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PubMed1990

Synchronization of motor units in human masseter during a prolonged isometric contraction.

1. The action potentials of concurrently active motor units in human masseter were recorded on several fine-wire intramuscular electrodes and synchronization was assessed by cross-correlation of their firing times. 2. Long recording sessions and a test of significance for peaks in histograms were used to enhance detection of synchrony. 3. Synchrony in masseter units was found to be very weak, and was observed consistently only in histograms with a large number of counts. However, synchrony was widespread within the motoneurone pool; of the unit pairs whose cross-correlation histograms contained more than 100 counts per 1 ms bin, 89% were significantly synchronized. 4. The strength of synchrony between pairs of units did not change consistently during 15 min of continuous activity. 5. It was concluded that most, if not all, low-threshold masseter motoneurones have some common synaptic inputs, although the origin of those inputs remains unclear.

Action Potentials↗

PubMed2004

Mechanomyographic responses in human biceps brachii and soleus during sustained isometric contraction.

The purpose of this study was to elucidate the responses of the mechanomyogram (MMG) from two apparently different muscles (biceps brachii and soleus) during a sustained voluntary contraction at 50% maximum voluntary contraction. The MMG and surface electromyogram (EMG) were recorded from human biceps brachii and soleus during sustained elbow flexion and plantar flexion, respectively. Results indicated that the slope coefficient of rise in EMG amplitude as a function of time for the biceps was significantly greater than that for the soleus ( P<0.001). On the contrary, the MMG amplitude of the biceps showed a significant increase during the initial phase of sustained contraction ( P<0.05); however, when exhaustion was approached the amplitude declined significantly ( P<0.05). In the soleus muscle the decrease in MMG amplitude toward exhaustion occurred to a much lesser extent than that observed in the biceps. This difference could be attributed to the nature of the fusion state of the underlying muscle fibers. That is, the great extent of fusion observed in the biceps may be as a result of a greater quantity of fatigable motor units. In addition, the absence of MMG reduction in the soleus would indicate the absence of fatigue-induced slowing of contractile machinery and/or the lack of full activation (tetanus) of muscle fibers even at the exhaustion phase of plantar flexion.

Adult↗

PubMed2000

Comparison of algorithms for estimation of EMG variables during voluntary isometric contractions.

Many algorithms have been described in the literature for estimating amplitude, frequency variables and conduction velocity of the surface EMG signal detected during voluntary contractions. They have been used in different application areas for the non invasive assessment of muscle functions. Although many studies have focused on the comparison of different methods for information extraction from surface EMG signals, they have been carried out under different conditions and a complete comparison is not available. It is the purpose of this paper to briefly review the most frequently used algorithms for EMG variable estimation, compare them using computer generated as well as real signals and outline the advantages and drawbacks of each. In particular the paper focuses on the issue of EMG amplitude estimation with and without pre-whitening of the signal, mean and median frequency estimation with periodogram and autoregressive based algorithms both in stationary and non-stationary conditions, delay estimation for the calculation of muscle fiber conduction velocity.

Algorithms↗

PubMed2000

Agonist-induced isometric contraction of smooth muscle cell-populated collagen gel fiber.

String-shaped reconstituted smooth muscle (SM) fibers were prepared in rectangular wells by thermal gelation of a mixed solution of collagen and cultured SM cells derived from guinea pig stomach. The cells in the fiber exhibited an elongated spindle shape and were aligned along the long axis. The fiber contracted in response to KCl (140 mM), norepinephrine (NE; 10(-7) M), epinephrine (10(-7) M), phenylephrine (10(-6) M), serotonin (10(-6) M), and histamine (10(-5) M), but not acetylcholine (10(-5) M). Phentolamine (10(-7) M) produced a parallel rightward shift of the NE dose-response curve. Moreover, NE-induced contraction was partially inhibited by nifedipine and completely abolished by the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, the myosin light chain kinase inhibitor ML-9, the Rho kinase inhibitor Y-27632, and papaverine. A [(3)H]quinuclidinyl benzilate binding study revealed that the loss of response to acetylcholine was due to the loss of muscarinic receptor expression during culture. The expression of contractile proteins in the fibers was similar to that in cultured SM cells. These results suggest that, although the fiber is not a model for fully differentiated SM, contractile mechanisms are maintained.

Acetylcholine↗

PubMed1989

Muscular tenderness evaluated by surface EMG frequency analysis of isometric contractions.

Myoelectric power spectrum analysis was used to compare muscular activity in the masseter and anterior temporal muscles between patients with tender masseter muscles and healthy controls. Surface EMG was recorded for 28 patients and nine controls while they were biting on a bite force transducer during a period of 10-15 s up to maximal effort (100% MVC). The mean frequency (MF) was calculated as a single estimate of the myoelectric power spectrum. Regression analyses of MF's for the masseter muscles versus the force level 0-60% MVC showed significantly less steep slopes of regression for the patients compared interindividually with the controls. As regards the anterior temporal muscles there was no significant difference between groups. One explanation for the difference in MF's for the masseter muscles between patients and controls is that the patients suffered an inhibition or difficulty in recruiting motor units. Different firing rate properties for motor units at high force levels (60-100% MVC) could be one reason for a significantly smaller decrease in MF for the patients' elevator muscles than for the controls.

Adult↗

PubMed1983

The control of isometric contractions in patients suffering from different types of hypotonia.

Different forms of hypotonia, resulting from various lesions in the somatosensory system, are discussed. In order to investigate the causes of hypotonia the forearm flexors of patients were stretched and the segmental EMG responses studied. Special attention was given to parkinsonian patients who were stereotaxically operated for the relief of tremor. These patients were studied before and after their operation and their reflex responses were compared to those of normal subjects. It was found that the M2 component was always significantly larger before operation when compared to normal subjects. This could be due to high-frequency firing of single motor units during the M2 interval, as shown with single unit analysis. After operation the M2 component was significantly reduced; clinically hypotonia was observed. It is thus suggested that hypotonia observable in parkinsonians following subthalamotomy and/or thalamotomy could be the result of the decreased M2-component. Pathways which may be interrupted by the lesion are discussed and possible mechanisms mediating unusually high M2-components are mentioned. The hypothesis is forwarded that the stereotaxic lesion may selectively interfere with the static gamma drive to muscle spindles.

Diencephalon↗

PubMed1998

Frequency response of spine extensors during rapid isometric contractions: effects of muscle length and tension.

During muscle contraction, electrical activity necessarily precedes force output, yet models that utilize processed electromyograms sometimes predict force as preceding EMG under rapid ballistic loading conditions. The purpose of this study was to define the frequency response transfer function of the upper and lower erector spinae musculature, at different lengths and tensions, using rectified, low pass filtered EMG. This would enable accurate estimates of force from the processed electromyogram, specifically during impulsive contractions. Abdominal and erector spinae EMG were measured in synchrony with impulsive low back moments in five men. EMG signals were rectified and low pass filtered repeatedly with cut-off frequencies from 1 to 3 Hz at 0.5 Hz increments in order to quantify the frequency response. It was found that EMG signals processed through a simple, Butterworth low pass filter could not produce the measured force output without an additional time shift. These shifts were quantified by cross-correlating EMG and force with increments of 1 ms. In order to define the transfer function of EMG to force, optimal cut-off frequencies were selected two ways: quantitatively by searching for maximum cross correlations coefficients, and qualitatively. Results indicated that the frequency response of both the upper and lower erector spinae can be modelled with a cut-off frequency between 2 and 2.5 Hz and that these values are not significantly modulated by changes in muscle length or tension.

Adult↗

PubMed2003

Motor-unit coherence during isometric contractions is greater in a hand muscle of older adults.

The purpose of this study was to quantify the strength of motor-unit coherence from the first dorsal interosseus muscle in young and old adults using data obtained in a previous study, where no differences in motor-unit synchronization between the two groups were observed. The strength of motor-unit coherence was quantified from 47 motor-unit pairs in 11 young adults (age 24.1 +/- 4.1 yrs) and from 48 motor-unit pairs in 14 old adults (age 70.4 +/- 5.9 yrs). The strength of motor-unit coherence was greater in old adults, particularly at low frequencies of 5-9 Hz (85% greater in old adults at 5 Hz). In addition, the older adults expressed an extra oscillation at approximately 12-13 Hz that was not present in the young subjects. These data demonstrate that common oscillatory inputs to motor neurons (motor-unit coherence) are enhanced in older adults despite no age-related difference in the strength of shared inputs (synchronization). Furthermore, the data emphasize that measures of motor-unit synchronization and coherence highlight different features of the same common input, and a coherence analysis may be a more sensitive tool to characterize shared input to motor neurons.

Adult↗

PubMedDate not supplied

A computer simulation study of isometric contraction of latissimus dorsi muscle used for cardiac assistance.

This study was designed to investigate the feasibility of a skeletal muscle pump employing latissimus dorsi muscle (LDM) for cardiac assistance. We developed and used a 2-dimensional mathematical model for LDM to investigate how the size of pneumatic balloons (30, 38, and 45 ml) and the three different locations (proximal, center, and distal) affect the pressure applied to the balloon by LDM. The computer simulation was performed by coding a visco-elastic and nonlinear 2-dimensional program that employed the finite element method (FEM). The muscle specific parameters of LDM were obtained from animal experiment results. The model is based on Hill's characteristic equation and composed of a contractile component and a passive element. The simulation results indicated that the intermediate and largest sized balloon lead to the highest and the lowest power (volume reduction per unit time interval), respectively. On the other hand, when the balloon is inserted in the distal LDM, the power is lower than in the other two positions, regardless of the balloon size. The above results suggest that the optimal size of the balloon should be selected depending on the muscle specific parameters of the actuator, and that the balloon should be inserted either in the proximal portion or center of the actuator.

Animals↗

PubMed1996

Acoustic and electrical activities during voluntary isometric contraction of biceps brachii muscles in patients with spastic cerebral palsy.

This study was designed to compare electromyogram (EMG) and acoustic myogram (AMG) recordings of biceps brachii muscles in patients with spastic cerebral palsy (CP). The maximal voluntary contraction (MVC) in the CP group was approximately one half of that of the normal group even after being normalized by the muscle cross-sectional area (CSA) (18.6 - 5.9 kNm/m2 in CP, 37.3 +/- 2.9 kNm/m2 in normal). Both CP and normal groups demonstrated a progressive increase in the root mean squared values per unit muscle CSA in the EMG (RMSEMG/CSA) as well as in the AMG (RMSAMG/CSA) with increasing force up to 50% MVC. The increasing magnitude of the RMSEMG/CSA with force was not significantly different between two subject groups. However, all the levels of force resulted in significantly smaller RMSAMG/CSA in the CP group compared to the normal group. The ratios of RMSAMG to RMSEMG in the CP group (0.75 +/- 0.03 m/s2/mV) were significantly smaller than those in the normal group (1.37 +/- 0.07 m/s2/mV) at force levels above 30% MVC. These results suggest that motor disabilities in CP patients are caused not only by primary neural impairment but also by secondary deterioration in muscular contractile properties, probably resulting from muscle fiber atrophy. This appears to be more selective in fast twitch fibers.

Acoustics↗

PubMed1988

Mechanisms resisting fatigue in isometrically contracting human skeletal muscle.

Human adductor pollicis was fatigued during circulatory occlusion by supramaximal stimulation via the ulnar nerve using intermittent trains of stimuli in ascending (1, 10, 20, 50 and 100 Hz) and descending (100, 50, 20, 10 and 1 Hz) frequencies to investigate the contribution of relaxation rate slowing and post-tetanic potentiation (PTP) to fatigue resistance. At 50 and 100 Hz force was initially well maintained despite a marked loss of excitation as indicated by EMG, demonstrating the operation of a high-frequency 'safety factor' which appeared independent of the pattern of stimulation. At 10 Hz, force was initially potentiated before declining during both activity series. Potentiation was greater during the descending frequency series and the rate of decline of force, or fatigability, was reduced. The 'extra' low-frequency potentiation at 10 Hz was not simply the result of PTP of twitch force, since this declined more during the descending than during the ascending series, nor the result of maximal relaxation rate changes which were identical for both fatiguing series. It is hypothesized that the extra potentiation and reduced fatigability at low stimulation frequencies, when preceded by high frequency, is the result of increased myofibrillar Ca2+ availability and/or sensitivity. These findings may have important practical implications in relation to functional electrical stimulation techniques as used in paraplegia and in other areas of muscle research where fatigue is to be minimized.

Action Potentials↗

PubMed2004

Influence of repeated isometric contractions on muscle deoxygenation and pulmonary oxygen uptake kinetics in humans.

The purpose of the present study was to compare simultaneously vastus lateralis (VL) deoxygenation and pulmonary O(2) uptake (O(2)) kinetics during fatiguing knee extension exercise with minimal cardiac load. Eight healthy subjects realized an intermittent bilateral knee-extension exercise (3-s contraction/3-s relaxation) at 40% of maximum voluntary contraction for 10 min. VL deoxygenation was recorded by near infrared spectroscopy at 2 Hz (NIRO-300, Hamamatsu Photonics, Japan) and O(2) was determined breath-by-breath (K4b(2), Cosmed, Italy). After a time delay of 16 +/- 5 s, deoxygenation kinetics at the onset of exercise followed an exponential time course at a significant faster rate than O(2) (time constant of 5.4 +/- 4.0 s vs. 31.6 +/- 10.4 s, P<0.01) reflecting a mismatch between local O(2) consumption and perfusion. Thereafter, a rise in O(2) of 223 +/- 123 ml min(-1) (consistent with the mathematical model, 259 +/- 126 ml min(-1)) was observed between minutes 2 and 10. During the same exercise time, changes in tissue oxygenation index decreased significantly and were individually correlated with the corresponding increased O(2) (P<0.05), suggesting that the majority of the slow rise of O(2) arose from the exercising limbs. Averaged heart rate increased from 67 +/- 11 to 116 +/- 20 beats min(-1) during exercise. Knee extension exercise may be relevant to estimate the cardiopulmonary and deoxygenation of working skeletal muscle responses for assessment of exercise limiting factors in clinical settings or in the exercise physiology.

Adult↗

PubMed2004

Changes in the P100 latency of the visual evoked potential and the saccadic reaction time during isometric contraction of the shoulder girdle elevators.

We investigated changes in the P100 latency of the visual evoked potential (VEP) and the saccadic reaction time (SRT) in relation to the degree of activity of the shoulder girdle elevators. Muscle force was set in 10% increments from 0% to 50% of the maximal voluntary contraction (MVC). The VEP was derived from a midline occipital electrode with reference electrodes on the ears when the right retina was stimulated through the eyelid by light emitting diodes while the eyes were closed. The P100 latency of the VEP was defined as the time from the stimulus onset to the main positive peak. The SRT was defined as the latency until the beginning of eye movement toward the lateral target, which was moved at random time-intervals. P100 latency was shortened until 30% of the MVC, and which it lengthened. The SRT changed in a pattern similar to that observed for the P100 latency. The ratio of the shortening in P100 latency relative to that of the SRT was approximately 20%. All data is presented as the mean value, plus the standard deviation. We believe that the information processing time in the neural pathway from the retina to the visual cortex was shortened up to a certain muscle force of the shoulder girdle elevators, and then this processing time lengthened. These findings indicate that shortening of information processing time in the neural pathway beyond the visual cortex is included in the shortening of the SRT.

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↗

PubMed1993

The effect of an occlusal stabilization splint and the mode of visual feedback on the activity balance between jaw-elevator muscles during isometric contraction.

The aim of the present study was to gain an insight into the influence of a vertical bite-rise (clenching in intercuspal occlusion vs. clenching on an occlusal stabilization splint), the mode of visual feedback (VF; obtained from the compound masseter signal, from the compound anterior temporalis signal, or from the compound signal of both masseter and anterior temporalis muscles) and the EMG clenching level (10% MVC and 50% MVC) on the muscle balance between the masseter and the anterior temporalis muscles. The muscle balance was quantified as the logarithmic value of the ratio between the summated mean rectified EMG activity of the masseter muscles and this activity of the anterior temporalis muscles. The muscle balance was influenced significantly by the mode of VF (p < 0.01), the muscle balance shifting toward the group of muscles from which VF was obtained. When VF was obtained from the masseter muscles, a decrease in the anterior temporalis EMG activity was observed when the vertical dimension was increased (p < 0.05-0.01). When VF was obtained from the anterior temporalis muscles, the activity of the masseter muscles was raised with respect to that of the anterior temporalis muscles during clenching with a vertical bite-rise (p < 0.05-0.01). When VF was obtained from both groups of muscles, the masseteric EMG activity increased, whereas the anterior temporalis EMG activity decreased. Hence, regardless of the mode of VF, a relatively lower activity level of the anterior temporalis muscles was achieved after insertion of an occlusal stabilization splint.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1995

Variability of grip strength during isometric contraction.

The use of measures of strength variability as a means of determining sincerity of effort is becoming a more common practice, particularly in medico-legal and rehabilitation settings. The stability of such variability measures, however, has not been documented. This research investigated, in two studies, the trial-to-trial variability of grip strength under maximal and submaximal effort conditions. In the first study, 63 subjects were asked to give 100% grip effort, and in the second study 40 subjects from the original group were asked to give 50% grip effort. The Jamar hand dynamometer was used to measure grip strength in both experiments, and a coefficient of variation (CV) was calculated for every three repeat measures at each handle position. Testing was conducted on two separate occasions for both experiments. Although the interoccasion reliability of grip strength was very high, in comparison, the CV was not stable over test occasions, with interoccasion reliability indices close to zero. Factors significantly influencing CV were effort level, with submaximal effort producing larger CVs, and gender, with females having greater strength variability. If the rule is applied that one or more CVs above the 7.5% cut-off value could indicate submaximal effort, then for this sample of subjects giving maximal effort, 97% of females and 64% of males would be misclassified. Applying a single CV classification cut-off value to a mixed sample of subjects appears to unfairly discriminate against the females. Further research into the factors associated with high CV values is essential before the CV can be used with any confidence in a clinical setting as a method for determining sincerity of effort.

Adult↗

PubMed1984

Energetics of isometric contraction in porcine carotid artery.

The rate of ATP utilization ( JATP ) and unloaded shortening velocity ( Vus ) of porcine carotid artery were studied during tension development and maintenance to assess the importance of mechanisms proposed to account for its high economy of tension maintenance. Previous work from our laboratory established that the rate of O2 consumption ( JO2 ) can be used to measure rates of tissue phosphagen utilization as early as 30 s after initiation of contraction. Studies at 37 degrees C of tissue JO2 indicated the following. 1) The basal JO2 was 0.069 +/- 0.017 mumol O2 X min-1 X g-1) (n = 10). During steady-state tension maintenance suprabasal JO2 was 0.079 +/- 0.012 mumol O2 X min-1 X g-1 for tissues stimulated with KCl (n = 11) and 0.213 +/- 0.022 (n = 3) for tissues stimulated with KCl + histamine. 2) For both stimulation conditions an initial elevated (peak) suprabasal JO2 was observed that correlated with the rate of tension development. The peak suprabasal JO2 was approximately twice the steady-state suprabasal JO2 measured 10 min after stimulation. 3) There was no significant change in JO2 during tension redevelopment after a rapid length step at 15-20 min after stimulation (n = 4). Studies of Vus indicated the following. 1) During steady-state tension maintenance Vus was 0.008 +/- 0.001 tissue lengths/s for tissues stimulated with KCl (n = 13) tissue lengths/s for tissues stimulated with KCl + histamine (n = 6). 2) An initial twofold peak in Vus was observed at 0.25-0.5 min in that decreased to steady-state levels by 4 min after KCl stimulation, whereas minor decreases in Vus still occurred until 10-20 min after KCl + histamine stimulation. 3) The measured transient in Vus , although biphasic, does not temporally correlate with the transient in JO2 even after the JO2 time course was corrected for delays resulting from O2 diffusion and the electrode response time. The mechanism underlying the observed maximum change of two- to fourfold in both JATP and Vus is not sufficient to account for the high economy of tension maintenance.

Animals↗

PubMed1993

Gain and threshold of the jaw-jerk reflex in man during isometric contraction.

The control of mandibular posture has been related to the activity of the anterior temporal muscles, whereas the masseter muscles have been viewed mainly as force producers. However, these groups of muscles, especially in the deep layers, are highly endowed with muscle spindles, so that a difference in function should imply a difference in the reflex sensitivity. By studying the jaw-jerk reflex by means of bipolar surface electromyogram, the reflex sensitivity was determined from relationships between reflex amplitude and jaw displacement from both groups of muscles in eight subjects. At a constant level of background muscle activity, and hence with a constant excitability of the alpha motoneurons, the reflex sensitivity can be determined from these relationships in terms of gain and threshold. In order to account for differences in thickness of the soft tissues overlying the various muscles studied, the reflex amplitude was normalized with respect to the level of maximal voluntary contraction (MVC). In experiments where the inter-electrode distance was 18 mm over both groups of muscles, the reflex gain of the anterior temporal muscles was larger than that of the masseter muscles (P < 0.05). The threshold value did not differ significantly from zero for either group of muscles. Normalization of the reflex amplitude with respect to MVC can be carried out correctly only if the reflexly activated muscle fibres are distributed uniformly within the muscle. In order to gain an insight into this distribution, control experiments were performed with three subjects in which the inter-electrode distance was varied, thus influencing the depth in the muscle from which active muscle fibres were recorded. The reflex gain of the masseter muscle with an inter-electrode distance of 22 mm, which records from deeper layers of the muscle as well as superficial ones, was larger than with a distance of 11 mm (P < 0.01-0.10 in the various subjects). No such differences were found for the anterior temporal muscle. It was concluded that the afferents of the spindles in the jaw-elevator muscles do not project uniformly upon the motoneurons, but involve mainly fibres in the deeper layers of the muscle. The difference in gain found between the masseter and the anterior temporal muscles with the same inter-electrode distance of 18 mm is likely to be due to a larger distance of the reflexly activated muscle fibres in the masseter muscle with respect to the electrodes rather than to a difference in reflex sensitivity between the muscles.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗