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PubMed1985

Torque-velocity relationship in isokinetic cycling exercise.

Seven healthy female subjects performed brief (less than 10 s) periods of maximal exercise on a constant-velocity cycle ergometer, over the functional range of pedaling velocities, and an isometric contraction with each leg. There was an inverse relationship between peak torque and pedal crank velocity in all subjects; isometric torque was (mean +/- SE) 19.8 +/- 8.3% greater than the torque recorded at the slowest velocity of 11 rpm. The torque-velocity relationship was described best by a single exponential equation: y = 189.6 X e-0.0834x, where y is peak torque in Newton . meters and x is crank velocity in revolutions per minute. Peak power was a parabolic function of crank velocity; the data were fitted suitably by a second-order polynomial equation: y = -0.0589x2 + 14.504x + 47.092, where y is peak power in watts and x is crank velocity in revolutions per minute. Maximal peak power occurred at crank velocities ranging from 120 to 160 rpm, when the torque was 0.36 +/- 0.06 of the maximal isometric tension. These results demonstrate the importance of recording velocity in measurements of dynamic maximal power.

Adult↗

PubMed1992

Muscle sounds during voluntary and stimulated contractions of the human adductor pollicis muscle.

The relationships between force, electromyography (EMG), and muscle sounds recorded by acoustic myography (AMG) were investigated for both voluntary and stimulated isometric contractions in the adductor pollicis muscle. Voluntary activity was performed at 10, 25, 50, 75, 85, and 100% of maximal voluntary contraction (MVC) force. Stimulated contractions were produced by supramaximal electrical stimulation of the ulnar nerve at the wrist at frequencies of 10, 20, 30, 50, 70, and 100 Hz. Contractions lasted for 4 s each, and were performed in random order with a 3-min rest between each. The voluntary and stimulation studies were performed in random order between subjects. Simultaneous recordings were obtained for force, force oscillation (from the differentiated force signal), and raw and integrated AMG (IAMG) and EMG (IEMG). During voluntary contractions, IAMG increased with force up to MVC (r2 = 0.99, P less than 0.001) in a curvilinear fashion and a similar relationship was seen between force and IEMG (r2 = 0.99, P less than 0.001). Conversely, during stimulated contractions as stimulation frequency increased, IAMG decreased in a fashion mirroring the frequency-force curve. The frequency of the AMG signal matched stimulation frequency and declines in total IAMG were due to reductions in amplitude of the AMG signal. The stimulation frequency-oscillation of force relationship was identical to that seen for stimulation frequency and IAMG. Integrated EMG increased linearly with stimulation frequency (r = 0.99). The stimulation results suggest that muscle sounds reflect oscillation of muscle fibers and that AMG signal characteristics are determined by motor control mechanisms rather than intrinsic contractile processes.

Acoustics↗

PubMed2001

Motor unit recruitment during lengthening contractions of human wrist flexors.

The purpose of this study was to revisit the question of recruitment of motor units during lengthening contractions because of conflicting views in the literature on this subject. Motor unit activity was recorded from the flexor carpi radialis muscle of four human subjects to compare the patterns of recruitment during lengthening and isometric contractions. Lengthening contractions were produced either when the subject voluntarily stopped opposing a background load or when an additional load was imposed on the already contracting muscle. In both cases, lengthening of the active muscle was produced at a variety of speeds, from quite slow to "as fast as possible." No differences in recruitment order were observed between isometric and lengthening contractions at any speed of lengthening contraction. It is concluded that all contractions in normal humans recruit motor units in an orderly fashion from small to large, according to the size principle of motor unit recruitment.

Adult↗

PubMed2006

Changes in the reflex excitability during and after a sustained, low-intensity muscle contraction.

The purpose of the study was to evaluate the soleus H-reflex amplitude during and after a low-intensity isometric contraction. Twelve healthy, untrained subjects performed a 10-minute isometric plantar flexion at 20% of their maximum voluntary contraction torque output. The electromyogram, H-reflex, and maximum M-wave (Mmax) of the soleus muscle was recorded during and 10 minutes after the end of the contraction. The results indicated that the H-reflex increased significantly (mean +/- SEM: 44.7 +/- 16.6%, p < 0.05), but when the fatigue protocol was over, the H-reflex was depressed for the first 3 minutes, relative to the H-reflex that was recorded before fatigue, when the muscle was relaxed. The Mmax did not change significantly during the whole experiment. Furthermore, the stimulation frequency (0.1 vs. 0.3 Hz) did not have any significant effect on the H-reflex modulation. The results of the current study suggest that the reflex excitability is increased as fatigue develops, whereas this increase turns to depression for the first minutes of the recovery phase. The functional significance of these changes and the neural mechanisms which might be responsible are discussed.

Adult↗

PubMed2002

Interaction of blood flow and oxygen delivery affects peak VO2 and fatigue in canine muscle in situ.

This investigation was designed to examine the interactive effects of blood flow (Q) and oxygen delivery (QO2) on peak oxygen consumption (VO2peak) and fatigue in the gastrocnemius-plantaris dog muscle in situ. Arterial oxygen content (CaO2) was manipulated by varying the fraction of inspired O2 to create a high- and a low-QO2 treatment at each of three levels of Q (approximately/=800, 1100 and 1500 ml x kg(-1) x min(-1)). Isometric contraction was achieved by stimulation with one tetanic contraction per second (200 ms at 70 Hz) at supramaximal voltage for 10 min to achieve VO2peak under each condition. Arterial and venous blood were sampled, and Q and tension development (TD) were measured at 1, 5 and 10 min into the contractions. Fatigue was calculated as the percentage (%) change in TD from min 1 to min 10. While VO2peak was highly correlated to QO2 (R = 0.97) in both the moderate- and high-Q conditions, both low-Q conditions had a significantly (P < 0.05) higher VO2peak than predicted by the QO2-VO2 relationship. In addition, VO2 peak was significantly greater in the low-Q versus the high-Q condition. Fatigue (% decrease in TD) was significantly different between the two low-Q conditions [Low-QO2=38.4 (6.9); Norm QO2=27.9 (1.8)] and the two moderate-Q conditions [low-QO2=31.1 (7.3); high-QO2 = 20.9 (4.1)] but not between the two high-Q conditions [normal-QO2 = 10.4 (6.9); high-QO2 = 11.9 (4.5)]. In the two conditions of equal QO2, fatigue was significantly less in the high-Q condition (10.4%) compared to the low (27.9%). Thus, it seems that there is a unique interaction between Q and QO2 which determine the VO2peak and rate of fatigue in contracting muscle.

Animals↗

PubMed1991

Measurement of muscle fiber conduction velocity in the human masseter muscle using the surface EMG cross-correlation technique.

During sustained isometric contraction of the jaw muscles, the surface electromyography (EMG) power spectrum compresses into lower frequencies. This compression has been mainly attributed to the change of the conduction velocity of the action potentials along the muscle fibers. In this study, the muscle fiber conduction velocity of the masseter muscle was measured by using the surface electrode array and the cross-correlation technique. The conduction velocity of the masseter muscle varied from 6.83 to 18.01 m/s among the eight subjects, and slightly increased with the contraction level.

Adult↗

PubMed1985

Effect of myoelectric signal processing on the relationship between muscle force and processed EMG.

The relationship between the force generated by an isometrically contracting muscle and the associated myoelectric signal has been the subject of extensive investigation in the past and conflicting results were reported regarding this relationship. The objective of the present study was to investigate some of the sources that may lead to such conflicting results. Two possible sources were examined. The first was the variability in the force-EMG relationship resulting from processing the EMG signal with different EMG signal processing techniques and the second was the variability in the force-EMG relation obtained from repeated muscle contractions. The results of the study indicate that slight variations in the force-EMG relationships can be attributed to the selection of different processing techniques. However, the variability in the force-EMG relation, obtained by using different EMG signal processors, was found to be significantly smaller than the variability in this relationship obtained from repeated muscle contractions.

Adult↗

PubMed2002

Superficial aponeurosis of human gastrocnemius is elongated during contraction: implications for modeling muscle-tendon unit.

Two questions were addressed in this study: (1) how much strain of the superficial aponeurosis of the human medial gastrocnemius muscle (MG) was obtained during voluntary isometric contractions in vivo, (2) whether there existed inhomogeneity of the strain along the superficial aponeurosis. Seven male subjects, whose knees were extended and ankles were flexed at right angle, performed isometric plantar flexion while elongation of superficial aponeurosis of MG was determined from the movements of the intersections made by the superficial aponeurosis and fascicles using ultrasonography. The strain of the superficial aponeurosis at the maximum voluntary contraction, estimated from the elongation and length data, was 5.6+/-1.2%. There was no significant difference in strain between the proximal and distal parts of the superficial aponeurosis. Based on the present result and that of our previous study for the same subjects (J. Appl. Physiol 90 (2001) 1671), a model was formulated for a contracting uni-pennate muscle-tendon unit. This model, which could be applied to isometric contractions at other angles and therefore of wide use, showed that similar strain between superficial and deep aponeuroses of MG contributed to homogeneous fascicle length change within MG during contractions. These findings would contribute to clarifying the functions of the superficial aponeurosis and the effects of the superficial aponeurosis elongation on the whole muscle behavior.

Adult↗

PubMed1995

Compound motor action potentials and mechanical failure during sustained contractions by electrical stimulation in paraplegic patients.

In nine paraplegic patients we recorded the torque output and compound motor action potentials (CMAPs) produced by the quadriceps muscle during an isometric contraction elicited by electrical stimulation. The torque, the peak to peak amplitude, the latency, the peak to peak duration and the total surface of the rectified CMAPs were computed over a period of 126 s. After a brief increase the mechanical output rapidly decreased and reached a stable minimum level by the end of 126 s. The final torque output values ranged from 7.1 to 54% of initial values. This torque decrease was related neither to length of time between injury and testing, nor to the thoracic level of the spinal cord injury. The peak to peak amplitude of the CMAPs changed over the course of stimulation. It was noted to increase over a period of time after which it decreased to a minimum level. The latency from the onset of stimulation to the onset of the CMAP varied to a relatively small extent compared to the peak to peak duration. Therefore, the conduction velocity along the muscle fibres appeared to be more affected by the test than by the conduction velocity along the nerve fibres and the transmission across the neuromuscular junction. The mechanisms involved in the changes in CMAPs and the change in torque output over the time course of stimulation are discussed.

Adult↗

PubMed2002

Objective measurement of muscle strength in children with spastic diplegia after selective dorsal rhizotomy.

OBJECTIVES: To examine changes in isometric muscle strength at the elbow, knee, and ankle at 6 months and 1 year after selective dorsal rhizotomy (SDR) and to determine if SDR altered the frequency of muscle cocontraction. DESIGN: Prospective outcome study of a consecutive sample. SETTING: Children's hospital. PATIENTS: Ten children with spastic diplegia (7 independent and 3 dependent ambulators who used assistive devices) and 8 age-matched controls. INTERVENTIONS: SDR; physical and occupational therapy; elbow, knee, and ankle measured for flexion and extension strength during three 10-second isometric contractions for each muscle group; and monitored cocontraction measured via muscle electrodes. MAIN OUTCOME MEASURES: Absolute and normalized values of isometric strength; and alterations in the frequency of cocontraction at 6 months and 1 year postoperatively. RESULTS: Children with spastic diplegia showed significantly weaker knee extensors, ankle dorsiflexors, and ankle plantarflexors than age-matched controls. There were no significant differences in strength between the 2 groups in the elbow flexors, elbow extensors, and knee flexors. Isometric strength did not increase or decrease significantly after SDR. Cocontraction during knee extension was normalized after SDR, whereas cocontraction during ankle plantarflexion was unchanged by SDR in the majority of children. CONCLUSION: SDR did not result in a significant decrease in muscle strength in ambulatory children with spastic diplegia. The normalization of the electromyographic patterns at the knee and not the ankle after SDR lends support to the premise that in children with cerebral palsy cocontraction is multifaceted, representing a volitional strategy to enhance control, as well as a disorder of the mechanisms that govern patterns of muscle activity.

Cerebral Palsy↗

PubMed1974

Dynamic properties of inferior rectus muscle of the rat.

1. Isometric responses of rat inferior rectus muscle to indirect and direct stimulation were compared, and conditions were found for selective direct stimulation of twitch fibres in vitro.2. Most of the twitch fibres were qualitatively ;fast'.3. The influence of length on isometric contractions and the relation between relative load and speed of sarcomere shortening of fast-twitch fibres were determined.4. The isometric twitch contraction and half-relaxation times of fast-twitch inferior rectus fibres were only about one half of those of rat extensor digitorum longus fibres in the same conditions, whereas the force: velocity properties of these two fibre groups were virtually the same. These results show that the relation between intrinsic speed of shortening and duration of the active state of the contractile material is not the same for rat extraocular and hind-limb muscles.

Journal Article↗

PubMed1982

The strength-endurance relationship in skeletal muscle: its application to helmet design.

The isometric strength and endurance of the neck muscles in man was measured in four male subjects during dorsal, ventral, and lateral flexion of the head in a helmet dynamometer. The purpose of these experiments was to quantify the maximum strength of the neck muscles in these directions and to examine the relationship between isometric strength and endurance for various submaximal isometric tensions. Isometric contractions were sustained to fatigue by these subjects at tensions of 25, 40, 55, 70, and 90% of the maximum isometric strength in each of the directions of movement. Generally, the endurance of the muscles involved in ventral flexion was less than that found in dorsal and lateral flexion of the head. However, the strength of the muscles involved was progressively greater for lateral, dorsal, and ventral flexion, respectively. The implication of these findings in helmet design and helmet loading are discussed.

Adult↗

PubMedDate not supplied

The mechanisms underlying Bay K 8644-stimulated phasic myometrial contractions.

OBJECTIVE: Phasic myometrial contractions appear to be produced by calcium transients resulting from the activation of the phosphatidylinositol-signaling pathway. Bay K 8644, an L-type calcium channel activator, produces an increase in frequency and intensity of phasic myometrial contractions. These studies were performed to test the hypothesis that Bay K 8644-stimulated contractions were mediated through mechanisms involving phosphoinositide-specific phospholipase C activation and cytosolic calcium oscillation-like mechanisms. METHODS: In vitro contraction studies and intracellular calcium imaging were performed on longitudinal strips of uterine tissue obtained from mature virgin Sprague-Dawley rats. Isometric contraction data were computer digitized, analyzed for contraction area, and normalized for cross-sectional area. Dose-response studies were performed using previously reported inhibitors of cytosolic calcium oscillation mechanisms. In addition, qualitative inositol-phosphate production studies were performed after prelabeling uterine tissue in vitro with 3H-inositol. Subsequently, the labeled inositol phosphates were separated and recovered using anion exchange chromatography. RESULTS: Bay K 8644 produced periodic calcium transients or oscillations along with a dose-related increase in contractile activity and a significant increase in inositol-phosphate production. In contrast, neomycin (an inhibitor of phospholipase C), adenine (an inhibitor of calcium-induced calcium release), nifedipine (an L-type calcium channel blocker), and EGTA (a calcium chelator) significantly inhibited Bay K 8644-stimulated contractile activity. CONCLUSIONS: These results are consistent with the hypothesis that Bay K 8644, through its facilitation of increased intracellular calcium, results in the activation of the phosphatidylinsitol-signaling pathway and cytosolic calcium oscillation-like phenomena, thereby resulting in the generation of phasic myometrial contractions.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1993

Effects of repeated eccentric contractions on structure and mechanical properties of toad sartorius muscle.

It has been proposed that lengthening of active muscle at long lengths is nonuniformly distributed between sarcomeres, with a few being stretched beyond overlap and most hardly being stretched at all. A small fraction of the overstretched sarcomeres may fail to reinterdigitate on subsequent relaxation, leading to progressive changes in the muscle's mechanical properties. Sartorius muscles of the toad Bufo marinus were subjected to repeated lengthening (eccentric) contractions at long lengths, while controls were passively stretched and then contracted isometrically or stretched at short lengths. The muscles undergoing eccentric contractions showed a progressive shift to the right of the length-tension curve, a fall in the yield point during stretch, an increase in slope of the tension response during stretch, and a fall in isometric tension. In control muscles, changes, if any, were significantly less. In electron micrographs, muscle fibers that had been subjected to a series of eccentric contractions showed sarcomeres with A bands displaced toward one half-sarcomere, leaving no overlap in the other half. Adjacent regions often looked normal. These results are all in agreement with the predictions of the nonuniform stretch of sarcomeres hypothesis.

Animals↗

PubMed1993

Electromyogram spectrum changes during sustained contraction related to proton and diprotonated inorganic phosphate accumulation: a 31P nuclear magnetic resonance study on human calf muscles.

The calf muscles of five clinically healthy men were submitted to isometric exercise and examined by 31P nuclear magnetic resonance (NMR) spectroscopy and electromyography (EMG) to evaluate the influence of proton (H+) and diprotonated forms of inorganic phosphate (H2PO4-) accumulation on EMG spectrum changes. The experiments were performed in a supra-conducting magnet (2.35 Tesla, 35-cm effective diameter) using a surface coil (7-cm diameter) positioned against the calf muscles. The EMG surface electrodes were applied on the gastrocnemius medialis muscle and acquisition of both NMR and EMG signals was synchronized. The exercise consisted of a sustained isometric contraction at 70% of the maximal voluntary contraction until exhaustion. A continuous decrease in phosphocreatine content and a large concomitant increase in H2PO4- was observed in the calf muscles of each subject. A significant increase in H+ concentration was also found when considering the whole population but intracellular acidosis was low for two subjects. Moreover, a quasilinear decrease in mean power frequency (MPF) was found during the test. Changes in MPF were correlated with variations in H+ and H2PO4- concentration but a more significant relationship was found when MPF changes were correlated with H2PO4- concentration. An interpretation of EMG spectrum changes in terms of an accumulation of by-products of anaerobic metabolism and an increase in the relative number of activated slow fibres is proposed.

Adult↗

PubMed2002

Temperature dependency of force loss and Ca(2+) homeostasis in mouse EDL muscle after eccentric contractions.

The goals of this study were first to determine the effect of temperature on the force loss that results from eccentric contractions in mouse extensor digitorum longus (EDL) muscles and then to evaluate a potential role for altered Ca(2+) homeostasis explaining the greater isometric force loss observed at the higher temperatures. Isolated muscles performed five eccentric or five isometric contractions at either 15, 20, 25, 30, 33.5, or 37 degrees C. Isometric force loss, caffeine-induced force, lactate dehydrogenase (LDH) release, muscle accumulation of (45)Ca(2+) from the bathing medium, sarcoplasmic reticulum (SR) Ca(2+) uptake, and resting muscle fiber free cytosolic Ca(2+) concentration ([Ca(2+)](i)) were measured. The isometric force loss after eccentric contractions increased progressively as temperature rose; at 15 degrees C, there was no significant loss of force, but at 37 degrees C, there was a 30-39% loss of force. After eccentric contractions, caffeine-induced force was not affected by temperature nor was it different from that of control muscles at any temperature. Loss of cell membrane integrity and subsequent influx of extracellular Ca(2+) as indicated by LDH release and muscle (45)Ca(2+) accumulation, respectively, were minimal over the 15-25 degrees C range, but both increased as an exponential function of temperature between 30 and 37 degrees C. SR Ca(2+) uptake showed no impairment as temperature increased, and the eccentric contraction-induced rise in resting fiber [Ca(2+)](i) was unaffected by temperature over the 15-25 degrees C range. In conclusion, the isometric force loss after eccentric contractions is temperature dependent, but the temperature dependency does not appear to be readily explainable by alterations in Ca(2+) homeostasis.

Animals↗

PubMed1991

Surface electromyogram spectral characterization and motor unit activity during voluntary ramp contraction in men.

The relationships were investigated between the surface electromyographic (SEMG) power spectrum analysed by the 20 order autoregressive model (AR spectrum) and underlying motor unit (MU) activity during isometric contractions increasing linearly from 0% to 80% maximal voluntary contraction. Intramuscular spikes and SEMG signals were recorded simultaneously from biceps brachii muscle; the former were analysed by a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and the latter subjected to AR spectral analysis. Results indicated that there was a positive correlation between the force output and the mean amplitude of the ISAF histogram but not with the mean frequency. These changes were accompanied by changes in relative power of the high frequency (100-200 Hz) peak (HL) in the AR spectrum. It was also found that there was a positive correlation between the mean amplitude of the ISAF histogram and the HL value. These data suggested that the power of the high frequency peak in the AR spectrum of the SEMG signal preferentially reflected the progressive recruitment of underlying MU according to their size. Differences between the AR spectrum and the spectrum estimated by fast Fourier transform algorithm have also been discussed.

Adult↗

PubMed1990

Drug effects on function in the ferret ischemic hindlimb.

In this article a new model of peripheral occlusive arterial disease is described. The lower hindlimb of an anesthetized ferret was fixed to a holder, the distal end of the Achilles tendon attached to an isometric force transducer, and a passive preload of 100 g was applied to the muscle preparation. The hindlimb was stimulated to contract isometrically via supramaximal electrical stimulation of the sciatic nerve. During the initial period, when femoral blood pressure equaled aortic blood pressure, net contractile force peaked within 1 or 2 min (372 +/- 24g, n = 20) and gradually declined to about 85% of peak over 20 min. Following 60 min of ischemia (induced by partial occlusion of the abdominal aorta), when blood pressure in the contralateral femoral artery was about 45 mm Hg, the 15-min area under the force-time curve (AUC) was 33.2 +/- 2.5% (n = 4) of the initial value. To validate the utility of this model in the search for treatment of peripheral vascular diseases, two drugs were tested. Pentoxifylline, which is clinically effective, attenuated the loss of function observed during ischemia in a dose-related manner, but nifedipine, which is without clinical benefit, had no effect at a dose that was extremely hypotensive. Because femoral perfusion pressure was controlled, systemic hemodynamic effects of test compounds are minimized as potential mechanisms of action, simplifying the evaluation of novel pharmacotherapy for treatment of ischemic diseases.

Animals↗