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PubMed2004

Properties of the two neuromuscular compartments in a split bipennate muscle.

Bipennate muscles may be split along their distal aponeurosis, dividing each into two compartments. These sub-muscle units may be used in tendon transfers. This paper presents the contractile properties of the two sub-units of the flexor carpi ulnaris in a macaca fascicularis, after it was split by up to 80% of its length. The sub-muscle units were electrically stimulated and found to have independent isometric contraction, with minimal contraction recorded from the non-stimulated sub-unit. Also, the sum of the forces measured from each unit when stimulated individually, was found to be greater than the force of the whole muscle, given the same isometric conditions. The distal aponeurosis which is common allows force transmission between the compartments. Splitting the muscle along this distal aponeurosis alters this function and the force capacity of the muscle, providing a new potential for using the sub-units as grafts for tendon transfers.

Animals↗

PubMed2002

Force enhancement above the initial isometric force on the descending limb of the force-length relationship.

Edman et al. (J. General Physiol. 80 (1982) 769) observed in single fibres of frog that the steady-state forces following active fibre stretch were greater than the purely isometric force obtained at the length from which the stretch was initiated. Operating on the descending limb of the force-length relationship, such a result can only be explained within the framework of the sarcomere length non-uniformity theory, if some fibre segments shortened during the fibre stretch. However, such a result was not found, leaving Edman's observation unexplained. Force enhancement above the initial isometric force has not been investigated systematically in whole muscle, and therefore it is not known whether this property is also part of whole muscle mechanics. The purpose of this study was to test if the steady-state forces following active stretch of cat semitendinosus were greater than the corresponding purely isometric forces at the muscle length from which the stretch was started. Cat semitendinosus was stretched by various amounts on the descending limb of the force-length relationship, and the steady-state forces following these stretches were compared to the corresponding isometric forces at the initial and final muscle lengths. In 109 of 131 tests, the steady-state forces following stretching were greater than the isometric forces at the initial muscle lengths. Force enhancement increased with increasing amounts of stretching, and force enhancement above the initial isometric force was more likely to occur following stretches of great compared to small amplitude. Passive forces following active muscle stretching were often significantly greater than the passive forces at the same muscle length following an isometric contraction or a passive stretching of the muscle. This observation was made consistently at the longest muscle lengths tested. It appears, therefore, that there is a passive force that accounts for part of the force enhancement above the isometric force at the initial muscle length, and that provides increased passive force when a muscle is actively, rather than passively, stretched at long muscle lengths. We conclude that cat semitendinosus demonstrates steady-state force enhancement above the corresponding purely isometric force at the initial muscle length on the descending limb of the force-length relationship for many contractile conditions, and that a unique, and so far undetected, passive, parallel element contributes to this force enhancement, particularly at long muscle lengths where muscle is assumed to be most vulnerable to injuries associated with sarcomere length instability.

Adaptation, Physiological↗

PubMed1999

The effect of propofol on human gastric and colonic muscle contractions.

UNLABELLED: Although propofol is widely used for sedation in the intensive care unit, there are limited data on its effects on gastrointestinal motility. For that reason, we studied the in vitro effects of propofol on human gastric and colonic smooth muscle. Grossly normal human gastric and colonic muscle strips were mounted in an organ bath set-up for isometric contraction and stimulated by acetylcholine (Ach), using a cumulative dose schedule in the absence or presence of different concentrations of propofol [1.7 x 10(-6) M (0.3 microg/mL) to 4.4 x 10(-4) M (78 microg/mL)]. Ach led to concentration-dependent contraction of both gastric and colonic muscle strips, whereas propofol, at a concentration 6.7 x 10(-6) M (1.2 microg/mL) and above, significantly depressed Ach-induced contraction in a concentration-dependent manner for both smooth muscle preparations. In addition, propofol, at a concentration 2.7 x 10(-5)M (4.8 microg/mL) and above, depressed spontaneous contractile activity of both smooth muscle preparations. Fat emulsion 10% (Intralipid), the solvent for propofol, had no effect on either the spontaneous activity or the Ach-induced contraction of gastric and colonic smooth muscles. IMPLICATIONS: The success of enteral feeding requires a normal gastrointestinal motility. We found that, at clinically relevant concentrations, propofol impaired gastrointestinal contractile activity. Further investigations are required to determine the clinical significance of this change.

Acetylcholine↗

PubMed2005

Variability of motor unit discharge and force fluctuations across a range of muscle forces in older adults.

Variability of motor unit discharge is a likely contributor to the greater force fluctuations observed in old adults at low muscle forces. We sought to determine whether the variability of motor unit discharge rate underlies the fluctuations in force during steady contractions across a range of forces in young (n = 11) and old (n = 14) adults. The coefficient of variation (CV) for discharge rate and force were measured during a force-matching task as the first dorsal interosseous muscle performed isometric contractions. The recruitment thresholds of the 78 motor units ranged from 0.04% to 34% of maximal voluntary contraction (MVC) force. The CV for discharge rate ranged from 7.6% to 46.2% and was greater (P < 0.05) for old adults (21.5% +/- 7.7%) than young adults (17.3% +/- 8.1%). Although the CV for force was similar for young and old subjects (2.53% +/- 1.6%) across all target forces, it was greater for old adults at the lowest forces. Furthermore, there was a positive relation (r2 = 0.20, P < 0.001) between the CV for force and the CV for discharge rate across the range of recruitment thresholds. This relation was significant for old adults (r2 = 0.30, P < 0.001), but not for young adults (r2 = 0.06, P > 0.05). Thus, the normalized variability (CV) of motor unit discharge was greater in old adults and was related to the amplitude of force fluctuations across a broader range of forces than previously examined. These findings underscore the contribution of variability of motor unit activity to motor output in normal human aging.

Action Potentials↗

PubMed1988

Analysis of electromyographic signals in human jaw closing muscles at various isometric force levels.

The effect of sustained isometric contraction on surface electromyograph (EMG) and force signals derived from these muscles was examined. Premolar-molar region force was measured with a small unilaterally positioned force transducer. Subjects produced and sustained 25, 50, 75 and 100 per cent isometric force levels, and measurements were made at the beginning and end of these efforts. There was no significant change in the resulting EMG/force ratio at any of the force levels. The EMG signal did exhibit a significant shift in its frequency both as the force level increased and during the sustained effort. Neuromuscular fatigue, when defined as a change in the EMG/force ratio, was not demonstrated even though there was a consistent change in the frequency of the EMG signal.

Adult↗

PubMed1991

The nature of suprasegmental influences on the late reflex activity in human forearm muscles.

The earliest component of the late EMG response (M2) to sudden stretch of an isometrically contracting muscle is influenced by suprasegmental mechanism, but whether these are tonic, phasic, or both, is uncertain. We have therefore investigated the nature of these mechanisms by varying the predictability of the direction of perturbation of forearm muscles held isometrically against a constant flexor or extensor force of 2.3 N. We found that the M2 response did not change appreciably regardless of whether the direction of perturbation was known in advance or which hand was used by the subject. These findings suggest that any tonic supraspinal influence of the M2 response is of little significance compared to the importance of phasic mechanisms and provide support for the existence of a cerebral arc subserving this reflex component.

Adult↗

PubMed1987

Inhibition of monosynaptic reflexes in the human lower limb.

1. Presynaptic inhibition of muscle spindle Ia afferents by afferents from the same and other muscles has been studied in the human lower limb. The experiments have utilized conditioning of test monosynaptic reflexes by vibration of both the test and other muscles. 2. The pattern of inhibition invariably includes autogenetic actions. 3. There are powerful effects from flexor to extensor Ia afferents. Actions from flexor to flexor, and from extensor to extensor, are weaker. Actions from extensors to flexors are very weak. 4. The strength of presynaptic inhibition from one muscle type to another weakens as the muscles considered become more anatomically distant. 5. The inhibition studied both by vibration and by electrical conditioning stimulation of nerves becomes weaker during voluntary isometric contraction of the test muscle. It is strongest at rest and during antagonist contraction. 6. Evidence is provided suggesting that descending control is the primary cause of this modulation of inhibition during contraction. 7. Stimulation of afferents in cutaneous nerves reduces group I presynaptic inhibition of Ia afferents.

Adolescent↗

PubMed1994

Human spinal lateralization assessed from motoneurone synchronization: dependence on handedness and motor unit type.

1. Motoneurone synchronization as a means of investigating synaptic connectivity was studied in the extensor carpi radialis muscles of the preferred and non-preferred arms of healthy right- and left-handed human subjects. The activities of pairs of motor units recorded during voluntary isometric contractions were analysed by cross-correlation to detect any synchronous motor unit firing in the form of central peaks in the cross-correlation histograms. 2. The synchronization peaks were compared first in the case of 273 motor unit pairs tested in the preferred and non-preferred arms of two left-handed subjects and two right-handed subjects. The percentage of synchronized motor unit pairs was found to be significantly higher in the preferred arm with synchronization peaks significantly larger and broader than in the non-preferred arm. The narrow peaks (< 7.5 ms) likely to reflect the activity of common inputs to motoneurones were also found to be significantly larger in the preferred arm of all four subjects. 3. The handedness-related differences in synchronization were definitely confirmed in a total of 275 pairs of motor units tested in the left extensor carpi radialis muscles of fourteen right-handed subjects using their non-preferred arm and six left-handed subjects using their preferred arm. In order to determine whether the differences in synchronization were dependent on the motor unit type, each motor unit was characterized on the basis of its recruitment threshold and on the basis of the contraction time of its twitch extracted from the overall muscle force using the spike-triggered averaging method. Two populations of motor units were distinguished, namely the 'slow' motor units (recruitment thresholds < 0.4 N, contraction times > 40 ms) and the 'fast' motor units (recruitment thresholds > 0.6 N, contraction times < or = 40 ms). 4. In the non-preferred arm, the synchronization peaks were always fairly narrow, whatever the motor unit's biomechanical properties; whereas in the preferred arm, broad peaks were found to be particularly common among the pairs including one or two fast motor units, which also showed the largest rate of synchronization occurrence. 5. The narrow peaks (< 7.5 ms) were found to be consistently larger in the preferred than the non-preferred arm whatever the categories of motor unit pairs. In both arms, however, the amplitude of the narrow peaks tended to increase as the recruitment threshold of the motor unit decreased and as their contraction time increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1990

Contractile properties of bronchial smooth muscle with and without cartilage.

The majority of in vitro studies on airway smooth muscle have used the trachealis (TSM) as a convenient substitute for muscle from airways that constitute the flow-limiting segment. The latter are technically difficult to work with. However, because the site of maximum resistance to airflow is at the third to seventh generations of the bronchial tree, the trachealis preparation is of limited value. Length-tension and force-velocity properties were therefore studied at optimal length (lo) of canine bronchial smooth muscle (BSM) from which cartilage had been carefully removed. Normalized maximum isometric tension or stress (Po x 10(4) N/m2) for BSM was 7.1 +/- 0.19 (SE), which was similar to that of BSM with cartilage (BSM+C, 6.8 +/- 0.21) but lower than for TSM (18.2 +/- 0.81). At length greater than lo, the BSM+C was stiffer than the BSM. The values of maximum shortening capacity (delta Lmax), obtained directly from isotonic shortening at a load equal to the resting tension at lo, were 0.76 lo +/- 0.03, 0.41 lo +/- 0.02, and 0.24 +/- 0.02 lo for TSM, BSM, and BSM+C, respectively. The BSM and BSM+C delta Lmaxs were different (P less than 0.05). Maximal shortening velocities (Vo) for BSM, elicited at 2, 4, and 8 s by quick release in the course of an isometric contraction were significantly higher than for the BSM+C. Vos showed gradual decreases in all three groups in the later phase of contraction, suggesting the operation of latch bridges.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Functional properties of spinal interneurons activated by muscular free nerve endings and their potential contributions to the clasp-knife reflex.

1. The goal of this study was to characterize the functional properties of spinal interneurons that are excited by muscular free nerve endings and to assess their contributions to the clasp-knife reflex. 2. The patterns of activity of 82 spinal interneurons that were excited by squeezing the Achilles tendon or manipulation of the muscle surfaces, preferential stimuli for muscular free nerve endings, were extracellularly recorded in lamina V-VII of the L5-S1 spinal cord in decerebrated and spinalized cats. 3. Interneurons were uniformly excited by increases in muscular length and force. Responses to muscle stretch exhibited gradual decay during maintained stretch, afterdischarge after stretch release, and adaptation to repeated stretch. Responses to isometric contraction induced by electrical stimulation of motor axons was also prolonged after contraction, but did not decay during maintained contraction. For similar increases in force, stretch evoked greater excitation than contraction, indicating that both stretch and contraction contributed to interneuronal activity. Overall, the time course and magnitude of the interneuronal responses to stretch and contraction paralleled the time course and magnitude of the clasp-knife reflex. 4. Interneurons were powerfully excited by muscular free nerve endings, which mediate the clasp-knife reflex, and by cutaneous receptors. Only occasionally were they excited by primary spindle or Golgi tendon organ afferents, which suggests that activation of muscular free nerve endings mediated the interneuronal responses to stretch and contraction. 5. Simultaneous recordings of interneuronal activity and the clasp-knife reflex revealed a broad correlation between interneuronal activity and clasp-knife inhibition. 6. Because the patterns of activity of free nerve ending-responsive interneurons during stretch and contraction were similar to the clasp-knife reflex, were closely correlated with clasp-knife inhibition during simultaneous interneuronal and reflex recordings, and were powerfully excited by muscular free nerve endings, it is likely that the interneurons described above contributed to the clasp-knife reflex. 7. In contrast, a small number (n = 16) of interneurons were recorded that were only weakly excited by muscular free nerve endings but strongly excited by group I afferents, exhibited less spontaneous and evoked activity, and had significantly different responses to stretch and contraction. These interneurons are less likely to have contributed to the clasp-knife reflex.

Animals↗

PubMed1985

Pressures recorded in ulnar neuropathy.

The pressure between the ulnar nerve and the arcade bridging the two heads of the flexor carpi ulnaris muscle was recorded peroperatively in ten patients with electrophysiologically confirmed ulnar neuropathy at the elbow. At rest, with the elbow extended, pressures ranged from 0 to 19 mm Hg but increased in flexion and during isometric contraction of the flexor carpi ulnaris muscle to maximal values above 200 mm Hg.

Adult↗

PubMed1992

Hypothermia enhances contractile responses of the guinea pig taenia caeci to acetylcholine, 5-hydroxytryptamine, caffeine and calcium.

Isometric contraction of isolated guinea-pig taenia caeci was induced with acetylcholine (ACh) and 5-hydroxytryptamine (5-HT) at 37 and 30 degrees C to investigate the effect of hypothermia on the response of smooth muscle to neurotransmitters. Lowering the temperature increased the amplitude of contraction in response to 10(-6) M ACh. Contraction in response to 10(-6) M 5-HT was also greater at 30 degrees C. 5-HT-contraction was not inhibited by atropine, but was inhibited by ketanserin. Calcium-contraction was also induced in an isosmotic high potassium solution. The amplitude of the contraction elicited by 5 x 10(-3) M Ca was significantly greater at 30 degrees C, and was inhibited by verapamil. The amplitude of the contractile response to 5 x 10(-3) M caffeine was also greater at 30 degrees C than at 37 degrees C. The finding that both calcium- and caffeine-contraction were enhanced at low temperature raises the possibility that intracellular calcium participate in cold-induced enhancement of contraction.

Acetylcholine↗

PubMed1984

The effects of shortening on myoplasmic calcium concentration and on the action potential in mammalian ventricular muscle.

When cardiac muscle shortens during a contraction, the duration of mechanical activity is abbreviated (shortening deactivation), but the duration of the action potential is prolonged. Neither of these phenomena is fully understood, but both may be related to changes in the myoplasmic free calcium concentration. In these experiments, isolated papillary muscles from cats and ferrets were allowed to contract under various mechanical conditions while myoplasmic calcium was monitored with aequorin, or in parallel experiments the membrane potential was recorded with microelectrodes or a sucrose gap. When shortening occurred, myoplasmic calcium was increased and the membrane potential was more positive than in isometric contractions. The changes in calcium apparently precede the depolarization. We propose that muscle shortening reduces calcium binding to the contractile proteins and leads to a rise in myoplasmic calcium, and that this rise in myoplasmic calcium activates an inward current leading to the observed changes in the action potential. These processes may be important contributory factors in some arrhythmias.

Action Potentials↗

PubMed2001

Ethanol-induced contractions in cerebral arteries: role of tyrosine and mitogen-activated protein kinases.

BACKGROUND AND PURPOSE: The relationship between alcohol consumption and stroke appears complex; moderate ingestion is associated with reduced stroke risk, while heavy intake is associated with increased stroke risk. Ethanol has been shown both experimentally and epidemiologically to induce hemorrhagic and ischemic strokes, which are associated with cerebral vasoconstriction. Ethanol is known to induce contraction in isolated cerebral arteries and intact microvessels from diverse mammalian animals. The relationships between ethanol-induced contractions in cerebral arteries, intracellular free Ca(2+) ([Ca(2+)](i)), tyrosine kinases (including the src family), and mitogen-activated protein kinases (MAPK) were investigated in the present study. METHODS: Cerebral arterial muscle tension and [Ca(2+)](i) were quantified by an isometric contraction technique and direct visualization of Ca(2+) in single cells. RESULTS: Ethanol induces concentration-dependent contractions in intact canine basilar arteries, which are attenuated significantly by pretreatment of the arteries with low concentrations of an antagonist of protein tyrosine kinases (genistein); an src homology 2 (SH2) domain inhibitor peptide; a highly specific antagonist of p38 MAPK (SB-203580); a potent, selective antagonist of MEK1/MEK2 (U0126); and a selective antagonist of mitogen-activated protein kinase kinase (MAPKK) (PD-98059). IC(50) levels obtained for these 5 antagonists are consistent with reported K:(i) values for these tyrosine kinase, MAPK, and MAPKK antagonists. Ethanol induces transient and sustained increases in [Ca(2+)](i) in primary single smooth muscle cells from canine basilar arteries, which are markedly attenuated in the presence of genistein, an SH2 domain inhibitor peptide, SB-203580, U0126, and PD-98059. Several specific antagonists of known endogenously formed vasoconstrictors do not inhibit or attenuate either the ethanol-induced contractions or the elevation of [Ca(2+)](i). CONCLUSIONS: The present study suggests that activation of protein tyrosine kinases (including the src family) and MAPK appear to play important roles in the ethanol-induced contractions and the elevation of [Ca(2+)](i) in smooth muscle cells from canine basilar arteries. The results could be used to suggest that selective antagonists of protein tyrosine kinases and MAPK may be useful both prophylactically and therapeutically in alcohol-induced strokes.

Animals↗

PubMed1997

Differential regulation of glucose and glycogen metabolism in vascular smooth muscle by exogenous substrates.

The aim of this study was to determine whether the pathways of glycolysis and glycogenolysis can be independently modulated by the provision of acetate or pyruvate as exogenous substrates. Hog carotid artery segments were allowed to replete glycogen stores to over 6 micromol/g of new 13C-labeled glycogen by incubation at 37 degrees C with 5 mM [1-13C]glucose for 6-16 h and then were isometrically contracted for 3 h with 80 mM KCl in the presence of 5 mM [2-13C]glucose and either 2 mM sodium acetate or 5 mM sodium pyruvate. Measurements were made of total lactate production, glucose utilization, glycogen utilization, isometric force, [2-13C]lactate and [3-13C]lactate production. Compared to experiments with glucose as the sole exogenous substrate, provision of pyruvate significantly decreased glucose utilization (by 28%) but insignificantly decreased glycogen utilization. In contrast, provision of acetate resulted in a statistically insignificant decrease in glucose utilization (by 23%) and an increase in glycogen utilization (by 20%). The fraction of [3-13C]pyruvate derived from glycogen that was converted to [3-13C]lactate was significantly decreased in the presence of acetate despite the enhanced glycogen utilization. Despite these alterations in cellular energy balance, isometric force generation and maintenance was similar for all experimental groups. This differential regulation of glycolysis and glycogenolysis may either reflect the compartmentation of these pathways or suggest a novel regulation of carbohydrate metabolism in vivo.

Acetates↗

PubMed1993

Effects of jaw clenching, jaw movement and static jaw position on facial skin sensitivity to non-painful electrical stimulation in man.

The effects of isometric jaw clenching, static jaw position and jaw movement on electrically evoked perception thresholds of the facial skin of the mental region were studied in healthy human subjects. Exercise consisted of brief (1 s) isometric contractions of jaw-closing muscles against a static load (30% of the maximum force), or continuous jaw movements at two different frequencies (1 and 3 Hz). A visual cue was used to indicate the start and end of the isometric exercise (duration 1 s.). Isometric jaw clenching induced a significant elevation of perception thresholds in the skin of the lower jaw just before and during the early electromyographic response of the jaw-closing muscles. This elevation was attenuated before the end of the exercise. Corresponding thresholds evoked by electrical stimulation applied to the dorsum of the hand were not changed by isometric jaw clenching. Changes in static jaw position did not have any effect on detection thresholds. Continuous 'masticatory-like' jaw movements produced a velocity-dependent reduction of sensitivity in the facial skin. The suppression was significantly stronger than that produced by isometric jaw exercise. An imagined isometric biting exercise, which presumably activated the supplementary motor cortex, did not cause any threshold elevations. The results indicate that isometric jaw clenching as well as cyclical jaw movements produce segmentally a phasic, rapidly attenuating masking of facial skin sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1984

Isometric exercise before and after beta-receptor blockade with propranolol and mepindolol sulfate.

The effect of isometric exercise on hemodynamic parameters before and after beta blockade were investigated in two groups of six male volunteers, ages 25-28 years. On the basis of a randomized scheme, the subjects received either 15 mg propranolol i.v. or 0.5 mg mepindolol sulfate i.v. Cardiac output (CO) was determined by means of the Swan-Ganz thermodilution technique, and blood pressure was measured invasively in the radial artery. Analysis of the results pointed to the following conclusions. Isometric contraction brings about a reflex constriction of arterial and venous vessels, and thus an increase in blood pressure, pulmonary artery wedge pressures (PAWP) and CO, as well as a reduction in stroke volume (SV). The circulatory reflex mechanism operative during isometric exercise cannot be significantly influenced by beta blockade with propranolol or mepindolol sulfate. The increase in heart rate during isometric exercise is therefore unlikely to be the result of sympathetic activation, but rather of other (vagal?) reflex mechanisms.

Adrenergic beta-Antagonists↗

PubMed1997

Electromyographic amplitude and frequency changes in the iliocostalis lumborum and multifidus muscles during a trunk holding test.

BACKGROUND AND PURPOSE: Muscle endurance is an important variable to measure in the assessment of back muscle function. This study investigated the electromyographic (EMG) activity and fatigue patterns of iliocostalis lumborum and multifidus muscles during a trunk holding test. SUBJECTS: Sixteen male subjects (mean age = 24.2 years, SD = 4.2, range = 20.6-31.9) without low back pain or known pathology were recruited in the study. METHODS: Surface EMG electrodes were used to record the activity of iliocostalis lumborum and mutifidus muscles during a 60-second isometric contraction. To reflect the activity level and fatigue rate of the muscles. EMG amplitude (root mean square [RMS] values) and a frequency variable (median frequency [MF]) were measured. RESULTS: The multifidus muscle displayed a higher level of activity, initial MF, and normalized MF slope than did the iliocostalis lumborum muscle. There was no difference, however, in the normalized RMS slope between the two muscles. The correlations between the normalized MF slope and the RMS slope of the two muscles were nonsignificant. CONCLUSION AND DISCUSSION: This study shows that monitoring frequency changes of the EMG signals may enable therapists to quantify the fatigue changes of individual muscles during the trunk holding test. The higher fatigue rate shown in the multifidus muscle compared with the iliocostalis lumborum muscle may be due to the higher activity level of the multifidus muscle during te trunk holding contraction. This greater activity of the multifidus muscle during the contraction might be explained by the functional differences between these two muscles.

Adult↗