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Interactions of 5'-N-ethylcarboxamide adenosine (NECA), aminophylline and dipropyl-phenyl-xanthine (XAC) on the isolated guinea-pig atria.

5'-N-Ethylcarboxamide adenosine (NECA) (2.2-22 nmol l-1) produced a concentration-dependent depression of both the isometric contraction and the atrial rate of the isolated, spontaneously beating guinea-pig atria. In the presence of a standard concentration of verapamil (73 nmol l-1), the dose-response curves for NECA, both for the isometric contraction and the atrial rate, were significantly shifted to the left. The inhibitory effect of NECA on the atria was almost completely reversed by the addition of calcium chloride (1.8 mmol l-1) into the organ bath. The depressive actions of NECA on the isolated atria were also antagonized by aminophylline (32 mumol l-1) and isoprenaline (24 nmol l-1). In the presence of XAC (Xanthine Amine Congener) (0.5 mumol l-1) the dose-response curve for the effect of NECA on the isometric contraction was significantly shifted to the right. In the concentration used, XAC produced no changes either in the isometric contraction or in the atrial rate. These experiments indicate that NECA may act on the adenosine A1-receptors present in the guinea-pig atria because its effects on the atria are antagonized by XAC, a more selective A1-receptor antagonist. It is concluded that NECA might act through depression of calcium channel function in the heart atria, and that the action of NECA most probably implicates A1-receptors.

Adenosine↗
PubMed1998

Z/I and A-band lattice spacings in frog skeletal muscle: effects of contraction and osmolarity.

A-band and Z-line/I-band lattice spacings were measured by small-angle X-ray diffraction from relaxed and isometrically-contracting whole frog sartorius muscles with lattice spacings reduced or swollen by changing the osmolarity of the bathing solution. A-band spacing increased by approximately 3% upon isometric contraction at reduced lattice spacings (245-356 mOsm) and decreased by approximately 1% at swollen spacings (172 mOsm), similarly to the behaviour of skinned muscles upon changing from the relaxed state to rigor. The Z/I lattice underwent a significant lattice expansion (3-8%) upon isometric contraction at all osmolarities, in qualitative agreement (but quantitative disagreement) with results from electron microscopy on mammalian skeletal muscle. Lattice areas calculated for the Z/I and A-band lattices indicate a barrel-shaped sarcomere in the resting state, which may provide a partial explanation for how longitudinal forces produced in the A-band can produce a radial expansive force in the Z-line during contraction. The radial component of cross-bridge stiffness was calculated from the A-band data for contracting muscle, using a lattice stability model incorporating structural, osmotic and electrostatic forces. The calculations gave a radial cross-bridge stiffness during contraction of about 9 x 10(5) N m-2, and outward radial force per thick filament in normal Ringer's solution of 6 x 10(-9) N, corresponding to a radial force per cross-bridge of 10(-11) N.

Animals↗
PubMed2005

Force control and disease severity in Parkinson's disease.

Several measures of isometric contractions reflect motor impairments in subjects with Parkinson's disease (PD), including long relaxation times and greater power in the 5 to 15 Hz electromyographic (EMG) bandwidth during the holding phase of contractions compared to those measures in healthy subjects. We sought to determine whether the impairments observed in subjects with PD in the performance of isometric contractions reflect disease severity. Twenty-eight subjects with PD performed isometric contractions at a torque level equal to 50% of the torque generated during a maximum voluntary contraction while off medication. Subjects were instructed to reach the target torque as fast as possible upon hearing the auditory "go" signal and to relax their muscles when a second auditory cue signaled the end of the hold phase. There was a significant positive correlation between torque relaxation time and Unified Parkinson's Disease Rating Scale (UPDRS)-Motor score. A significant positive correlation was also observed between the proportion of power in the 5 to 15 Hz frequency bin of the agonist EMG signal and UPDRS-Motor score, and a significant negative correlation between the proportion of power in the 15 to 30 Hz frequency bin and UPDRS-Motor score. These measures provide objective quantification of the severity of motor impairment that can be used to investigate the efficacy of different interventions in individuals with PD.

Acoustic Stimulation↗
PubMed1985

The effects of two forms of isometric training on the mechanical properties of the triceps surae in man.

The training effects of rhythmic and sustained isometric contractions on the contractile characteristics of the triceps surae have been investigated in four healthy subjects over a period of 8 weeks. One leg (ST) of each subject was trained by performing repeated daily sustained (1 min) isometric contractions at 30% of maximal voluntary contraction force (MVC), and the other using rhythmic isometric contractions at 100% of MVC. The protocol was so arranged that the total area under the training force/time curves was the same for each limb. Electrically evoked maximal twitch (Pt) and tetanic (Po) tensions of the triceps surae were measured weekly on both legs using a standard procedure. The results showed that the rhythmic regime increased the MVC at the rate of 5.5% per week and the sustained training increased MVC at the rate of 3.3% per week. Twitch and tetanic tensions were not altered by either regime. However, only training using sustained contractions produced a progressive increase in endurance as measured by performance in a fatigue test. The results suggest that increases in voluntary strength resulting from short term isometric training are not necessarily associated with a rise in the intrinsic involuntary capacity of muscle fibres to generate force.

Adult↗
PubMed2003

Neural adaptations with sport-specific resistance training in highly skilled athletes.

The aim of this study was to assess the effects of variations in the volume and intensity of resistance training in highly skilled athletes on neural adaptive mechanisms: the maximality and pattern of neural drive. The maximality of muscle activation was measured using a high-resolution sample and hold amplifier to record interpolated twitches. The pattern of neural drive was measured by analysing isometric torque-time curves and electromyographic (EMG) characteristics during the performance of rapid isometric contractions at maximal effort. The volume and intensity of training were varied at 4-weekly intervals to systematically emphasize the development of strength, power and motor performance in 14 highly skilled track and field athletes (e.g. discus, hammer, javelin, shot put and weight). Knee extension strength increased significantly by 15% during steady maximal isometric contractions and by 24% during rapid isometric contractions at maximal effort after the 16-week training programme (P < 0.05). Increases in EMG amplitude and rate of EMG activation indicated that improvements to the pattern of neural drive occurred with sport-specific resistance training (P < 0.05). The maximality and pattern of neural drive did not change in the control group.

Adaptation, Physiological↗
PubMed1999

Reduced short and long latency reflexes during voluntary tracking movement of the human wrist joint.

In six healthy human subjects we compared changes in the strength of Hoffmann (H), short latency (30-55 ms) and long latency (55-100 ms) stretch reflexes of flexor carpi radialis (FCR) muscle during movement and isometric contractions. In one set of experiments, stretches were imposed to the wrist during voluntarily tracked sinusoidal movement and during matched isometric contractions to compare short and long latency stretch reflex responses. In the second set, H-reflexes were compared during similar matched conditions. All reflexes decreased significantly (P < 0.05) during the voluntary tracking movement. The H-reflex was reduced during the wrist flexion, on average, by 33% of its value obtained during the isometric condition. Compared with their values during isometric conditions, the short latency stretch reflex and long latency stretch reflex during movement were reduced by 52 and 40%, respectively. From the pattern changes of the stretch reflexes and the H-reflex, a movement-induced presynaptic inhibition combined with pronounced muscle spindle unloading is proposed to play an important role in decreasing the strength of the stretch reflexes during the tracking task as compared with a matched isometric contraction.

Adult↗
PubMed2001

Training-related changes in the maximal rate of torque development and EMG activity.

This study monitored the effects of a short-term elbow flexor training program on surface electromyographic (SEMG) spike activity. The experimental paradigm consisted of three test sessions separated by 2-week intervals. At the beginning of each session, participants (N=13) performed five maximal effort isometric contractions of the elbow flexors to serve as baseline. After 5 min of rest, the participants then engaged in a 30-trial isometric fatigue protocol during which maximal elbow flexion torque was measured with a load-cell, and the maximal rate of change in the torque (dtau/dt(max)) was obtained from the differentiated torque-time curve. Bipolar electrodes were used to monitor the SEMG spike activity of the biceps brachii. Mean spike amplitude (MSA) and mean spike frequency (MSF) were calculated for the torque development and constant-torque phases of the isometric contraction, termed Segment 1 and Segment 2, respectively. Mean power frequency (MPF) was also calculated for Segment 2. The five baseline contractions of the second and third sessions were compared with those of the first session and analyzed for training-related changes. Training increased dtau/dt(max) but failed to change maximal elbow flexion torque or MSA. However, there was an increase in the MSF during the torque development phase of the contraction (Segment 1). Both MSA and MSF were greatest during the constant-torque phase of the isometric contraction (Segment 2). There was a strong linear correlation (r=0.90, P<0.05) between MSF and MPF during (Segment 2). We hypothesize that the increase in dtau/dt(max) is due to enhanced motor-unit rate-coding. The demonstrated correlation between MSF and MPF measures will allow investigators to use spike analysis to examine the frequency content of the SEMG signal under non-stationary conditions.

Biomechanical Phenomena↗
PubMed2000

The organization of quick corrections within a two-joint synergy in conditions of unexpected blocking and release of a fast movement.

OBJECTIVE: Within this study, we tested a hypothesis that common organization of central commands to the elbow and the wrist joints within a two-joint synergy is associated with a similar organization of pre-programmed corrections to unexpected changes in the external conditions. DESIGN AND METHODS: The subjects (n = 7) performed series of very fast movements or isometric contractions against a pad at the level of the palm or at the level of the forearm. Some trials within a series of movements were unexpectedly blocked at the initial position leading to an isometric contraction, while some trials within a series of isometric contractions were unexpectedly released leading to a movement. Movement kinematic and electromyographic (EMG) patterns were analyzed. RESULTS: In cases of unexpectedly blocked or unexpectedly released trials, differences in the EMG patterns between perturbed and unperturbed trials were seen at latencies between 50 and 80 ms. Two patterns were observed representing interindividual differences among subjects. One included reciprocal changes in the flexor-extensor pairs controlling both joints. The other pattern included reciprocal changes in the wrist flexor-extensor pair and unidirectional changes in the EMGs of biceps and triceps. The patterns were reproducible within each individual subject across tasks and conditions. In particular, the same pattern in the wrist flexor-extensor pair was seen when the pad was applied to the palm and when it was applied to the forearm, despite the fact that early wrist joint deviations were in opposite directions. CONCLUSIONS: It is argued that the observed early EMG changes may be unrelated to local joint kinematics and stretch reflexes, and represent consequences of control patterns for fast corrective movements that are organized with respect to the motion of the endpoint of the limb similarly to the organization of voluntary movements. Within this framework, the organization of joints of a multi-joint limb into a synergy implies a simultaneous, automatic organization of pre-programmed reactions into a similarly organized synergy.

Adult↗
PubMed1998

Reliability of spectral EMG parameters of healthy back extensors during submaximum isometric fatiguing contractions and recovery.

Spectral EMG parameters are being used as an objective evaluation of low back rehabilitation programs. The reliability of these spectral parameters is important in determining the validity of this evaluation tool. Two groups of eight subjects, with no history of back pain, were measured: the first group every day for one week; the second group one day per week for four weeks. During each session, subjects performed a 30-second isometric fatiguing contraction of the back extensors at 60% MVC followed by a 60-second rest and a 10-second repeat contraction. The intraclass correlation coefficient (ICC) did not demonstrate high reliability (ICC < 0.6) for the rate of median power frequency (MedPF) change (Hz/s), the magnitude of the recovery (Hz) and differences between the dominant and the non-dominant sides within a subject. However, the initial MedPF (Hz) of the fatigue and repeat contractions demonstrated excellent reliability (ICC > 0.8) with five or more repeated measures. The practical implication of this work is that the experimental condition must have sufficiently large changes in MedPF (signal)--at least larger than the variability inherent in the MedPF (noise)--to constitute a valid measure.

Adult↗
PubMed2002

Differences in activation patterns between eccentric and concentric quadriceps contractions.

Previous studies analysing electromyograms (EMGs) from indwelling electrodes have indicated that fast-twitch motor units are selectively recruited for low-intensity eccentric contractions. The aim of this study was to compare the frequency content of surface EMGs from quadriceps muscles during eccentric and concentric contractions at various contraction intensities. Electromyograms were recorded from the rectus femoris, vastus lateralis and vastus medialis muscles of 10 men during isokinetic (1.05 rad x s(-1)) eccentric and concentric knee extension contractions at 25%, 50%, 75% and 100% of maximal voluntary contraction (MVC) for each contraction mode. Additionally, isometric contractions (70 degrees) were performed at each intensity. The mean frequency and root mean square (RMS) of the surface EMG were computed. Mean frequency was higher for eccentric than concentric contractions at 25% (P < 0.01), 50% (P < 0.01) and 75% (P < 0.05) but not at 100% MVC. It increased with increasing contraction intensity for isometric (P < 0.001) and concentric (P < 0.01) contractions but not for eccentric contractions (P = 0.27). The EMG amplitude (RMS) increased with increasing contraction intensity similarly in each contraction mode (P < 0.0001). Higher mean frequencies for eccentric than concentric contractions at submaximal contraction intensities is consistent with more fast-twitch motor units being active during eccentric contractions.

Adult↗
PubMed2007

Force fluctuations during the Maximum Isometric Voluntary Contraction of the quadriceps femoris in haemophilic patients.

In the general population, the degenerative processes in joints are directly related to adult age, and osteoarthrosis represents the most frequent musculoskeletal alteration. In the haemophilic patient, the degenerative processes in the joint begin at very early ages, and are directly related to musculoskeletal bleeding episodes, which are occasionally subclinical and constitute haemophilic arthropathy. In the haemophilic patient, arthropathy constitutes the most frequent, severe and disabling pathology, and its assessment includes muscular force-related parameters. We have studied the value of Maximum Isometric Voluntary Contraction in the quadriceps femoris of 46 subjects, 28 haemophiliacs (16 severe, eight moderate and four mild) and 18 healthy individuals with a view to establishing appropriate values of force and to restoring physical therapy recommendations. The maximum force values were significantly greater (P < 0.001) in the healthy individuals group. The mild haemophiliacs group also presented significant differences of force (P < 0.05) in relation to the severe and moderate haemophilic patient groups. The mild and severe haemophilia patients presented greater fluctuations of force (P < 0.001) than the control group, the haemophilia group have a minor skill to produce constant force. The seriousness of the arthropathy in the knee is directly related to diminished values of maximum force. Our work evidences that patients with severe haemophilia present a greater degree of arthropathy in relation to moderate and mild haemophilia patients. Haemophilic arthropathy is associated with muscular atrophy and strength deficit. In haemophilic patients, the deficit of maximum force and the presence of fluctuations may suggest an increased risk of bleeding during physical activities and the need to programme specific physical therapy guidelines which increase muscular power through resistance training.

Adolescent↗
PubMed1975

Very high tension with very little ATP breakdown by active skeletal muscle.

The tension developed and the chemical change occurring during stretching at velocities up to 2l0 sec-1 and during isometric contraction of dinitrofluorobenzene-treated and untreated frog sartorius muscle have been measured. At all velocities of stretching the tension was greater than during shortening at the same velocity and the mean rate of chemical change was less than or equal to the rates found in similar studies of shortening. For velocities 0.18, 0.33 and 0.66l0 sec-1, the mean rate of chemical change was significantly less (P less than 0.05) than during shortening at the same velocity. At the highest velocity of stretching the tension at l0 was about 0.8 P0 and the mean rate of chemical change was about the same as that for isometric contraction. At low velocity (0.1 l0 sec-1) the tension was as high as 1.4 P0 and the mean rate of chemical change during the stretches at the three slowest velocities was significantly less (P less than 0.05) than during isometric contraction; the lowest mean rate of chemical change amounted to only about 25% of that for isometric contraction. The fact that the chemical change was so low suggests that during stretching crossbridges can form links and develop tension without breaking down ATP.

Adenosine Triphosphate↗
PubMed1998

Reproducibility of electromyographic recordings of submaximal concentric and eccentric muscle contractions in humans.

OBJECTIVES: to determine the intratester and intertester reliability of measures of electromyographic activity (EMG) of submaximal concentric and eccentric contractions and to compare the reliability of normalized and non-normalized measures of EMG. There were 10 subjects, of 22-33 years old. METHODS: Subjects performed submaximal concentric and eccentric contractions at 60 degrees/s, and maximal isometric contractions (MIC) of their knee extensors. The target power of the submaximal contractions was 90%+/-10% of the subject's maximal concentric power. EMG was recorded via bipolar surface electrodes from 3 of the quadriceps femoris muscles. The rmsEMG for submaximal contractions that were within the target power range were determined. The rmsEMG for the submaximal contractions were normalized to the rmsEMG of the maximal isometric contractions. Intraclass correlation coefficients (ICC version 1.1) were calculated to determine intratester and intertester reliability. RESULTS: For non-normalized rmsEMG, ICC values for intratester reliability ranged from 0.62 to 0.91 for concentric and from 0.84 to 0.97 for eccentric contractions. ICC values for intertester reliability ranged from 0.66 to 0.96 for concentric and 0.78 to 0.90 for eccentric contractions. CONCLUSIONS: Non-normalized rmsEMG of submaximal concentric and eccentric isokinetic contractions were found to be reliable. Normalization did not lead to consistently improved reliability.

Adult↗
PubMed1992

Interlimb coordination in Parkinson's disease.

This study examined the degree to which Parkinson's disease (PD) patients could "spatially link" the upper limbs to facilitate the performance of bimanual simultaneous movements. Six right-handed PD patients, and seven normal age- and sex-matched controls performed three different tasks: (a) an isotonic elbow flexion as rapidly as possible through an angle of 30 degrees; (b) an isometric contraction of the flexor muscles at the elbow joint to 40% and 60% of maximal volitional force (MVF) for a period of 5 s; (c) an isometric contraction for 2.5 s with one limb, then simultaneously performing an isotonic flexion with the contralateral limb while maintaining the isometric contraction for 2.5 s more. As expected, PD patients were significantly slower in performing the isotonic movement and produced lower peak velocities than the controls. More importantly, the two groups were differentially affected during the bimanual condition. In normals, movement time decreased and peak velocity increased in the bimanual condition. In contrast, PD patients showed increased movement times and sometimes decreased peak velocities in the bimanual condition. The results suggest that normal subjects utilize bilateral outflow to symmetrical muscle groups to synchronize the two limbs in the bimanual task, whereas PD patients dissociate the two limbs.

Aged↗
PubMed1998

Changes in fusimotor activity during repetitive lengthening muscle contractions in decerebrate cats.

Responses of fusimotor neurons to lengthening vs isometric contractions have been studied in decerebrate cats. Spike discharges of fusimotor neurons to the medial gastrocnemius muscle were recorded from this muscle nerve filament during sequences of contractions and/or stretches of the lateral gastrocnemius and soleus muscles. The sequences lasted for 250-450s (duty cycle 4:2 s). Isometric contractions were elicited by electrical stimulation (40 Hz, 1.3 times motor threshold) of the muscle nerves. Lengthening contractions were elicited in the same way while the muscles were stretched by 4 mm at a velocity of 1 mm/s. Of 25 fusimotor neurons studied, 23 responded to muscle contractions with an increase in firing rate, subsiding towards the end of the sequence. The increase was either modulated with each subsequent contraction or smooth throughout the sequence. Approximately 64% of fusimotor neurons, responding to muscle contractions, responded in a similar way to the sequences of muscle stretches, applied alone. Responses to sequences of the lengthening contractions were significantly larger, on average, than those to the isometric ones, but smaller than the sum of the responses to the contractions and stretches applied separately. On the other hand, they were also larger in fusimotor units, showing no overt responses to muscle stretches alone.

Animals↗
PubMed1990

Identification of a frequency response model of joint rotation.

A second order, linear oscillator transfer function model is fit to the measured transfer function relating the abduction-adduction rotation of the first finger to the applied moment. Nearly constant isometric contractions of the first palmar and dorsal interossei are maintained by the subjects during the measurements. The stiffness and damping components of the identified models increase significantly with increasing isometric contraction when compared to those recorded under relaxed contraction. Muscle fatigue causes the natural frequency, damping ratio and stiffness of the joint rotation to decrease under full isometric contraction, and it causes the natural frequency and stiffness to increase when the muscles are relaxed.

Action Potentials↗