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At least 397 records · Page 22Linked to original sources
PubMed1989

Changes of muscular sound during sustained isometric contraction up to exhaustion.

The sound (SMG) generated by the biceps muscle during isometric exercise at 20, 40, 60, and 80% of maximum voluntary contraction (MVC) up to exhaustion has been recorded by a contact transducer and integrated (iSMG), together with the surface electromyogram (EMG) in eight young untrained men. At the onset of exercise, iSMG and integrated surface EMG (iEMG) amplitude increased linearly with exercise. iSMG remained constant for 253 +/- 73 (SD), 45 +/- 16, 21 +/- 5, and 0 s at the four levels of contraction. Then iSMG increased linearly at 20% MVC, fluctuated at 40% MVC, and decreased exponentially at 60 and 80% MVC. iSMG exhaustion-to-onset ratio was 5.0 at 20%, 1.0 at 40%, and 0.2 at 60 and 80% MVC. On the contrary, independently of exercise intensity, iEMG increased with time, being 1.4 higher at exhaustion than at the onset. The nonunivocal iSMG changes with time and effort of exercise suggest that the sound may be a useful tool to acquire different information to EMG and output force during muscle contraction up to fatigue.

Adult↗
PubMed1976

The responses of human muscle spindle endings to vibration during isometric contraction.

1. An human subjects, vibration of amplitude 1-5 mm and frequency 20-220 Hz was applied to the tendons of muscles in the leg to examine the effects on the discharge of primary and secondary endings during manoeuvres designed to alter the level of fusimotor drive. 2. In four experiments, the peroneal nerve was completely blocked with lidocaine proximal to the recording site in order to de-efferent spindle endings temporarily. The responses to muscle stretch and vibration, as seen in multi-unit recordings and in single unit recordings, were similar during the block as in the relaxed state prior to the block. Thus, these experiments provided no evidence for a functionally effective resting fusimotor drive. 3. The responses to vibration of nine primary endings and four secondary endings were examined during isometric voluntary contractions of the receptor-bearing muscles. Providing that the endings were responding submaximally in the relaxed state, voluntary contraction enhanced the response to vibration, suggesting co-activation of the fusimotor system sufficient to compensate for mechanical unloading. Unloading effects were observed during contractions of neighbouring synergistic muscles, indicating a close spatial relationship between the co-activated skeletomotor and fusimotor outflows. 4. Recordings were obtained from ten primary endings and seven secondary endings during isometric reflex contractions resulting from the vibratory stimulus (TVR contractions). For twelve endings, the appearance of the tonic vibration reflex in the receptor-bearing muscle resulted in a significant decrease in the response to vibration, suggesting that the endings were unloaded by the extrafusal contraction. On voluntary suppression of the reflex contraction spindle responses reverted to their previous levels. 5. These results suggest that the tonic vibration reflex, like the tendon jerk reflex, operates predominantly or exclusively on alpha motoneurones and that it does not utilize the same cortically originating efferent pathways as are used in the performance of voluntary contractions.

Action Potentials↗
PubMed1995

Pressure-induced changes in the isometric contractions of single intact frog muscle fibres at low temperatures.

Effects of increased hydrostatic pressure (range 0.1-10 MPa) on isometric twitch and tetanic contractions of single intact muscle fibres, isolated from frog tibialis anterior muscle, were examined at 4-12 degrees C. The tension changes produced on exposure to steady high pressures are compared with those produced on exposure to low concentrations of caffeine (0.5 mM, subthreshold for contracture) and when pressure is rapidly released during a contraction. The peak twitch tension was potentiated by pressure accompanied by increased rate of tension rise and increased duration; the pressure sensitivity of twitch tension was approximately 8% MPa-1. The correlation between the rate of tension rise and peak tension in caffeine-induced twitch tension potentiation was quantitatively similar to that in pressure-induced twitch potentiation. Experiments involving the rapid release of pressure (approximately 2 ms) during twitch contractions demonstrate that high pressure need only be maintained for a brief period during the early part of tension development to elicit full twitch potentiation. The tetanic tension was depressed by pressure (approximately 1% MPa-1). Results demonstrate that the major effect of increased hydrostatic pressure on intact muscle fibres, which results in tension potentiation, is complete very early during contraction and is similar to that of caffeine.

Animals↗
PubMed1996

Haemodynamic changes induced by submaximal isometric contraction in painful and non-painful human masseter using near-infra-red spectroscopy.

Although mechanisms underlying chronic muscle pain are poorly understood, one prevalent theory is that it is due, in part, to localized hypoxia. The purpose of this study was to evaluate this theory using non-invasive near-infra-red spectroscopy that monitors relative changes in intramuscular haemoglobin (Hb) concentration and oxygen saturation levels. Data were collected for the human masseter muscle during and following three isometric 30-s trials at 50% maximum voluntary contraction. Ten females, with a history of chronic muscle pain in the jaw, and eight matched healthy females without muscle pain (controls) participated. Results showed that, upon initiation of masseter muscle contraction, there was a rapid reduction in the intramuscular Hb concentration concomitant with a reduction in oxygen saturation levels. After cessation of the contraction, the Hb concentration increased rapidly and then fell toward the baseline. Significant differences in the recovery profile for oxygen saturation were found between the first trial and the following two trials for both the muscle pain- and control group. Looking at the first trial only, and adjusting for covariates of height, weight and bite-force in the analysis, revealed a marginally significant postcontraction difference between the two groups with a lower level of oxygen saturation during recovery in the group with chronic muscle pain. Significant group differences were found in Hb concentrations without any significant trial effect. It is likely that the well-known changes in intramuscular blood flow that occur during and after contraction in human muscles are reflected in these altered relative Hb concentrations. The group with chronic muscle pain showed a clearly reduced magnitude of the Hb concentration change in the postcontraction recovery period. The results support the concept that patients with chronic muscle pain have a slower intramuscular reperfusion during the recovery phase after sustained isometric contractions.

Adult↗
PubMed2004

Biomimetic model of skeletal muscle isometric contraction: II. A phenomenological model of the skeletal muscle excitation-contraction coupling process.

This paper describes a new macroscopic, phenomenological model of the skeletal muscle excitation-contraction coupling process, as represented by four principal and consecutive compartments (biophysical, biochemical, and biomechanical phases) characteristic of isometric excitation-contraction coupling in mammalian skeletal muscle, and coupled by a system of simultaneous, first-order linear ordinary differential equations. The model is based upon biological compartmental transport kinetics and irreversible thermodynamic energy transformation, and represents a distinct improvement over other biomimetic models. The model was derived using physiological parameter data published in the literature, and validated using MATLAB R12.

Calcium↗
PubMed1984

EMG patterns in antagonist muscles during isometric contraction in man: relations to response dynamics.

We studied the EMG activity of biceps and triceps in human subjects during isometric force adjustments at the elbow. Rapid targeted force pulses exhibited stereotyped trajectories in which peak force was a linear function of the derivatives of force and the time to peak force was largely independent of its amplitude. These responses were associated with an alternating triphasic pattern of EMG bursts in agonist and antagonist muscles similar to that previously described for rapid limb movements. When the instructions demanded rapid force pulses, initial agonist bursts were of constant duration, and their magnitude was strongly related to peak force achieved. The timing of EMG bursts in antagonist pairs was closely coupled to the dynamics of the force trajectory, and the rising phase of the force was determined by both agonist and antagonist bursts. When peak force was kept constant and rise time systematically varied, the presence and magnitude of antagonist and late agonist bursts were dependent on the rate of rise of force, appearing at a threshold value and then increasing in proportion to this parameter. It is proposed that antagonist activity compensates for nonlinearity in muscle properties to enable the linear scaling of targeted forces which characterizes performance in this task.

Adult↗
PubMed2000

Different neuromuscular recruitment patterns during eccentric, concentric and isometric contractions.

Aim. The purpose of this study was to determine the neuromuscular fatigue profiles during 100 s isometric (ISO), concentric (CON), and eccentric (ECC) activity.Methods. Twelve subjects (age 25.1+/-3.7 years, mass 70.1+/-8.2 kg, mean+/-SD) performed ISO, CON and ECC maximal voluntary contractions and 100 s endurance trials on an isokinetic dynamometer. Raw EMG data were recorded throughout each trial from the rectus femoris of the right limb. Corresponding data for integrated electromyography (IEMG), percentile frequency shifts (MPFS) and peak torque output were divided into five 5 s epochs and subsequently normalised with the first epoch being the reference point, in order to assess changes over time.Results. There were no significant differences between ECC, CON and ISO peak torque output (211+/-63 vs 169+/-41 vs 177+/-61 Nm; ECC, CON, ISO) and IEMG activity (280+/-143 vs 305+/-146 vs 287+/-143 mV; ECC, CON, ISO) during maximal contractions. Serial reductions in torque output were greatest in ISO in which torque output during the final epoch was 31+/-13% of initial values, similar to the final torque values in CON (58+/-15%), but significantly less than ECC (108.6+/-38.6%; P<0. 001) values. In CON and ECC, IEMG was maintained (95+/-27% and 93+/-21%; CON and ECC), whereas IEMG for ISO decreased to 38+/-13% of initial values. The greatest reduction in MPFS occurred in CON (69+/-10%) compared to ISO (78+/-9%; P<0.05) and ECC (93+/-6%; P<0.001).Conclusion. These data demonstrate distinct neuromuscular fatigue profiles for the different types of muscle contraction. Whereas eccentric activity was largely fatigue resistant, isometric and concentric contractions displayed different neuromuscular fatigue profiles.

Adult↗
PubMed1981

Estimation of myodynamic parameter values from observations on isometrically contracting muscle groups.

A new method is presented which makes it possible to estimate from a series of experimental observations of isometric maximum-effort muscle torque, a set of myodynamic parameter values for each of a number of muscles contributing collectively to the total torque output. The parameters that can be estimated are: the individual maximum isometric forces; the spreads of the length-tension curves; the relative maximum isometric tendon extensions; and the optimum muscle lengths; most of which parameters could not be estimated previously. The method is described for both penniform and fusiform muscles, and is demonstrated using the human triceps muscle as an example. The values obtained by this method are in general agreement with comparable values obtained by in vitro methods.

Adult↗
PubMedDate not supplied

Isometric contractions of normal and spastic human skeletal muscle.

Isometric contractile properties of human elbow flexor muscles were examined in 27 normal subjects (7 females and 20 males). Contractions of elbow flexors were elicited by application of electrical pulses to the biceps motor points. The tension development was recorded at the wrist with the elbow angle fixed at 90 degrees--100 degrees. The data for the time to peak and time to half relaxation of the twitch, the twitch tension-to-tetanus tension ratio, and the posttetanic twitch potentiation are reported. The contractile properties of spastic elbow flexor muscles from 7 subjects who had been hemiparetic for periods ranging from 4 months to 60 months were found to be similar to those of muscles in normal subjects and unaffected muscles in hemiparetic subjects.

Adolescent↗
PubMed1986

Muscle ammonia metabolism during isometric contraction in humans.

The changes in ammonia (NH3) and amino acid contents in human skeletal muscle during isometric exercise (2/3 maximal voluntary contraction force) to fatigue have been investigated. Biopsies from musculus quadriceps femoris were obtained at rest, fatigue, and 1 and 4 min recovery. Muscle NH3 (n = 9) increased from 1.3 +/- 0.3 (mean +/- SE) mmol/kg dry muscle (dm) at rest to 3.6 +/- 0.6 at fatigue (P less than 0.01) and remained elevated during recovery, whereas the lactate increase after contraction decreased rapidly during recovery. Total adenine nucleotide (TAN) content decreased from 28.7 +/- 0.5 mmol/kg dm at rest to 25.1 +/- 0.6 at fatigue (P less than 0.001). Muscle glutamine did not change after contraction (P greater than 0.05), whereas glutamate decreased (P less than 0.001), and alanine increased (P less than 0.001). In vivo AMP deaminase activity (measured by the rate of TAN decrease) was positively correlated with the percentage of fast-twitch fibers (r = 0.92; P less than 0.001) and the ATP turnover rate (r = 0.75; P less than 0.001) but was not related to the muscle lactate content (r = 0.27; P greater than 0.05). Phosphocreatine decreased to 6.1 +/- 0.7 mmol/kg dm (range = 1-11) after contraction. It is concluded that during exercise activation of AMP deaminase in vivo occurs when a high ATP turnover rate is coupled with a low phosphocreatine level, muscle pH is of minor importance for direct activation of AMP deaminase in vivo, and increases in NH3 do not have an important influence on glycolysis.

AMP Deaminase↗
PubMed2001

Reserve capacity for ATP consumption during isometric contraction in human skeletal muscle fibers.

Maximum velocity of the actomyosin ATPase reaction (V(max) ATPase) and ATP consumption rate during maximum isometric activation (ATP(iso)) were determined in human vastus lateralis (VL) muscle fibers expressing different myosin heavy chain (MHC) isoforms. We hypothesized that the reserve capacity for ATP consumption [1 -- (ratio of ATP(iso) to V(max) ATPase)] varies across VL muscle fibers expressing different MHC isoforms. Biopsies were obtained from 12 subjects (10 men and 2 women; age 21--66 yr). A quantitative histochemical procedure was used to measure V(max) ATPase. In permeabilized fibers, ATP(iso) was measured using an NADH-linked fluorometric procedure. The reserve capacity for ATP consumption was lower for fibers coexpressing MHC(2X) and MHC(2A) compared with fibers singularly expressing MHC(2A) and MHC(slow) (39 vs. 52 and 56%, respectively). Tension cost (ratio of ATP(iso) to generated force) also varied with fiber type, being highest in fibers coexpressing MHC(2X) and MHC(2A). We conclude that fiber-type differences in the reserve capacity for ATP consumption and tension cost reflect functional differences such as susceptibility to fatigue.

Adenosine Triphosphatases↗