment. The results showed that following rhythmic isometric exercise there was a small decrease in low (10 and 20 Hz) and high (40 Hz) frequency tetanic tension which was associated with % MVC. However, there was no change in the 20/40 ratio of tetanic forces, MVC or the contraction times and force of the maximal twitch. In contrast, following sustained isometric exercise tetanic forces were markedly reduced, particularly at low frequencies of stimulation. The 20/40 ratio decreased and the induced muscle weakness was greater at 30% than 60% or 100% MVC. The performance of sustained isometric contractions also effected a decrease in contraction time of the twitch and MVC. The results are in accord with previous findings for dynamic work (Davies and White 1982), and show that if isometric exercise is performed rhythmically the effect on tetanic tensions is small and there is no evidence of a preferential loss of electrically evoked force at either high or low frequencies of stimulation following the contractions. For sustained contractions, however, the opposite is true, the ratio of 20/40 Hz forces is markedly reduced and following 30% sustained MVC there is a significant (p less than 0.05) change in the time to peak tension (TPT) of the maximal twitch.

Adult↗
PubMed2005

Knee angle-dependent oxygen consumption during isometric contractions of the knee extensors determined with near-infrared spectroscopy.

Fatigue resistance of knee extensor muscles is higher during voluntary isometric contractions at short compared with longer muscle lengths. In the present study we hypothesized that this would be due to lower energy consumption at short muscle lengths. Ten healthy male subjects performed isometric contractions with the knee extensor muscles at a 30, 60, and 90 degrees knee angle (full extension = 0 degrees ). At each angle, muscle oxygen consumption (m.VO2) of the rectus femoris, vastus lateralis, and vastus medialis muscle was obtained with near-infrared spectroscopy. m.VO2 was measured during maximal isometric contractions and during contractions at 10, 30, and 50% of maximal torque capacity. During all contractions, blood flow to the muscle was occluded with a pressure cuff (450 mmHg). m.VO2 significantly (P < 0.05) increased with torque and at all torque levels, and for each of the three muscles. m.VO2 was significantly lower at 30 degrees compared with 60 degrees and 90 degrees and m.VO2 was similar (P > 0.05) at 60 degrees and 90 degrees . Across all torque levels, average (+/- SD) m.VO2 at the 30 degrees angle for vastus medialis, rectus femoris, and vastus lateralis, respectively, was 70.0 +/- 10.4, 72.2 +/- 12.7, and 75.9 +/- 8.0% of the average m.VO2 obtained for each torque at 60 and 90 degrees . In conclusion, oxygen consumption of the knee extensors was significantly lower during isometric contractions at the 30 degrees than at the 60 degrees and 90 degrees knee angle, which probably contributes to the previously reported longer duration of sustained isometric contractions at relatively short muscle lengths.

Adult↗
PubMed1985

Comparison of two modalities when exerting isometric contractions.

In order to verify whether the method employed to obtain an isometric contraction has any effects on the time for which the contraction can be sustained as well as the consequent cardiocirculatory responses, nine male subjects, from 23 to 42 years of age, carried out isometric contractions of the elbow flexor muscles against either a suspended weight (W) or a fixed strain gauge (SG). The maximum voluntary contraction (MVC) appears to be identical in both instances; and, for a given percentage of MVC, the limit-time of the isometric contraction and its accompanying heart rate changes reveal no significant differences in the two cases.

Adult↗
PubMed2001

Comparison of EMG activity of medial and lateral hamstrings during isometric contractions at various cuff weight loads.

Since both the medial head (MH) and lateral head (LH) of the hamstring muscles contribute to knee flexion, this study investigated whether the relative electrical activity of these heads remained constant with respect to each other or changed during isometric contractions at five different resistance levels. The relative electrical activity of these two heads was determined by comparing their integrated EMG (iEMG). Forty-two volunteers with no history of right lower extremity injury or disease, between the ages of 18 and 35, were studied. Following motor point location, surface electrodes were placed over the MH and LH. Subjects, positioned prone, flexed the knee to 45 degrees using a sawhorse as a tactile cue to help maintain this position. Three recordings, 8 s in length, were taken at each subject's maximum isometric contraction and then using cuff weights of 1, 3, 5, and 7% of their body weight. The average of the three recordings was used in the analysis. During maximum isometric contraction (at 45 degrees of knee flexion), the LH contributes a significantly greater percent of the total iEMG (63.4%) than the MH (P < 0.0001). Furthermore, within the four submaximal levels tested, the LH's contribution was significantly greater than the MH (P < 0.01). During submaximal isometric contractions, the LH percent contribution to total iEMG was less than its contribution during maximal isometric contraction, all P values < 0.005. As a result, during these same submaximal isometric contractions, the MH contribution to total iEMG was greater than its contribution during maximal isometric contraction, all P values < 0.005).

Adolescent↗
PubMed1985

Discharge characteristics of motor units and the surface EMG during fatiguing isometric contractions at submaximal tensions.

The RMS amplitude of the surface electromyogram (EMG) and the frequency of discharge of motor units was examined throughout the duration of isometric contractions of the adductor pollicis muscles sustained to fatigue at tensions of 25, 40, and 55% of the maximum voluntary strength (MVC) of eight male subjects during fatiguing isometric contractions. The maximum strength of the muscle and the EMG above the adductor pollicis muscles was also assessed during 3 s of voluntary and electrically induced isometric contractions interposed at 25, 50, 75, and 100% of the duration of the fatiguing contractions. At the point of fatigue from submaximal isometric contractions, the RMS amplitude of the surface EMG was highest for contractions at 55 as compared to 40 and 25% MVC. The lower RMS amplitude of the EMG during contractions at lower as compared to higher tensions at the point of fatigue was paralleled by a lower discharge frequency of the alpha motor neurons in the fatigued muscle during contractions at 25% as compared to 40 and 70% MVC. The reduction in discharge frequency was probably of a sufficient order of magnitude to account for the lower amplitude of the EMG at the end of fatiguing isometric contractions at lower tensions.

Aerospace Medicine↗
PubMed2003

Muscle performance following fatigue induced by isotonic and quasi-isometric contractions of rat extensor digitorum longus and soleus muscles in vitro.

AIM: To study the effect of contraction mode on fatigue development. METHODS: Muscle fatigue was induced by isotonic and quasi-isometric contractions in rat soleus (SOL) and extensor digitorum longus (EDL) muscles, using identical stimulation protocol (60 Hz, 400 ms s-1) for 100 s in SOL and 60 s in EDL. Fatigue was quantified as the decline in peak values of shortening, shortening velocity, relaxation and work during the isotonic contractions, and, correspondingly, of force, rate of force development, relaxation and work during the quasi-isometric contractions. Maximal test contractions (60 Hz, 1.5 s) performed before and after fatigue were analysed for decline in force development (Fmax), rate of force development (dF/dtmax) and relaxation (-dF/dtmax). RESULTS: Fmax declined to significantly lower values after isotonic than after quasi-isometric fatiguing contractions (fatigued in percentage of unfatigued): 58.5 +/- 6.4% vs. 64.4 +/- 7.0% in SOL, and 30.4 +/- 4.1% vs. 33.3 +/- 3.6% in EDL, respectively. The same pattern was seen for dF/dtmax which decreased to: 46.3 +/- 9.9% vs. 52.3 +/- 8.5% in SOL, and 19.1 +/- 4.3% vs. 22.3 +/- 3.2% in EDL after isotonic and quasi-isometric contractions, respectively. Similarly, when comparing fatigue development during the two contraction modes, the respective fatigue variables decreased more rapidly and to lower levels during isotonic vs. quasi-isometric contractions. During maximal test contractions, the dynamic fatigue variables (+/-dF/dtmax) declined to significantly lower levels than Fmax. CONCLUSIONS: Fatigue development was significantly larger during isotonic vs. quasi-isometric contractions. The use of force as the only experimental fatigue variable may underestimate the functional impairment of fatigued muscle, neglecting the fatigue effect on time and length dimensions.

Animals↗