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PubMed1990

Differences in coordination of elbow flexor muscles in force tasks and in movement tasks.

Motor-unit activity in m. biceps brachii, m. brachialis and m. brachioradialis during isometric contractions has been compared with motor-unit activity during slow voluntary (extension and flexion) movements made against external loads. During these slow movements the recruitment threshold of m. biceps motor units is considerably lower than it is during isometric contractions but the recruitment threshold of both m. brachialis and m. brachioradialis motor units is considerably higher. For all three elbow flexor muscles the motor-unit firing frequency seems to depend on the direction of movement: the firing frequency is higher during flexion movements (3 deg/s) and lower during extension movements (-3 deg/s) than during isometric contractions. The relative contribution of the biceps to the total exerted flexion torque during slow voluntary movements is estimated to increase from 36% to about 48% and that of the brachialis/brachioradialis is estimated to decrease from 57% to about 45% compared to the relative contribution of these muscles during isometric contractions. This difference in the relative contribution of the three major elbow flexor muscles is shown to be caused by differences in the central activation in force tasks and movement tasks.

Elbow↗
PubMed1991

Comparison of adenosine receptor agonists with other vasodilators on noradrenaline-, potassium- and phorbol ester-contracted rabbit aorta.

Isometric contractions of rabbit isolated aortic spirals were induced by noradrenaline (1 microM), potassium chloride (51 mM) or phorbol-12,13-dibutyrate (1 microM). A cumulative concentration-response curve for adenosine, N-ethylcarboxamidoadenosine (NECA), verapamil, sodium nitroprusside, isoprenaline or forskolin was then constructed. Sodium nitroprusside displayed selectivity towards noradrenaline- compared with potassium-contracted tissues. In contrast, verapamil selectively relaxed the potassium-contracted tissues. Adenosine preferentially relaxed the noradrenaline-contracted aorta. Adenosine, in the presence of the transport inhibitor dipyridamole (10 microM), and NECA were without effect upon potassium-contracted tissues. This absolute selectivity for noradrenaline-contracted aortas indicates that adenosine A2-receptor agonists do not interfere with calcium influx through voltage-operated channels but inhibit calcium influx via receptor-operated channels or its intracellular release. The inhibition by dipyridamole of a small relaxation by adenosine in potassium-contracted tissues indicates that this was due to stimulation of an additional intracellular site. The selectivity profile of adenosine and NECA was similar to that of forskolin and isoprenaline indicating that their relaxations were due to a common activation of adenylate cyclase and cAMP accumulation. Only sodium nitroprusside- and forskolin-relaxed aortas contracted with phorbol dibutyrate. Adenosine and NECA also failed to cause relaxation of noradrenaline-contracted aortas preincubated with the phorbol ester indicating that it may desensitize A2-receptors.

Animals↗
PubMed2003

Electromyographic fatigue threshold of erector spinae muscle induced by a muscular endurance test in health men.

PURPOSE: To identify the electromyographic fatigue threshold in the erector spinae muscle. METHODS: Eight 19 to 24-year-old male volunteers participated in this study, in which surface electrodes were used, as well as a biological signals acquisition module (Lynx) with a sampling frequency of 1000 Hz, a 1000 times gain, a 20 Hz high pass filter and a 500 Hz low pass filter. The test consisted of repeated isometric contractions of the erector spinae muscle in a 45 degrees hip flexion posture, with 30%, 40%, 50% and 60% of the maximum voluntary isometric contraction. RESULTS: A positive correlation of the RMS (root mean square) value as a function of time was found for most of the subjects with 40% (N = 6), 50% (N = 7) and 60% (N = 8) loads of the maximum voluntary isometric contraction. CONCLUSIONS: It was concluded, from this study, that the proposed protocol provides evidence, through the electromyographic signal, of the development of fatigue in the erector spinae muscle with loads of 40%, 50% and 60% of the maximum voluntary isometric contraction. The protocol also allows the electromyographic fatigue threshold and its probable applicability in the diagnosis of this phenomenon during repetitive activities to be determined.

Adult↗
PubMed1994

X-ray diffraction measurements of the extensibility of actin and myosin filaments in contracting muscle.

We have used a small angle scattering system assembled on the high flux multipole wiggler beam line at CHESS (Cornell) to make very accurate spacing measurements of certain meridional and layer-line reflections from contracting muscles. During isometric contraction, the actin 27.3 A reflection increases in spacing from its resting value by approximately 0.3%, and other actin reflections, including the 59 and 51 A off-meridional reflections, show corresponding changes in spacing. When tension is augmented or diminished by applying moderate speed length changes to a contracting muscle, changes in spacing in the range of 0.19-0.24% (when scaled to full isometric tension) can be seen. The larger difference between the resting and isometric spacings suggests either nonlinearity at low tension levels or the presence of a component related to activation itself. Myosin filaments also show similar increases in axial period during slow stretch, in addition to the well known larger change associated with activation. An actin spacing change of 0.25-0.3% can also be measured during a 2 ms time frame immediately after a quick release, showing that the elastic behavior is rapid. These observations of filament extensions totaling 2-3 nm per half-sarcomere may necessitate some significant revision of the interpretation of a number of mechanical experiments in muscle, in which it has usually been assumed that virtually all of the elasticity resides in the cross-bridges.

Actins↗
PubMed1985

An investigation of alpha-adrenoceptor responsiveness in the vas deferens of spontaneously hypertensive rats.

alpha-Adrenoceptor-mediated responses were investigated in isolated vasa deferentia from spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NWR). There was no significant difference between NWR and SHR in the inhibition of the isometric contraction to single pulse field stimulation by alpha 2-selective agonists in prostatic portions, nor by alpha 2-selective agonists and the alpha 1-selective agonist, amidephrine in epididymal portions in the presence of nifedipine to prevent postjunctional actions of alpha-1-selective agonists. There was no significant difference between NWR and SHR in the potency of amidephrine in causing a postjunctionally mediated potentiation of the isometric contraction to single pulse field stimulation in prostatic portions but the maximum potentiation was significantly reduced in SHR. However, the maximum potentiation of the isometric contraction by the calcium entry facilitator, Bay K 8644, was not significantly different between NWR and SHR. The maximum direct contraction to amidephrine, but not to Bay K 8644, was also significantly reduced in SHR. The irreversible alpha 1-adrenoceptor antagonist, phenoxybenzamine was more potent in SHR than NWR in reducing the maximum potentiation by amidephrine of the stimulation-evoked isometric contraction and in reducing the maximum direct contraction to amidephrine. It is concluded that there is a reduced postjunctional alpha 1-mediated responsiveness in the vas deferens of SHR due probably to a reduction in receptor number.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1998

Evoked isometric muscle contractions in myopathies: analysis of pathophysiological properties by different stimulus patterns.

Isometric twitches of the adductor pollicis muscle following ulnar nerve stimulation were investigated in healthy subjects (n = 35) and patients with different types of myopathies (myotonic dystrophy, n = 19; limb girdle muscular dystrophy, n = 10; metabolic myopathy, n = 6). The changes within the rising part (i.e. within the contraction time, CT) of the isometric twitches after single stimuli were similar in myotonic and limb girdle dystrophies: the first part of CT, which lasts until the maximal contraction rate is achieved, was shortened, whereas the following second part of CT, which lasts until the maximal twitch force is achieved, was normal. In metabolic myopathies the first part was normal, whereas the second part was prolonged. The relaxation was prolonged in all types of myopathies, particularly in metabolic myopathies. Using double stimuli with short interstimulus intervals (ISI), the absolute refractory period of the muscle contraction (healthy subjects: 1.35 +/- 0.16 ms) was shortened in patients with myotonic dystrophy (1.02 +/- 0.11 ms). In the other types of myopathies, the absolute refractory period was only shortened provided that the single twitch force was clearly reduced. A similar dependence on a reduced single twitch force was also found with regard to the maximal force development with two stimuli and the corresponding ISI: the force contributed by a second stimulus was pathologically enhanced if the single twitch force was clearly reduced. The ISI related to the maximal force with two stimuli was shifted towards very short values (healthy subjects: 10.5 ms, myotonic dystrophy: 4.6 ms, limb girdle dystrophies: 5.0 ms). Our results can be attributed to altered kinetics of calcium release and uptake by the sarcoplasmic reticulum in myopathies.

Adolescent↗
PubMed1993

Ligament forces at the knee during isometric quadriceps contractions.

A mathematical model of the knee in the sagittal plane was used to investigate the ligament forces resulting when a posteriorly directed external force, applied to the tibia, resists extension of the knee under increasing isometric quadriceps contractions. The model is based on simple geometric representations of the bones, ligaments and muscles at the knee. An elementary mechanical analysis was used to predict which ligament, the anterior or posterior cruciate, was loaded at a given flexion angle and known line of action of the external force. Ligament force, as a proportion of the external force, was calculated first assuming the ligaments to be represented by single, inextensible lines. Modelling the ligaments as continuous arrays of extensible fibres then showed that tibio-femoral translations and ligament forces increased non-linearly with increasing muscle forces and approached asymptotic values which depended on flexion angle. In most positions of the joint, the calculated asymptotic ligament force values were less than the reported ultimate strength of human ligament, despite quadriceps forces of over three times body weight. The possibility of these asymptotic values of ligament force may explain why, at certain flexion angles, large forces can be developed by the muscles at the knee without ligament rupture.

Biomechanical Phenomena↗
PubMed1994

The use of the relation between relative force and endurance time.

The relation between relative force (i.e., the actual force relative to the maximal voluntary force) and endurance time (i.e., the time the force can be maintained) is employed in a number of methods for ergonomics research. The aim is to assess the development of localized muscular fatigue as a consequence of isometric contractions and to compare activities regarding both the intensity and the duration of the isometric contractions exerted. The main assumption underlying this approach is that the relation is identical for different muscles and in different situations. Several anatomical and physiological factors contributing to variation in the relation are discussed and the consequences of this variation for the validity of the methods employing the relation are quantified. From this analysis, it appears, that the use of the relation between relative force and endurance time with the assumption mentioned leads to unreliable results. Therefore assessment of isometric contractions in labour situations will have to be based on fundamentally different methods.

Ergometry↗
PubMed2001

Saccadic reaction times during isometric voluntary contraction of the shoulder girdle elevators and vibration stimulation to the trapezius.

We investigated the effect of increases in muscle afferent information from the shoulder girdle elevators on saccadic reaction time. Saccadic reaction time was measured under conditions of isometric voluntary contraction of the shoulder girdle elevators and vibratory stimulation of the trapezius. Saccadic reaction time was defined as the latency until the beginning of eye movement toward the lateral target, which was moved at random time-intervals in jumps of 20 degress amplitude. Eye movement was measured using the electro-oculogram technique. Muscle contraction force was set in 10% increments from 0% to 30% of maximal voluntary contraction (MVC), and vibration frequency was maintained at 100 Hz. Under voluntary contraction, the saccadic reaction time gradually shortened up to 30% MVC. Under vibration stimulation at 0% MVC, the reaction time shortened to the same degree as that under voluntary contraction at 30% MVC. Under conditions of combined vibratory stimulation and voluntary contraction, the reaction time was essentially identical to these values; namely, no additive effect in shortening of the reaction time was recognized. The results demonstrated that saccadic reaction times were remarkably shortened by increases in muscle afferent information from the neck extensors. We have discussed reasons for the lack of an additive effect and factors limiting shortening of reaction times.

Adult↗
PubMed1994

X-ray diffraction evidence for the extensibility of actin and myosin filaments during muscle contraction.

To clarify the extensibility of thin actin and thick myosin filaments in muscle, we examined the spacings of actin and myosin filament-based reflections in x-ray diffraction patterns at high resolution during isometric contraction of frog skeletal muscles and steady lengthening of the active muscles using synchrotron radiation as an intense x-ray source and a storage phosphor plate as a high sensitivity, high resolution area detector. Spacing of the actin meridional reflection at approximately 1/2.7 nm-1, which corresponds to the axial rise per actin subunit in the thin filament, increased about 0.25% during isometric contraction of muscles at full overlap length of thick and thin filaments. The changes in muscles stretched to approximately half overlap of the filaments, when they were scaled linearly up to the full isometric tension, gave an increase of approximately 0.3%. Conversely, the spacing decreased by approximately 0.1% upon activation of muscles at nonoverlap length. Slow stretching of a contracting muscle increased tension and increased this spacing over the isometric contraction value. Scaled up to a 100% tension increase, this corresponds to a approximately 0.26% additional change, consistent with that of the initial isometric contraction. Taken together, the extensibility of the actin filament amounts to 3-4 nm of elongation when a muscle switches from relaxation to maximum isometric contraction. Axial spacings of the layer-line reflections at approximately 1/5.1 nm-1 and approximately 1/5.9 nm-1 corresponding to the pitches of the right- and left-handed genetic helices of the actin filament, showed similar changes to that of the meridional reflection during isometric contraction of muscles at full overlap. The spacing changes of these reflections, which also depend on the mechanical load on the muscle, indicate that elongation is accompanied by slight changes of the actin helical structure possibly because of the axial force exerted by the actomyosin cross-bridges. Additional small spacing changes of the myosin meridional reflections during length changes applied to contracting muscles represented an increase of approximately 0.26% (scaled up to a 100% tension increase) in the myosin periodicity, suggesting that such spacing changes correspond to a tension-related extension of the myosin filaments. Elongation of the myosin filament backbone amounts to approximately 2.1 nm per half sarcomere. The results indicate that a large part (approximately 70%) of the sarcomere compliance of an active muscle is caused by the extensibility of the actin and myosin filaments; 42% of the compliance resides in the actin filaments, and 27% of it is in the myosin filaments.

Actins↗
PubMed1984

Regulation of creatine kinase during steady-state isometric twitch contraction in rat skeletal muscle.

In vivo 31P-NMR saturation transfer measurements of the creatine kinase exchange flux in the direction creatine phosphate----ATP were made in the gastrocnemius muscle of rats at rest and during steady-state isometric twitch contraction at frequencies from 0.25 to 2 Hz. There was no correlation between creatine kinase exchange flux and either free [ADP] or oxygen consumption, both of which increase with stimulation frequency. The flux was found to be nearly constant over all conditions at about 16 mM X s-1, 10-times greater than the highest estimated ATP turnover in this study. The kinetic properties of skeletal muscle creatine kinase in vivo are similar to, but not completely predictable from, the equilibrium exchange fluxes measured on the isolated enzyme. These results are not consistent with strong functional coupling between ATP synthesis and mitochondrial creatine kinase.

Animals↗
PubMed1987

Calcium and contractions of isolated smooth muscle cells from rat myometrium.

Smooth muscle cells were isolated from estrogenized rat myometrium by collagenase digestion. Electron microscopic examination and measurement of cell lengths by image-splitting micrometry were carried out after fixation with acrolein. Mean lengths of cells before and after isolation were 81.7 and 66.9 micron, respectively. Responses of cells were compared with contractions of isolated strips recorded isometrically. Effects of carbachol and KCl were examined in 2 mM Ca, 2 mM Ca + 4 mM EGTA, and 2 mM Ca + 10(-8) M nitrendipine solution. Carbachol and KCl produced concentration-dependent shortening of isolated cells maximal at 30 s after addition. The concentrations of carbachol required to produce shortenings were about 100-fold less than those required to produce isometric contractions; but no major difference was observed in the concentration dependence of cell shortening and isometric contraction produced by potassium-induced depolarization. In 2 mM Ca solution, there was a phasic response, followed by a tonic response such that more than 50% of maximum cell shortening was maintained for 10 min. However, in 2 mM Ca + 4 mM EGTA or 10(-8) M nitrendipine, the tonic contraction was abolished and cells rapidly relaxed after 30 s. If carbachol was added to cells after varying times in the EGTA-containing solution, the ability to initiate a contraction declined exponentially with a half-time of 160 s. Effects of depolarization by KCl were examined in 2 mM Ca plus nitrendipine and 2 mM Ca + 4 mM EGTA solution. Shortening occurred in 2 mM Ca solution by depolarization but not if nitrendipine was added. Though shortening was not observed in 2 mM Ca + 4 mM EGTA solution by KCl, subsequent addition of carbachol induced shortening. These results suggested that there was an intracellular Ca store site from which Ca was released by carbachol and which was not affected by depolarization in the absence of external Ca. No evidence was obtained that the contraction persists in Ca2+-free medium in isolated cells, which is in agreement with previous findings in small muscle strips in which only a similar transient response was obtained.

Animals↗
PubMed2000

Perceived exertion during isometric quadriceps contraction. A comparison between men and women.

BACKGROUND: 1) To examine the validity and accuracy of the CR-10 scale for evaluating perceived exertion, and 2) to assess gender differences in perceived exertion across different levels of contraction intensity. METHODS EXPERIMENTAL DESIGN: cross-sectional, comparative design. SETTING: Human Performance and Fatigue Laboratory, Eastern Washington University. SUBJECTS: 30 healthy, college age volunteers (15 males, 15 females). MEASURES: All subjects were assessed for isometric torque and perceived exertion of the quadriceps femoris muscles, via the CR-10 scale. One low anchor was applied under resting conditions with the knee flexed to 60 degrees, and a high anchor was applied during a maximal voluntary muscle contraction (MVC). SUBJECTS performed five-second isometric contractions equivalent to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% of their MVC, in a random order, and were assessed for perceived exertion by visually observing the CR-10 scale. One sample "t"-tests and 95% confidence intervals were calculated for perceived exertion at each relative torque level. A single factor ANOVA with repeated measures was performed across al levels of exercise intensity. Linearity for perceived exertion was assessed via regression analysis. RESULTS: Perceived exertion at each exercise intensity were as follows: 10%: 1.87+/-1.14, 20%: 2.43+/-1.19, 30%: 3.5+/-1.36, 40%: 3.97+/-1.52, 50%: 4.73+/-1.28, 60%: 5.53+/-1.28, 70%: 6.73+/-1.62, 80%: 7.57+/-1.72, and 90%: 8.6+/-1.52. The increase in perceived exertion across the intensity spectrum was found to fit both linear and quadratic trends. There were no gender differences in perceived exertion across all levels of exercise intensity. CONCLUSIONS: The findings demonstrate that the CR-10 scale closely approximates perceived exertion of the quadriceps femoris muscles during sub-maximal, static contractions, and is not gender specific.

Cross-Sectional Studies↗
PubMed2001

Blood pressure and heart rate response to isometric exercise: the effect of spinal cord injury in humans.

The systolic (SP) and diastolic (DP) blood pressure and heart rate (HR) responses during a fatiguing isometric contraction of either the handgrip or quadriceps muscles was measured in 139 paraplegic patients divided into five groups (having injuries at T4, T12 and L1-L3 and suffering complete paraplegia and having injuries at T4 but showing incomplete hemiplegia or paraplegia) and 25 controls. While all handgrip exercise was maintained by voluntary effort, only the controls were able to make a voluntary contraction of their quadriceps muscles; the paraplegics had isometric contractions of their quadriceps muscles elicited by functional electrical stimulation (FES). The endurance of the handgrip exercise was not significantly different among the controls and any one of the five experimental groups (control group endurance = 151 s, average paraplegic group endurance = 141 s) while it was higher in the leg muscles (P < 0.05 controls 133 s, paraplegics 111 s). All subjects showed a linear increase in SP and DP throughout the duration of the handgrip exercise, pressure increasing from rest to fatigue by 55-56 mmHg. The controls, subjects with a complete and incomplete spinal cord injuries at T4 showed similar SP and DP increases during leg exercise, both the SP and DP response was eliminated in the T12 and L1-L3 injury groups. The HR increased during fatiguing isometric handgrip contractions from an average resting value of 82 to a maximum of 121 beats x min(-1) in both the controls and paraplegic subjects. However, while the HR increased to a slightly higher level of 127 beats x min(-1) at the end of the contraction of the leg muscles in the controls, the increase in HR during FES exercise was modest, increasing by an average of only 6 beats x min(-1) in the paraplegic group.

Adult↗
PubMed1989

Effects of opiates during baroreceptor and ergoreceptor induced changes in blood pressure.

Various opioids were used to investigate the role they might play in the cardiovascular responses to fatiguing isometric contractions. Changes in blood pressure were measured in cats anaesthetised with alpha-chloralose. Fatiguing isometric contractions of the hind limb muscles (ergoreceptor activation) were generated using a microprocessor controlled stimulator (50 Hz, 0.2 ms, 200-800 mV). Baroreceptor inactivation was elicited by carotid artery occlusion. Muscle contraction caused an increase in mean arterial pressure of 51 (SEM 12) mm Hg and carotid occlusion an increase of 56(9) mm Hg above resting levels in control conditions. Injection of dynorphin (0.5-5.0 micrograms.5 microliters-1) into the cerebral aqueduct just rostral to the 4th ventricle eliminated the pressor response to muscular contraction (mean arterial pressure at rest, 80-118 mm Hg: on fatigue, 72-129 mm Hg) but did not affect the pressor response to carotid occlusion in the same cats. Similarly, injections of met-enkephalin (1-100 micrograms.5 microliters-1) or beta-endorphin (10-100 micrograms.5 microliters-1) eliminated the ergoreceptor induced changes in mean arterial pressure during isometric contractions but had no effect on the changes caused by carotid occlusion. Pressor responses to nerve crush were not eliminated. These results support the suggestion that a catecholaminergic-opioidergic pathway in part mediates the cardiovascular responses to ergoreceptor afferent but probably not baroreceptor afferent input.

Animals↗
PubMed1994

Effects of myofascial release leg pull and sagittal plane isometric contract-relax techniques on passive straight-leg raise angle.

Experimental evidence does not currently exist to support the claims of clinical effectiveness for myofascial release techniques. This presents an obvious need to document the effects of myofascial release. The purpose of this study was to compare the effects of two techniques, sagittal plane isometric contract-relax and myofascial release leg pull for increasing hip flexion range of motion (ROM) as measured by the angle of passive straight-leg raise. Seventy-five nondisabled, female subjects 18-29 years of age were randomly assigned to contract-relax, leg pull, or control groups. Pretest hip flexion ROM was measured for each subject's right hip with a passive straight-leg raise test using a fluid-filled goniometer. Subjects in the treatment groups received either contract-relax or leg pull treatment applied to the right lower extremity; subjects in the control group remained supine quietly for 5 minutes. Following treatment, posttest straight-leg raise measurements were performed. A one-way analysis of variance followed by a Newman-Keuls post hoc comparison of mean gain scores showed that subjects receiving contract-relax treatment increased their ROM significantly more than those who received leg pull treatment, and the increase in ROM of subjects in both treatment groups was significantly higher than those of the control group. The results suggest that while both contract-relax and leg pull techniques can significantly increase hip flexion ROM in normal subjects, contract-relax treatment may be more effective and efficient than leg pull treatment.

Adolescent↗
PubMed1970

Energy production in cardiac isotonic contractions.

The energy output of rabbit papillary muscle is examined and it is shown that there is more energy liberated in an afterloaded isotonic contraction than in an "equivalent" isometric contraction. This statement holds true regardless of whether equivalence is based on the proposition that tension or the time integral of tension is the best index of muscle energy expenditure. Besides the external work performed there is additional heat production in isotonic contractions and this heat increases as the afterload is decreased. The additional heat is more evident when tension rather than the time integral of tension is made the determinant of energy expenditure. It is shown in single contractions that the rate of isotonic heat production, regardless of afterload size, never exceeds the heat rate recorded in an isometric contraction at the same initial length. Experiments reveal no simple linear correlation between isotonic energy output and contractile element work. Problems associated with the compartmentalization of the energy output of a contraction are discussed.

Analysis of Variance↗