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PubMed1997

Influence of posture and training on the endurance time of a low-level isometric contraction.

Classically, the critical force of a muscle (the relative force below which an isometric contraction can be maintained for a very long time without fatigue) is comprised of between 15 and 20% of its maximum voluntary contraction (MVC). However, some authors believe that the value is below 10% MVC. If such is the case, signs that accompany the establishment of muscle fatigue (EMG changes, continuous increase in systolic blood pressure [SBP] and heart rate [HR]) would have to appear more rapidly and with a higher intensity if the muscle is already partially fatigued at the start of maintaining a contraction at 10% MVC. Twelve healthy untrained participants carried out two isometric contractions with the digit flexors: one (test A) began with a maximum contraction sustained for 4 min followed without interruption by a contraction at 10% MVC for 61 min; the other (test B) was a contraction maintained at 10% MVC for 65 min. For test B, after an initial increase of 4 bpm with respect to at rest, HR remained stable until the end of contraction, SBP progressively increased by 24 mm Hg in 28 min, then remained unchanged until the end, and there were no significant changes in EMG (absence of spectral deviation towards low frequencies). For test A, in spite of the initial maximum contraction, changes in the parameters being studied (total maintenance time, HR, SBP, EMG) during maintenance at 10% MVC were identical to those for test B. The results show that (1) when the number and intensity of the co-contractions are minimized by applying an appropriate posture, it is possible to sustain an isometric contraction at 10% MVC for at least 65 min without the appearance of signs of muscle fatigue; (2) the critical force of the digit flexors is higher than 10% MVC.

Adult↗
PubMed1987

Human motor-unit recruitment during isometric contractions and repeated dynamic movements.

Spike-triggered averaging was used to determine the twitch tensions and contraction times of motor units in the abductor pollicis brevis muscle of two human subjects for two directions of isometric contraction: abduction and opposition of the thumb. During isometric contractions in each direction, the threshold force for motor-unit recruitment and the twitch amplitude were correlated linearly. These data suggested that an orderly pattern of recruitment, according to increasing twitch size, describes the function of the human abductor pollicis brevis muscle for each contraction direction. Rank order of motor-unit recruitment in each isometric contraction direction was correlated, but not identical. All units contributed tension in each direction of contraction, so no clear evidence was found for task-dependent motor units. In two subjects, motor-unit recruitment order during isometric contraction of the first dorsal interosseous and abductor pollicis brevis muscles was then compared with that of motor-unit pairs in both muscles during repetitive dynamic movements. Recruitment according to increasing twitch size was largely preserved during the repetitive opening and closing of scissors. The recruitment reversals that were observed were usually between pairs of units with similar thresholds.

Electromyography↗
PubMed1992

Agonist-induced myosin phosphorylation during isometric contraction and unloaded shortening in airway smooth muscle.

We measured myosin phosphorylation during isometric contraction at optimal length (Lo) and unloaded shortening induced by K(+)-depolarization, electrical stimulation, carbachol, histamine, and phorbol dibutyrate (PDB) in bovine trachealis. Peak myosin phosphorylation during unloaded shortening was lower than that during isometric contraction in response to all stimuli. The lower peak myosin phosphorylation during unloaded shortening appeared to be a stretch-sensitive response because myosin phosphorylation was either equally low or further reduced during the second unloaded shortening of preshortened tissues. Similar to peak myosin phosphorylation, steady-state myosin phosphorylation was also lower during unloaded shortening in carbachol-induced contractions. However, steady-state phosphorylation during unloaded shortening and isometric contraction were not significantly different in histamine- and PDB-induced contractions. Since the coupling between Ca2+ and myosin phosphorylation was not stretch sensitive, these results suggest the coexistence of stretch-sensitive and stretch-insensitive signal transduction mechanisms in the airway smooth muscle cell membrane, and the stretch-insensitive signal transduction mechanism might involve protein phosphorylation by protein kinase C.

Animals↗
PubMed1992

Modulation of somatosensory evoked potentials during isometric contraction in relation to hand use.

The aim of the study was to assess the effect of the hand used on the modulation of somatosensory evoked potentials (SEPs) during isometric contraction. SEPs to median nerve stimulation were assessed during dynamic (increase and decrease of force) and hold phases of isometric contractions. The experiments were carried out with the right or left hand. The changes in the SEP amplitude were found to vary with the hand used. When isometric contractions were performed by the right hand the amplitudes of the later components diminished during the dynamic phases, except for the component N140-P190 which increased. The amplitude of N55-P100 diminished during both the dynamic and hold phases. When isometric contractions were performed by the left hand the amplitudes of the early components diminished during the dynamic phases. These results show that the changes in SEP during isometric contraction depend on the hand used.

Adult↗
PubMedDate not supplied

Electromyographic analysis of movement following a maximal isometric contraction.

A post-contraction sensory discharge effect following an isometric contraction is commonly cited as the explanation for perception of movement or distortion of limb proprioception during post-contraction movements. The purpose of this study was to compare motor output of agonist neuromuscular activity between a control positioning movement and a post-contraction movement. Terminal positioning accuracy and electromyographic (EMG) data of the triceps brachii during rapid elbow extension were recorded during normal movements and during movements following a maximal isometric contraction. Separate t-tests revealed the post-contraction positioning resulted in significant undershooting of the target position, and also the integrated EMG (IEMG) for the post-contraction conditions was significantly less than the control condition. No difference was observed in mean peak frequencies between the control and post-contraction conditions. Apparently, the post-contraction target undershooting is a result of less total motor output rather than a change in neuromuscular frequency firing rates.

Adolescent↗
PubMed1980

The influence of temperature on the amplitude and frequency components of the EMG during brief and sustained isometric contractions.

The influence of temperature on the amplitude and frequency components of the EMG power spectra of the surface EMG recorded over the forearm muscles was examined in five male and five female subjects during brief and fatiguing isometric contractions of their handgrip muscles. Brief (3 s) isometric contractions were exerted at tensions ranging between 10 and 100% of each subject's maximum strength while fatiguing contractions were exerted at tensions of 25, 40, and 70% of their maximum strength. The temperature of the muscles during those contractions was varied by placing the forearms of the subjects in a controlled temperature water bath at temperatures of 10, 20, 30, and 40 degrees C. The results of these experiments showed that the center frequency of the power spectra of the surface EMG was directly related to the temperature of the exercising muscles during brief isometric contractions. During fatiguing isometric contractions, the amplitude of the EMG increased while the center frequency of the EMG power spectra decreased for all tensions examined.

Adult↗
PubMed1956

Electron microscopical study of skeletal muscle during isotonic (afterload) and isometric contraction.

Bundles of the curarized semitendinosus muscle of the frog were fixed during isotonic (afterload) and isometric contraction and the length of the A and I bands investigated by electron microscopy. The sarcomere length, during afterload contraction initiated at 25 per cent stretch, varied depending on the afterload applied between 3.0 and 1.2 micro, i.e. the shortening amounted to 5 to 50 per cent. The shortening involved both the A and I bands. Between a sarcomere length of 3.0 to 1.7 micro (shortening 5 to 35 per cent) the A bands remained practically constant at about 1.5 micro (6 to 8 per cent shortening); the length of the I bands decreased from 1.4 to 0.3 micro (80 per cent shortening). Below a sarcomere length of 1.7 to 1.2 micro the A bands shortened from 1.5 to 1.0 micro (from 6 to 8 to 25 per cent). At sarcomere lengths 1.6 to 1.2 micro the I band was replaced by a contraction band. During isometric contraction the A bands shortened by about 8 to 10 per cent; the I bands were correspondingly elongated.

Animals↗
PubMed1997

Motor unit recruitment and discharge behavior in movements and isometric contractions.

The purpose of this study was to contrast the discharge patterns of the same motor units during movements and during isometric contractions that were produced with comparable torque-time characteristics. Subjects performed elbow flexion and extension movements with predetermined acceleration characteristics. The average acceleration and deceleration profiles for the movements were reproduced in the isometric setting by presenting the kinematic profiles as templates for torque production. Trained subjects were able to match the first agonist (AG1) and antagonist (ANT) electromyographic (EMG) bursts, but tended to produce a smaller second agonist burst (AG2) in the isometric contraction. Twenty-five motor units from triceps brachii were studied. The same motor units (with one exception) were recruited and subsequently discharged in a similar fashion in both the isometric and movement tasks in the AG1 and ANT EMG bursts, with fewer motor unit discharges in the AG2 burst in the isometric contraction. The central control mechanisms appear to be the same for the acceleration phase of movement and isometric contraction, but differ during the deceleration phase.

Adult↗
PubMed1994

The relative activation of muscles during isometric contractions and low-velocity movements against a load.

Surface electromyographic (EMG) and motor unit activity were measured in human arm muscles during isometric contractions and during movements against an elastic load. The direction of force applied proximal to the wrist and movement direction of the wrist were varied in a horizontal plane. During isometric contractions the direction in which the largest EMG activity was measured corresponded to the direction in which motor units had the smallest recruitment threshold, for each muscle. The same was found for movements against an elastic load. However, this direction was different for isometric contractions and for movements. Because the magnitude and sign of these changes varied for different muscles, this resulted in a different relative activation of muscles for the two conditions. The amplitude of the surface EMG during contractions against an elastic load was generally significantly larger than that for isometric contractions against the same load. For m. brachioradialis isometric conditions yielded occasionally increased EMG activity. The change in EMG activity could be attributed completely to changes in motor unit recruitment thresholds leading to proportionate changes in the number of recruited motor units. However, the initial firing rate of motor units at recruitment was the same under both conditions and, therefore, did not contribute to changes in amplitude of surface EMG activity.

Arm↗
PubMed1999

Swelling of sarcoplasmic reticulum in the periphery of muscle fibres after isometric contractions in rat semimembranosus lateralis muscle.

The decline in isometric force, swelling of sarcoplasmic reticulum and loss of desmin was measured in semimembranosus lateralis muscle of male Wistar rats immediately after a short series of brief (500 ms) maximal isometric contractions. For the active muscle, the series ended below (protocol A) and just over muscle optimum length (protocol AA). In one protocol, the muscle remained passive and was extended to lengths just over muscle optimum length (protocol P). After all experimental protocols, no loss of desmin was observed and sarcomere appearance was normal. Protocol A produced swelling (87%) of the sarcoplasmic reticulum but no decline in isometric force. Protocol AA produced larger swelling (147%) of the sarcoplasmic reticulum and an isometric force decline (<49%) at short muscle lengths. Swelling of sarcoplasmic reticulum was observed mainly in the periphery of muscle fibres. Protocol P did not result in swelling of the sarcoplasmic reticulum and isometric force decline. It is concluded that swelling of the sarcoplasmic reticulum in the periphery of muscle fibres after brief maximal isometric contractions is associated with muscle force and not muscle length.

Animals↗
PubMed1975

Muscular reflex stimuli to the cardiovascular system during isometric contractions of muscle groups of different mass.

1. The cardiovascular responses to voluntary isometric contractions performed by human subjects are determined by the proportion of maximal tension achieved by the muscles contracting, and not by the mass of the contracting muscles, nor by the absolute tension achieved (Lind & McNicol, 1967; confirmed here). When two or more muscle groups contract simultaneously at different relative tensions, the increments in heart rate and blood pressure are the same as when the muscle group at the higher relative tension contracts alone at that tension (Lind & McNicol, 1967). It is known that there are both central and reflex stimuli to the cardiovascular system in exercise, and the present study examines whether the muscular reflex stimuli are related to the proportion of maximal tension achieved or to the mass of contracting muscle. 2. Isometric hind-limb contractions were induced in anaesthetized dogs and cats by stimulation of spinal ventral roots. Pressor responses to contraction of both hind limbs were greater than responses to contractions of either hind limb alone. No differences were observed between heart rate responses to single or combined hind-limb contractions. 3. When human subjects perform isometric contractions, a pressor response can be maintained beyond the conclusion of the exercise by occluding muscle blood flow. This response is generally attributed to a reflex set up in the muscle by the action of chemical factors on afferent nerves. When comparable pressor responses were evoked by comparable proportional efforts with either the whole hand or the little finger, it was found here that the pressor responses remaining during the period of post-exercise occlusion were greater when the greater mass of muscle had been exercised. 4. It is concluded that the muscular reflex drive in isometric exercise is related to the bulk of contracting muscle.

Adult↗
PubMed2002

Structural features of cross-bridges in isometrically contracting skeletal muscle.

Two-dimensional x-ray diffraction was used to investigate structural features of cross-bridges that generate force in isometrically contracting skeletal muscle. Diffraction patterns were recorded from arrays of single, chemically skinned rabbit psoas muscle fibers during isometric force generation, under relaxation, and in rigor. In isometric contraction, a rather prominent intensification of the actin layer lines at 5.9 and 5.1 nm and of the first actin layer line at 37 nm was found compared with those under relaxing conditions. Surprisingly, during isometric contraction, the intensity profile of the 5.9-nm actin layer line was shifted toward the meridian, but the resulting intensity profile was different from that observed in rigor. We particularly addressed the question whether the differences seen between rigor and active contraction might be due to a rigor-like configuration of both myosin heads in the absence of nucleotide (rigor), whereas during active contraction only one head of each myosin molecule is in a rigor-like configuration and the second head is weakly bound. To investigate this question, we created different mixtures of weak binding myosin heads and rigor-like actomyosin complexes by titrating MgATPgammaS at saturating [Ca2+] into arrays of single muscle fibers. The resulting diffraction patterns were different in several respects from patterns recorded under isometric contraction, particularly in the intensity distribution along the 5.9-nm actin layer line. This result indicates that cross-bridges present during isometric force generation are not simply a mixture of weakly bound and single-headed rigor-like complexes but are rather distinctly different from the rigor-like cross-bridge. Experiments with myosin-S1 and truncated S1 (motor domain) support the idea that for a force generating cross-bridge, disorder due to elastic distortion might involve a larger part of the myosin head than for a nucleotide free, rigor cross-bridge.

Actins↗
PubMed1993

Persistent mechanical effects of decreasing length during isometric contraction of ovarian ligament smooth muscle.

When isometrically-contracting strips of ovarian ligament smooth muscle were suddenly shortened by 10-20% of their length, force fell rapidly and then redeveloped along an exponential time course. The amount of force recovered fell short of that expected in an isometric contraction at the new length, and this force deficit was proportional to the magnitude of the length step (approximately 80% of force was recovered after a 10% shortening). A sudden imposed decrease in length was more effective in reducing subsequent force than was isotonic shortening. Early in the recovery phase the stiffness of the muscle was decreased to less than its expected value; stiffness recovered to expected levels on an exponential time course approximately three to four times faster than force recovery itself. Force-velocity curves made during the redevelopment phase showed a reduced maximal force (Fmax) and an increased maximal shortening velocity (Vmax) when compared with control contractions matched in force, time and length. The curves crossed at approximately 10% of Fmax. During isometric relaxation the muscles showed an increase in their expected stiffness; prior imposed shortening (as above) reduced the relaxation stiffness increase in proportion to the prior force deficit. The persistent effects of early events on the later phases of the contraction, as well as the increase in shortening velocity with very light loads, are consistent with the hypothesis that the sudden shortening detaches crossbridges and that same fail to reattach during force recovery. During isotonic shortening of unperturbed muscle some slowly-cycling crossbridges may act as an internal load and reduce shortening velocity.

Animals↗
PubMed1987

Exercise after anterior cruciate ligament reconstruction. The force exerted on the tibia by the separate isometric contractions of the quadriceps or the hamstrings.

The anterior or posterior drawer force exerted on the tibia by the separate isometric contractions of the quadriceps or hamstrings at various angles of knee flexion was examined in 20 healthy males. A two-dimensional model was analyzed using roentgenographic films. In separate isometric contractions of the quadriceps, the mean value of the anterior drawer force was equivalent to 14% of the tension of the quadriceps at 5 degrees knee flexion. The value decreased with the increase in the angle of knee flexion. The mean angle at which the anterior drawer force became zero was 45.3 degrees +/- 12.5 degrees. When the angle was increased further, the posterior drawer force gradually increased. In separate isometric contractions of the hamstrings, the posterior drawer force was exerted at all angles of flexion. Thus, during the early stage of rehabilitation after the anterior cruciate ligament injury, the quadriceps exercise by isometric muscle contraction should be performed with the knee flexion at more than 70 degrees (mean +/- 1.96). Exercise of the hamstrings by isometric muscle contractions can be carried out regardless of flexion angle.

Adult↗
PubMed1974

Blood flow in resting (contralateral) arm and leg during isometric contraction.

1. Blood flow in resting forearm and calf were measured plethysmographically in healthy young men during isometric contraction performed both as a handgrip and as a dorsiflexion of the foot. The isometric contraction was maintained for 2 min at one third maximal voluntary contraction for the handgrip and half maximal voluntary contraction for the dorsiflexion of the foot. In some experiments the possible influence on blood flow of inadvertent muscle activation in the resting limb was checked by recording the e.m.g.2. Both handgrip and dorsiflexion of the foot produced substantial increases in heart rate and mean arterial pressure, the pressure rise being almost linear with time throughout the contraction.3. Isometric contraction produced a rapid increase in forearm blood flow which reached a maximum on average two and a half times the resting value after 1 min and then declined slightly. Similar increases in forearm blood flow were produced by handgrip and dorsiflexion of the foot.4. Unlike in the resting forearm, the blood flow in the resting calf increased only slightly during the first minute of contraction and then decreased again to the resting level.5. The flow increase in the resting limbs could not be ascribed to inadvertent muscle activation as judged from the e.m.g. recordings.6. It is concluded that isometric muscle contraction produces a rapid increase in blood flow in the resting forearm, but only a very slight flow increase in the calf. Since the flow increased faster than the arterial pressure it must to a certain extent be induced by active neurogenic vasodilatation. Similarly, the relative flow decrease during the latter half of the contraction concomitantly with a progressively rising arterial pressure suggests that the neurogenic effect on the resistance vessels changes character, becoming more vasoconstrictive, and this might be related to increased effort in sustaining the contraction.

Adult↗
PubMed1996

The increase of duration of isometric contraction may not relate to change of relative oxygenation of forearm muscle.

The aim of this study is to ascertain, how an increase in duration of isometric contraction influences tissue oxygenation of forearm muscles, undergoing physical training. Four subjects underwent 6 weeks of training of the left forearm muscles by performing isometric contractions. Subjects repeated 10 s contractions, 10-15 times in the first, and 20 s contractions in the second 3 week period. Relative oxygenation of forearm muscles was measured by using. Near Infrared Spectroscopy (CWS2000, NIM Incorporated, Philadelphia). The training increased the duration of isometric contraction at 20 kp of experimental forearm muscles by 41 +/- 25 s, which was more (P < 0.05) than in control forearm (8 +/- 4 s). This increase was not reflected in changes of maximum relative deoxygenation of experimental muscles, which decreased by only -6.9 +/- 14.4%. The results show that an increase in duration of isometric contraction may not depend on the oxygenation of muscle tissue at fixed force of 20 kp.

Exercise↗
PubMed1989

Force, endurance time, and cardiovascular responses in voluntary isometric contractions of different muscle groups.

A study was made to reinvestigate the general validity of the force/endurance relation of isometric contractions. Relative force development, mean bioelectrical muscle activity and circulatory responses (blood pressure, interbeat interval) during isometric contractions of different muscle groups were compared. 6 men performed maximum isometric contractions of foot plantar- and dorsalflexors and hand flexors lasting 10 minutes. The hydrostatic blood pressure was varied by placing subject's arms and legs in two different positions, high and low. 9 Women performed plantarflexion and dorsalflexion with maximum strength and at constant load of 60% MVC to exhaustion. The results confirm that the force/endurance relationship during isometric exercise is not a biological constant, applicable for all muscles. The fibre composition of the muscle and the blood pressure varied by different hydrostatic components are discussed as factors causing differences in force/endurance relationship.

Adult↗
PubMed1998

[The role of intra- and extracellular calcium sources in the rhythmic inotropic regulation of isometric contractions in the chick embryo myocardium].

Isometric contractions and force-frequency relationships (FFR) were assessed in isolated ventricular preparations of 3- and 4-day chick embryos (EM) and posthatched (PH) chicks. Pacing protocols for the FFR assessment were applied in normal buffer, buffer with 50% and 25% of normal Na+ concentration, and in buffer with ryanodine. PH myocardium showed greater peak force and more prominent FFR than EM. 50% low Na+ superfusion induced positive inotropic effect and increase in the FFR in PH and EM. 25% low Na+ superfusion induced negative inotropic effect and suppressed FFR more prominent in EM. Ryanodine more suppressed FFR in PH myocardium. The observed age-dependent differences substantiate the fact that Na-Ca exchange plays a leading role in the FFR regulation in early embryonic myocardium and sarcoplasmic reticulum plays a leading role in PH myocardium.

Animals↗