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PubMed1993

PDC activity and acetyl group accumulation in skeletal muscle during isometric contraction.

The activity of pyruvate dehydrogenase complex (PDC) was studied in the human quadriceps femoris muscle during isometric contraction induced by intermittent electrical stimulation at 20 Hz. Muscle biopsy samples were obtained at rest and after 10, 20, and 46 contractions. The active form of PDC (PDCa) increased from a mean value of 26% of the total PDC at rest to mean values of 46, 78, and 80%, respectively. Muscle biopsy samples were also obtained at rest, after 46 contractions with limb blood flow intact or occluded, and after 2 min of oxidative recovery. In another experiment, muscle biopsy samples were obtained at rest, after 10 min of resting ischemia, and after 46 contractions with limb blood flow occluded. The transformation of PDC to PDCa was nearly complete, regardless of whether the blood flow was intact or occluded. However, the accumulation of acetyl groups observed during stimulation with intact blood flow was abolished when the blood flow was occluded. The absence of NADH oxidation during anoxia had no effect on the contraction-induced transformation of PDC to PDCa, but it inhibited the flux through the enzyme reaction.

Acetyl Coenzyme A↗

PubMed1989

Transient increase in glucose 1,6-bisphosphate in human skeletal muscle during isometric contraction.

Changes in glucose 1,6-bisphosphate and regulators of glucose-1,6-bisphosphate synthase and phosphatase during isometric contraction have been determined. Biopsies were obtained from the quadriceps femoris muscle before and after 20 s of contraction and at fatigue. Glucose 1,6-bisphosphate increased by 35% after 20 s of contraction (P less than 0.001) with no further change at fatigue (P greater than 0.05 versus 20 s). Pi, fructose 1,6-bisphosphate and glycerate 3-phosphate, all inhibitors of the synthase, increased significantly during the first 20 s (P less than 0.05-0.001), whereas muscle pH (decrease in which inhibits synthase) decreased continuously. The decrease in the total adenine nucleotide pool, which is stoichiometric with the increase in IMP (an activator of phosphatase), was not significant after 20 s, but was 15% at fatigue (P less than 0.001). The rapid increase in glucose 1,6-bisphosphate, despite increases in the inhibitors of synthase, suggests that the synthase was activated, possibly by the substrate glycerate 1,3-bisphosphate and/or a yet unknown activator(s). The lack of any further change in glucose 1,6-bisphosphate during the latter part of contraction may be due to concomitant activation of the synthase and phosphatase.

Adult↗

PubMed1990

Muscle afferent responses to isometric contractions and relaxations in humans.

1. One hundred and two single afferents from the finger extensor muscles of humans were studied with the microneurography technique. 2. The afferents were provisionally classified as primary muscle spindle afferents (62/102), secondary spindle afferents (22), and Golgi tendon organ afferents (18) on the basis of their responses to four tests: 1) ramp-and-hold stretch, 2) 20- and 50-Hz small-amplitude sinusoidal stretch superimposed on ramp-and-hold stretch, 3) maximal isometric twitch contraction, and 4) stretch sensitization. 3. The response profiles of the three unit types were analyzed during slowly rising isometric contraction terminating with an abrupt relaxation. About 75% (61/84) of all muscle spindle afferents increased their discharge during isometric contraction, whereas the discharge was reduced for the remaining afferents. All Golgi tendon organs increased their discharge during the contraction. 4. The level of extrafusal contraction at which a spindle afferent increased its discharge rate often varied from trial to trial, speaking against a fixed fusimotor recruitment level of the individual spindle ending. 5. In 70% of the spindle afferents, a distinct burst of impulses appeared when the subject rapidly relaxed after the isometric contraction. The burst was more common and usually much more prominent with primary than secondary afferents, often reaching instantaneous discharge rates well above 100 Hz. 6. Whereas all Golgi tendon organ afferents displayed an increased discharge during the contraction phase, only one of them exhibited a rate acceleration close to the relaxation phase. However, this response could clearly be identified as being of different nature than the spindle bursts.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗

PubMed1996

Comparing composite scores based on maximal voluntary isometric contraction and on semiquantitative manual motor testing in measuring limb strength in patients with ALS.

Maximal voluntary isometric contraction (MVIC) is a standardized tool for measuring disease progression in patients with ALS. After normalization, summation, and averaging, it generates composite scores (CS) ("megascores"). In a cross-sectional study, these scores were highly correlated with "average strength" composite scores based on semi-quantitative manual motor testing (SQMMT): r = 0.697 (p < 0.0001). Each 10% difference in the raw SQMMT-CS corresponds to a 0.56 difference in the MVIC-CS (95% confidence limits [CL], 0.38, 0.74). Controlling for height (a highly significant covariate, p < 0.0001), each 10% difference in SQMMT-CS corresponds to a 0.39 difference in MVIC-CS (95% CL, 0.24, 0.54).

Adult↗

PubMed1999

Comparison of maximal voluntary isometric contraction and Drachman's hand-held dynamometry in evaluating patients with amyotrophic lateral sclerosis.

Maximal voluntary isometric contraction (MVIC) is a standard tool for assessment of muscle strength in treatment trials for amyotrophic lateral sclerosis (ALS). There is need for more practical bedside techniques especially for severely disabled patients. Hand-held dynamometry (HH-Dyn) is an inexpensive and easy-to-handle device. MVIC was measured in five proximal muscle groups bilaterally and compared with HH-Dyn in 43 ALS patients. After a training period we found good intrarater correlation for HH-Dyn (r = 0.99), with a low coefficient of variation. Measurements tended to become more accurate after repeated testing due to practice effects in examiners and patients. Overall correlation between HH-Dyn and MVIC was good [r = 0.85 (P < 0.01)]. Strength-range-specific analysis showed a significant linear correlation up to 20 kg (44 lbs.) [r = 0.57 (P < 0.01)]. However, we found a tendency to underestimate muscle strength above 10 kg by HH-Dyn as compared with MVIC, but this became meaningful only above a force of 20 kg. HH-Dyn provides a strength estimate with a precision close to MVIC in weak muscle groups (MRC grade 4). With standardization and appropriate training, HH-Dyn is a useful bedside test, providing an alternative to MVIC for follow-up assessment in ALS.

Adult↗

PubMed2001

Effect of human exposure to altitude on muscle endurance during isometric contractions.

The aim of this study was to evaluate the influence of exposure to altitude on muscle endurance during isometric contractions. Six sedentary subjects were studied. Surface electromyograph (sEMG) activity was recorded from the right biceps brachii (BB) during exhausting isometric exercise at 80% maximal voluntary contraction. Experiments were performed before, during and 6 months after a 12 day stay at the EV-K2 laboratory (Nepal, 5,050 m above sea level). From the sEMG signals from BB, the median frequencies (fmed) were computed for consecutive 1 s epochs. The sEMG was also analysed using a non-linear tool, the recurrence quantification analysis, and the percentage of determinism (%DET) was then calculated. The haemoglobin saturation significantly decreased at altitude. The mean (SD) BB endurance time decreased from 22.4 (4) s to 18.3 (4.7) s (P < 0.05). After exposure to altitude a significant variation in fmed and %DET slopes was observed. We concluded that during the first period of acclimatisation at altitude there was an impairment of isometric muscle endurance performance and there was also evidence of a modified myoelectric activity pattern suggesting a greater fatigability of the neuromuscular system.

Adult↗

PubMed1982

Energetics of isometric contraction in dystrophic fast and slow muscles.

The amount of phosphoryl creatine (PC) hydrolysed during a ten-second isometric contraction was measured in the biceps brachii (fast) and soleus (slow) muscles of adult normal and dystrophic mice (Re 129 strain) following inhibition of glycolysis and oxidative phosphorylation. The dystrophic muscles were found to have a lower isometric economy (tension-time integral per mumol PC) than the normal muscles. This was particularly so in the case of the fast biceps brachii muscle which is affected by dystrophy to a greater extent than the slow soleus muscle. The isometric economy of the dystrophic muscles was lower even when the results were based on a total creatine rather than on a weight basis. This suggests there may be some defect in the contractile proteins of dystrophic muscle. The normal soleus muscle was found to be approximately three times more economical in maintaining tension than the normal biceps brachii muscle. This indicates the adaptation of slow muscles such as the soleus are used for maintenance of posture.

Animals↗

PubMed2001

Deformation and three-dimensional displacement of fibers in isometrically contracting rat plantaris muscles.

In this study, the deformation of different fibers of the rat m. plantaris during "isometric" contractions at different muscle lengths was considered. Because the m. plantaris has an obviously inhomogeneous architecture, its fibers on the medial side of the muscle belly are judged to be shorter than those on the lateral side of it. It was expected that longitudinal deformation of different fibers would vary accordingly. A 3D video analysis of contracting muscle showed that longitudinal strain of fibers as a function of muscle length does not differ between fibers on different sides of the muscle. Apart from longitudinal shortening, the fibers were also displaced laterally during a contraction. The fibers displaced during a contraction in a direction perpendicular to their longitudinal axis. The displacement of the fibers occurred asymmetrically, resulting in a helical deformation of the whole muscle. It is concluded that the asymmetric displacement and the helical deformation must result from transversal forces between the fibers. It is hypothesized that these transversal forces cancel out differences in longitudinal strains that might exist between fibers.

Animals↗

PubMed1983

Skeletal muscle in alloxan diabetes. A comparison of isometric contractions in fast and slow muscle.

Diabetes was produced by withholding insulin treatment from previously alloxanized female rats. Isometric contraction was assessed in soleus and extensor digitorum longus (EDL) muscles removed 2 h to 32 days after insulin withdrawal. Directly induced contractions were measured in vitro at 20 degrees C. In soleus muscles from severely diabetic rats, average twitch and tetanic forces were normal or slightly greater than that of controls of similar age, whereas in EDL, marked decreases appeared in both twitch and tetanic forces. Soleus muscle from severely diabetic rats was not depolarized as already reported in EDL. After 16 and 32 days in the diabetic state, soleus muscles from moderately diabetic rats generated average tetanic forces that were equal to that found in age-matched controls, whereas EDL tetanic forces were significantly (P = less than 0.01) weaker. Average specific twitch force in diabetic soleus muscles was greater than age-matched controls after 16 and 32 days in the diabetic state. In diabetic soleus muscle, significant increases in the average half relaxation time and twitch duration were seen after prolonged (16 and 32 days) periods of diabetes. No changes were seen in the same temporal parameters of the twitch in diabetic EDL muscle. A greater atrophy appeared in EDL than in soleus after 16 and 32 days of uncontrolled diabetes.

Animals↗

PubMed2004

Metabolically assessed muscle fibre recruitment in brief isometric contractions at different intensities.

This study investigated the recruitment of type I, IIA and IIAX fibres after seven isometric contractions at 40, 70 and 100% maximal voluntary knee extension torque (MVC, 1 s on/1 s off). Biopsies of the vastus lateralis muscle were collected from seven subjects at rest and immediately post-exercise. Fibre fragments were dissected from the freeze-dried samples and characterized as type I, IIA and IIAX using mATPase staining. Phosphocreatine (PCr) and creatine (Cr) content were measured in the remaining part of characterized fibres. A decline in the ratio of PCr to Cr (PCr/Cr) was used as an indication of activation. The mean peak torques were, respectively, 39 (2), 72 (2) and 87 (6)% MVC. Cumulative distributions of type I and IIA fibres were significantly shifted to lower PCr/Cr ratios at all intensities (Kolmogorov-Smirnov test, P<0.05). The cumulative distribution of type IIAX fibres showed a significant leftward shift only at 87% MVC ( P<0.05). A hierarchical order of fibre activation with increasing intensity of exercise was found, with some indication of rate coding for type I and IIA fibres. Evidence for activation of type IIAX fibres was only found at 87% MVC.

Adenosine Triphosphatases↗

PubMed1982

Isometric contractions of motor units and immunohistochemistry of mouse soleus muscle.

1. Isometric contractions of motor units, isolated functionally by ventral root splitting in vivo, were recorded from mouse soleus muscle. 2. Motor unit tensions varied over a narrow symmetrical range and averaged 4.7% of whole muscle tension, corresponding to twenty-one motor units per muscle. 3. There was considerable variation between muscles in isometric twitch times-to-peak and even greater variation for the motor units. The distribution of motor unit times-to-peak was apparently unimodal and could be fitted by a single normal population. A slightly better fit was, however, obtained with two normal populations, as suggested by the histochemistry. 4. Twitch time-to-peak decreased in proportion to axonal conduction velocity in individual animals. The whole population of motor units could be fitted by a linear relation between time-to-peak and the reciprocal of conduction time in the motor axon. Motor unit tension was also linearly related to the reciprocal of conduction time. 5. Histochemistry showed clear division between Type I and Type IIa fibres. Type I fibres reacted strongly with antibody against slow myosin of cat soleus muscle; Type IIa gave a reaction no stronger than the background. The division was as clear as in the cat or rat.

Animals↗

PubMed2005

Experimental muscle pain increases mechanomyographic signal activity during sub-maximal isometric contractions.

This study was designed to investigate the local effect of experimental muscle pain on the MMG and the surface EMG during a range of sub-maximal isometric contractions. Muscle pain was induced by injections of hypertonic saline into the biceps brachii muscle in 12 subjects. Injections of isotonic saline served as a control. Pain intensity and location, MMG and surface EMG from the biceps brachii were assessed during static isometric (0%, 10%, 30%, 50% and, 70% of the maximal voluntary contraction) and ramp isometric (0-50% of the maximal voluntary contraction) elbow flexions. MMG and surface EMG signals were analyzed in the time and frequency domain. Experimentally induced muscle pain induced an increase in root mean square values of the MMG signal while no changes were observed in the surface EMG. Most likely this increase reflects changes in the mechanical contractile properties of the muscle and indicates compensatory mechanisms, i.e. decreased firing rate and increased twitch force to maintain a constant force output in presence of experimental muscle pain. Under well-controlled conditions, MMG recordings may be more sensitive than surface EMG recordings and clinically useful for detecting non-invasively increased muscle mechanical contributions during muscle pain conditions.

Adaptation, Physiological↗

PubMed2002

Spinal cord modulation associated with isometric contractions.

OBJECTIVE: The dual-strategy hypothesis explains single-joint voluntary movement by dividing movements into two different strategies and suggesting that different excitation pulses modulate these movements. The existence of this excitation pulse was evaluated by quantifying magnitude and timing changes in the H-reflex (changes in spinal excitability) prior to a voluntary contraction. These changes in spinal excitability were assessed during a ballistic plantar flexion isometric contraction, where both the target size and force level was manipulated. METHODS AND MATERIALS: Subjects were seated in a modified chair with a force transducer placed under the metatarsal heads to measure ankle force output. Following a visual stimulus subjects were trained to produce a plantar flexion force of 25% and 50% of a maximum voluntary contraction, within target sizes of 5% and 15% of the selected force level. Soleus motor neuron reflex excitability was analyzed by measuring changes in the H/M ratio. The H-reflex was randomly elicited by tibial nerve stimulations at 15, 30, 45, 60, 75 and 90 ms prior to the recorded average soleus premotor time for each of the force and target size conditions. RESULTS: A two-way repeated-measures analysis of variance indicated a significant effect among target sizes for the time of change in spinal excitability, slope of facilitation (rate of rise of spinal excitability), and peak facilitation. A significant difference was also established between force levels for the slope and peak facilitation, but there was no difference with time of facilitation. CONCLUSIONS: These results indicate that changes in both target size and force level can influence slope and peak of facilitation. However, only target size appears to affect the time of facilitation. Results clearly support the existence of an excitation pulse that is regulated by the type of movement.

Adult↗

PubMed1995

The role of metabolites in strength training. II. Short versus long isometric contractions.

The role of intramuscular metabolite changes in the adaptations following isometric strength training was examined by comparing the effect of short, intermittent contractions (IC) and longer, continuous (CC) contractions. In a parallel study, the changes in phosphate metabolites and pH were examined during the two protocols using whole-body nuclear magnetic resonance spectroscopy (NMRS). Seven subjects trained three time per week for 14 weeks. The right leg was trained using four sets of ten contractions, each lasting 3 s with a 2-s rest period between each contraction and 2 min between each set. The left leg was trained using four 30-s contractions with a 1-min rest period between each. Both protocols involved isometric contractions at 70% of a maximum voluntary isometric contraction (MVC). The MVC, length:tension and force:velocity relationships and cross-sectional area (CSA) of each leg were measured before and after training. The increase in isometric strength was significantly greater (P = 0.041) for the CC leg (median 54.7%; P = 0.022) than for IC (31.5%; P = 0.022). There were no significant differences between the two protocols for changes in the length:tension or force:velocity relationships. There were significant increases in muscle CSA for the CC leg only. NMRS demonstrated that the changes in phosphate metabolites and pH were greater for the CC protocol. These findings suggest that factors related to the greater metabolite changes during CC training results in greater increases in isometric strength and muscle CSA.

Adult↗

PubMed2003

Multiple features of motor-unit activity influence force fluctuations during isometric contractions.

To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in 10 young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation (CV) for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. The model that increased the level of short-term synchrony with excitatory drive provided the closest fit to the experimentally observed relation between the CV for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge-rate variability. Most of the power in the force power spectra for the models was contained in the frequency bins below 5 Hz. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz: 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model because of greater variability in discharge rate. These results indicate that the variation in force fluctuations across the entire operating range of the muscle cannot be explained by a single mechanism that influences the output of the motor-unit population.

Adult↗

PubMed1991

Activity index and isometric contraction velocity of human jaw muscles.

To determine the relative contribution of the masseter and anterior temporalis muscles to global isometric bite force, activity indices were constructed on the basis of the peak mean voltage (microV) and the integrated voltage (microV.s) of bipolar surface electromyograms obtained during brief maximum voluntary teeth clenching (MVC). The index that was based on integrated myoelectrical activity showed that the masseter muscle contributed the major part of the isometric MVC force. The index that was based on instantaneous peak myoelectrical activity also showed that activity in the masseter muscle predominated over that in the anterior temporalis muscle. In addition, the latter index showed a negative linear association with the initial (0-50% MVC) isometric contraction velocities of the masseter and anterior temporalis muscles. Both indices appear to be promising clinical diagnostic tools.

Action Potentials↗

PubMed1984

Temperature dependence of isometric contractions of cat fast and slow skeletal muscles.

The influence of temperature (range 38-20 degrees C) on the isometric contractions of flexor digitorum longus (fast-twitch) and soleus (slow-twitch) muscles of the cat hind leg was examined in situ and with supramaximal nerve stimulation. The maximum tetanic tension decreased by 5-7% on cooling from 38 to 27 degrees C and by about 17% on further cooling to 20 degrees C. The results were similar between the two muscles. The twitch tension increased by 100% in flexor digitorum longus and decreased by about 40% in soleus when the temperature was lowered from 38 to 20-24 degrees C. The results are compared with those reported for fast- and slow-twitch muscles of the rat.

Animals↗

PubMed2002

Positive proprioceptive feedback elicited by isometric contractions of ankle flexors on pretibial motoneurons in cats.

Pretibial flexor motoneurons were recorded intracellularly in anesthetized cats during unfused isometric contractions of a subpopulation of motor units from either tibialis anterior (TA) or extensor digitorum longus (EDL) muscles. The contractions elicited excitatory postsynaptic potentials in 23 of 28 pretibial flexor motoneurons. No effect was observed in the remaining motoneurons. In control experiments, the effects of electrical stimulation of afferents within the TA nerve were investigated to help identify afferents responsible for the contraction-induced positive feedback. This feedback was ascribed to actions of Ia fibers because the pattern of the contraction-induced excitatory potentials was consistent with the known pattern of Ia discharge; in control experiments, electrical stimulation of group I fibers elicited only monosynaptic excitatory potentials; and the distribution of both the contraction-induced positive feedback among motor nuclei as well as the electrically evoked Ia excitatory monosynaptic potentials were restricted to homonymous and synergic motoneurons. Observation of the Ia contraction-induced positive feedback was facilitated by the absence of Ib autogenic inhibition. This contraction-induced Ia excitatory feedback in ankle flexors might either reinforce Ia-induced reflexes when these muscles are lengthened or help to lift the leg over an obstacle.

Anesthesia↗