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PubMed2005

Sex differences in glycolysis during brief, intense isometric contractions.

We have previously observed less muscle fatigue in women than men under conditions of intact circulation, but similar fatigue across the sexes during local ischemia. Thus, we hypothesized that women utilize their aerobic metabolic pathways to a greater extent than do men. To test this hypothesis, we examined the extent to which different pathways of intramuscular adenosine triphosphate (ATP) production were utilized by men and women during maximal voluntary isometric contractions. Force production during 15-s and 60-s contractions were recorded in parallel sessions. In one session, central activation was assessed with electrical stimulation. In the other, phosphorus magnetic resonance spectroscopy was used to quantify muscle oxidative capacity, and the contributions of glycolysis and oxidative phosphorylation to ATP synthesis during the 60-s contraction. Fatigue and central activation were similar in men and women during both the 15-s and 60-s contractions. The rate constants of phosphocreatine recovery following the 15-s contraction were similar in men and women, indicating similar oxidative capacities. Men exhibited greater acidosis and peak glycolytic rates compared with women during the 60-s contraction, with no differences observed in creatine kinase flux or the percent of oxidative capacity utilized. We conclude that men exhibit greater in vivo glycolysis during brief, intense isometric contractions. Although this metabolic difference did not contribute to any observable differences in fatigue in the present study, these results highlight a potentially important mechanism to explain sex-related differences in muscle function.

Adenosine Triphosphate↗

PubMed1990

Isometric features of orthostatic tremor: an electromyographic analysis.

A patient is described with "orthostatic" tremor. Electromyography revealed tremor bursts of 15 Hz in the lower extremities while standing and with isometric activation of the muscles, but the bursts disappeared with isotonic activation of muscles. Similar tremor was recorded in the arms with isometric, but not isotonic activation. Review of previously reported cases confirms these findings. The clinical and electrophysiologic features of this tremor distinguish it from other recognized forms of tremor.

Aged↗

PubMed2003

Muscle reoxygenation rate after isometric exercise at various intensities in relation to muscle oxidative capacity.

The purpose of this study was to determine whether the reoxygenation rate (Reoxy-rate) immediately after static exercise at various submaximal intensities would be related to muscle oxidative capacity. Seven healthy male subjects performed isometric handgrip exercise for 10 sec at 30%, 60% and 90% of maximal voluntary contraction (MVC). The Reoxy-rate and muscle oxygen consumption during exercise (muscle VO2EX) were monitored by near infrared continuous wave spectroscopy (NIRcws). The muscle oxidative capacity was evaluated by the time constant for phosphocreatine resynthesis (PCrTc) using 31-phosphorus magnetic resonance spectroscopy (31P-MRS). The Peak blood flow of brachial artery after exercise (BABFpeak) was measured using Doppler ultrasound. There was no correlation between PCrTc and Reoxy-rate at 30% and 60% MVC. In contrast, Reoxy-rate at 90% MVC was positively correlated to PCrTC (r = 0.825, p < 0.05). The muscle VO2EX increased 5.9, 8.8 and 12.6-fold of the resting on average at 30%, 60% and 90% MVC, respectively, and the muscle VO2EX at 90% MVC was significantly higher than that at 30% and 60% MVC. On the other hand, BABFpeak increased only just 1.9, 2.4 and 2.7-fold of the resting on average at 30%, 60% and 90% MVC, respectively (Fig. 4). These results suggest that the higher oxidative capacity muscle shows slower muscle reoxygenation after 10 sec isometric exercise at 90% MVC because the Reoxy-rate after this type of exercise may be influenced more by muscle VO2 than by O2 supply. In contrast, 60% MVC and lower exercise intensities may not be severe enough to influence the muscle VO2 dependent Reoxy-rate.

Exercise↗

PubMed1993

The first thin filament layer line decreases in intensity during an isometric contraction of frog skeletal muscle.

During isometric contraction/activation of full-overlap and non-overlap live frog skeletal muscles, the intensity of the first thin filament layer line at the axial spacing of approximately 1/37 nm-1, when separated from the partially overlapping first thick filament layer-line at approximately 1/43 nm-1, remained unchanged in the inner radial region (0.02-0.08 nm-1) where a large intensity increase is observed in the rigor state. The intensity decreased in the outer radial region (0.08-0.18 nm-1) where this layer line is expected to peak in the resting state. The intensity decrease in the outer region became larger with increasing filament overlap; on activation of the non-overlap muscle, it was about half that of the full-overlap muscle. Thus the first thin filament layer line decreases in intensity and any indication of the rigor-like intensification is not observed at all during contraction. This intensity decrease can be attributed to the same structural changes giving rise to the intensity increase of the second thin filament layer line. The results indicate that the configuration of the myosin heads interacting with actin during contraction differs significantly from that of the rigor state. Four-fold rotational symmetry of the thin filament structure as a whole becomes more pronounced during isometric contraction of the overlap muscle.

Actin Cytoskeleton↗

PubMed1993

Structural change of crossbridges of rabbit skeletal muscle during isometric contraction.

Structural changes of crossbridges during isometric contraction have been studied by electron microscopy. Chemically skinned rabbit fibres were rapidly frozen either in activating solution or in ATP-free (rigor) solution, freeze-substituted and embedded. Longitudinal sections of muscle fibres show that the number of crossbridges in active fibres (isometric contraction) is approximately the same as in rigor fibres. Crossbridges of the active and rigor states differ in their shapes, angles and manner of arrangement on the thin filaments. In rigor many crossbridges are wide near the thin filaments and narrow near the thick filament shafts; in active fibres they have more uniform width along their length. The angle of the crossbridges in active fibres is somewhat variable. The average angle is approximately 90 degrees to the filament axis. The crossbridges are arranged on the thin filament retaining the 14.3 nm thick filament periodicity. The crossbridges in rigor are tilted and their arrangement near the thin filament reveals the 36 nm actin periodicity. The variability in the shapes of the crossbridges in active fibres is still higher when we look at them in cross-sections of muscle fibres. The crossbridge shapes in the cross-sections were classified and the relative frequency of different shapes was determined. The shapes that are commonly observed in active fibres are similar in that the majority of the mass of the crossbridges is farther away from the thin filament than the crossbridges in rigor fibres.

Actins↗

PubMed1994

Time-resolved X-ray diffraction studies of myosin head movements in live frog sartorius muscle during isometric and isotonic contractions.

Using the facilities at the Daresbury Synchrotron Radiation Source, meridional diffraction patterns of muscles at ca 8 degrees C were recorded with a time resolution of 2 or 4 ms. In isometric contractions tetanic peak tension (P0) is reached in ca 400 ms. Under such conditions, following stimulation from rest, the timing of changes in the major reflections (the 38.2 nm troponin reflection, and the 21.5 and 14.34/14.58 nm myosin reflections) can be explained in terms of four types of time courses: K1, K2, K3 and K4. The onset of K1 occurs immediately after stimulation, but that of K2, K3 and K4 is delayed by a latent period of ca 16 ms. Relative to the end of their own latent periods the half-times for K1, K2, K3 and K4 are 14-16, 16, 32 and 52 ms, respectively. In half-times, K1, K2, K3 lead tension rise by 52, 36 and 20 ms, respectively. K4 parallels the time course of tension rise. From an analysis of the data we conclude that K1 reflects thin filament activation which involves the troponin system; K2 arises from an order-disorder transition during which the register between the filaments is lost; K3 is due to the formation of an acto-myosin complex which (at P0) causes 70% or more of the heads to diffract with actin-based periodicities; and K4 is caused by a change in the axial orientation of the myosin heads (relative to thin filament axis) which is estimated to be from 65-70 degrees at rest to ca 90 degrees at P0. Isotonic contraction experiments showed that during shortening under a load of ca 0.27 P0, at least 85% of the heads (relative to those forming an acto-myosin complex at P0) diffract with actin-based periodicities, whilst their axial orientation does not change from that at rest. During shortening under a negligible load, at most 5-10% of the heads (relative to those forming an acto-myosin complex at P0) diffract with actin-based periodicities, and their axial orientation also remains the same as that at rest. This suggests that in isometric contractions the change in axial orientation is not the cause of active tension production, but rather the result of it. Analysis of the data reveals that independent of load, the extent of asynchronous axial motions executed by most of the cycling heads is no more than 0.5-0.65 nm greater than at rest.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Identification of time-varying dynamics of the human triceps surae stretch reflex. I. Rapid isometric contraction.

We have examined the time variations of stretch reflex dynamics throughout rapid voluntary changes in the isometric contraction level of the human triceps surae muscles. This was achieved by superimposing a small stochastic displacement upon many such changing contractions and then identifying the time-varying relationship between the perturbation and the evoked electromyograms (EMGs). An "ensemble" time-varying system identification technique was used to estimate these input-output dynamics as a set of impulse response functions, one for each time before, during, and after the change in contraction level, with a temporal resolution equal to the data acquisition rate. Three main findings resulted. First, stretch reflex gain (relating joint velocity to EMG) was significantly modulated during changes in voluntary contraction level, increasing as the subject contracted the muscles and decreasing as the subject relaxed. Second, stretch reflex dynamics did not change with contraction level, even when its gain varied substantially. Third, the time course of the gain changes closely followed the level of the EMG, even though the subjects used rather different activation and deactivation patterns. These results suggest that, for the behavior studied (i.e., rapid changes in isometric contraction level), stretch reflex gain and motoneuron pool activation level were controlled by a common descending command rather than being independently specified.

Adult↗

PubMed1993

Motor unit recruitment in human medial gastrocnemius muscle during combined knee flexion and plantarflexion isometric contractions.

Previous work on multifunctional muscle has suggested that motor unit recruitment during a combined force task is the result of an interactive effect of weighted inputs acting simultaneously on the motoneuron pool. The present study shows that a similar effect describes motor unit activation in a two-joint muscle as forces are combined at both proximal and distal attachments. The recruitment thresholds of single motor units in medial gastrocnemius muscle were determined during combined knee flexion and plantarflexion isometric contractions. Slow isometric ramp contractions in knee flexion were produced while maintaining various background levels of plantarflexion force. The combination of knee flexion and plantarflexion forces at which a motor unit initially discharged was used to characterize recruitment as represented by the slope of the regression line fit to the individual data points. Each subject completed two experiments; one at each of two knee joint angles, with the ankle joint fixed at 90 degrees. The effect of knee angle was assessed by comparing the slopes of the regression lines that characterized motor unit recruitment at each knee angle. Motor units in medial gastrocnemius were recruited when the linear sum of the forces exerted in plantarflexion and knee flexion exceeded a certain threshold of combined force. Specifically, the apparent force threshold of recruitment in knee flexion decreased as the level of force maintained in plantarflexion increased. Further, evidence is provided indicating that the linear relationship describing recruitment in two-joint muscle is dependent upon joint angle.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

The relation between the direction dependence of electromyographic amplitude and motor unit recruitment thresholds during isometric contractions.

The activation of muscles can be studied by measuring the activity of a representative set of single motor units in a muscle or by measuring the surface electromyographic (EMG) activity of a muscle that results from the contribution of a large number of motor units. In this study we have developed a model showing how the direction dependence of the amplitude of the EMG activity during isometric contractions can be understood from the recruitment thresholds of single motor units when force is applied in various directions within a plane. The model predicts that the direction with the largest EMG activity (called the "preferred direction") corresponds to the direction in which the largest number of motor units is recruited. If one assumes homogeneous activation of a population of motor units, this preferred direction can be shown to be equivalent to the direction in which the recruitment threshold of the motor units is smallest. The experimental data show that, for most muscles in the human arm, the amplitude of surface EMG activity for a constant, isometric force at the wrist was proportional to the cosine of the angle between the muscle's preferred direction and the direction of force. As predicted by the model, the preferred direction coincided with the direction in which the recruitment threshold of motor units was smallest. Musculus deltoideus anterior had a more complicated directional sensitivity for surface EMG activity that could not be well fit by a single cosine function. This effect could be explained by the finding of two subpopulations of motor units within that muscle, each with a different recruitment behavior.

Adult↗

PubMed1986

Relative activation of two human elbow flexors under isometric conditions: a cautionary note concerning flexor equivalence.

We examined the electromyographic (EMG) activity of two human elbow-flexor muscles, biceps brachii and brachioradialis, during isometric contractions. The task required subjects to match the EMG level of one of the muscles (the control muscle) to one of four target levels (5, 10, 15, or 20% of maximum) at various elbow angles. A new technique was developed for the target-matching task. The activity of the other muscle (the test muscle) was simultaneously recorded during the task. For the notion of flexor equivalence to be supported, the EMG levels for the two muscles should have covaried. This was not the case. The results revealed three features: (1) while the control-muscle EMG remained constant across joint angles, the test-muscle EMG varied with joint angle, and the trend of this variation differed among subjects; (2) in nine out of ten subjects the trend of test-muscle EMG variation with joint angle was reversed when the other muscle served as the test muscle; and (3) the test-muscle EMG associated with the four target levels was subject-, muscle-, and angle-dependent. These results caution against the generalization of the flexor equivalent concept to isometric conditions. In particular, the activity of one muscle is not a reliable indicator of the activity of other muscles subserving the same joint action.

Computers↗

PubMed1995

Intramuscular laser-Doppler flowmetry in the supraspinatus muscle during isometric contractions.

To study the regulation of microvascular blood flow in a compartment muscle, laser-Doppler measurements of muscle microcirculation were recorded in the supraspinatus muscle in eight volunteers during and following submaximal isometric muscle contractions. The subjects performed isometric shoulder abductions at five contraction levels from 5% to 50% maximal voluntary contraction for 1 min each and a sustained 30 degrees shoulder abduction for 20 min. The subjects' perceived exertion increased from "no perceived exertion" to "near maximal exertion" during the 20-min period with 30 degrees shoulder abduction. Microcirculation increased during all 1-min contractions. Following the contractions at 20%, 30% and 50% MVC post-exercise reactive hyperaemia was seen for a period of at least 1 min. The reactive hyperaemia increased in magnitude in response to increasing contraction level. The results showed the same time-history of the blood flow at microvascular level as previously seen in larger peripheral vessels in response to muscle contractions. During the 20-min contraction microcirculation increased in line with the findings during the brief contractions. However, in contrast to the brief contractions no postexercise reactive hyperaemia occurred following the prolonged contraction. Lack of postexercise reactive hyperaemia following the prolonged shoulder abduction would suggest insufficient regulation of the vascular resistance. Alternatively, lack of hyperaemia could be taken as an indication of sufficient microcirculation during the preceding contraction. From previous studies on intramuscular pressure and metabolism the latter alternative would seem unlikely.

Adult↗

PubMed1995

Asymmetric control of bilateral isometric finger forces.

We examined the ability to match the voluntary isometric finger flexion forces of the dominant and nondominant hand in humans, as well as the influence of unilateral visual feedback during this task. Right- and left-handed subjects were trained to produce a "low" force level (50 +/- 25 g) and a "high" force level (200 +/- 50 g) with the right and left index finger, separately. Following the training session, subjects were instructed to match the isometric forces of both fingers simultaneously within the required range (either low or high) so that they were perceived to be identical. The results showed an asymmetry, whereby greater forces were exerted with the index finger of the dominant hand. The asymmetry was independent of the subjects' maximum finger flexion strength. When unilateral visual feedback represented the force output of the dominant hand, the asymmetry was no longer present. In contrast, when it represented the force output of the nondominant hand, the asymmetry was not compensated. We hypothesize that these findings are the result of anatomical or physiological asymmetries inherent in the motor system controlling the production of force.

Adult↗

PubMed1995

Electromechanical delay in isometric muscle contractions evoked by voluntary, reflex and electrical stimulation.

Electromechanical delay (EMD) in isometric contractions of knee extensors evoked by voluntary, tendon reflex (TR) and electrical stimulation (ES) was investigated in 21 healthy young subjects. The subject performed voluntary knee extensions with maximum effort (maximal voluntary contraction, MVC), and at 30%, 60% and 80% MVC. Patellar tendon reflexes were evoked with the reflex hammer being dropped from 60 degrees, 75 degrees and 90 degrees positions. In the percutaneous ES evoked contractions, single switches were triggered with pulses of duration 1.0 ms and of intensities 90, 120 and 150 V. Electromyograms of the vastus lateralis and rectus femoris muscles were recorded using surface electrodes. The isometric knee extension force was recorded using a load cell force transducer connected to the subject's lower leg. The major finding of this study was that EMD of the involuntary contractions [e.g. mean 22.1 (SEM 1.32) ms in TR 90 degrees; mean 17.2 (SEM 0.62) ms in ES 150 V] was significantly shorter than that of the voluntary contractions [e.g. mean 38.7 (SEM 1.18) ms in MVC, P < 0.05]. The relationships between EMD, muscle contractile properties and muscle fibre conduction velocity were also investigated. Further study is needed to explain fully the EMD differences found between the voluntary and involuntary contractions.

Adult↗

PubMed1992

The time course of thyroid-hormone-induced changes in the isotonic and isometric properties of rat soleus muscle.

Rat thyroidectomy resulted in changes in a number of parameters used to characterise the mechanical and histochemical status of skeletal muscle. Thus thyroidectomy resulted in a prolongation of soleus slow-twitch muscle isometric contraction time and half-relaxation time with a reduced maximum velocity of shortening and maximum rate of development of tetanic tension but no significant change in twitch: tetanus ratio i.e. the ratio of twitch force/unit area to tetanic force/unit area. In addition the percentage of IIA fibres was reduced and the percentage of type I fibres increased. Triiodothyronine, administered to hypothyroid rats, brought about a speeding of these parameters again with no change in twitch: tetanus ratio. There was an increase in the percentage of IIA fibres with a concomitant reduction in the percentage of type I fibres. These changes were induced over 18 days and resulted in isotonic and isometric properties almost identical to those of soleus muscles from chronically hyperthyroid rats; speeding could be detected as early as 2 days after triiodothyronine had been given. Consideration is given to the possibility that changes in myosin isoforms and/or the kinetics of changes in intracellular calcium concentration in activation and relaxation could account for the time course of the observed changes.

Animals↗

PubMed1992

Hypertrophy without increased isometric strength after weight training.

Eight men (20-23 years) weight trained 3 days.week-1 for 19 weeks. Training sessions consisted of six sets of a leg press exercise (simultaneous hip and knee extension and ankle plantar flexion) on a weight machine, the last three sets with the heaviest weight that could be used for 7-20 repetitions. In comparison to a control group (n = 6) only the trained group increased (P less than 0.01) weight lifting performance (heaviest weight lifted for one repetition, 29%), and left and right knee extensor cross-sectional area (CAT scanning and computerized planimetry, 11%, P less than 0.05). In contrast, training caused no increase in maximal voluntary isometric knee extension strength, electrically evoked knee extensor peak twitch torque, and knee extensor motor unit activation (interpolated twitch method). These data indicate that a moderate but significant amount of hypertrophy induced by weight training does not necessarily increase performance in an isometric strength task different from the training task but involving the same muscle group. The failure of evoked twitch torque to increase despite hypertrophy may further indicate that moderate hypertrophy in the early stage of strength training may not necessarily cause an increase in intrinsic muscle force generating capacity.

Adult↗

PubMed1988

Glycogen degradation during isometric exercise at low contraction force.

The glycogen content was measured in biopsy sample of human vastus lateralis muscle during prolonged isometric contraction with low force generation. In the first experiment 15% of the maximum voluntary contraction force (MVC) was held for 10 min. Glycogen utilization was 68.1 mmol glucosyl units.kg-1 dry muscle (d.m.). The study was continued by intermittent contractions of 50 s duration and 10 s rest repeated for 50 min. This resulted in a total glycogen utilization of 167.5 mmol glycosyl units.kg-1 d.m. The study was repeated with a force set of 7.5% MVC starting with 20 min continuous contraction followed by the same intermittent contractions for a further 100 min. The glycogen decrease was 15 mmol after the continuous contraction and totally 50 mmol after 2 h with the lower force. Thus the glycogen degradation rate even at low contraction force was related to the force level, being 6 times higher when the force was increased from 7.5 to 15% MVC. With prolonged isometric work periods at work loads corresponding to 15% MVC or higher depletion of the glycogen store can limit work performance capacity.

Adult↗

PubMed1988

Isometric and isokinetic muscle strength, anthropometry and physical activity in 8 and 13 year old Swedish children.

Isometric muscle strength of the hand-grip and of trunk flexion and extension, and isokinetic torque of elbow and knee flexion and knee extension were assessed in a random sample of 8 and 13 year old Swedish children. The results were compared with respect to sex and age in absolute terms and relative to weight, height2 and estimates of lean body mass and cross-sectional muscle area. Daily physical activity was also estimated. The muscle strength variables were in general found to be very similar in the 8 year old boys and girls. In the 13 year old group the boys were generally stronger than the girls, in both absolute and relative terms, except for similar torque values during knee extension. The absolute and relative muscle strength and torque values were higher in the older than in the younger children, with the exception of trunk strength per unit of body weight and of lean body mass, which were similar in boys of both ages and significantly lower in the older than in the younger girls. No significant correlation was found between the estimates of physical activity and isometric and isokinetic muscle strength and torque.

Adolescent↗

PubMed1988

Electrically evoked isometric and isokinetic properties of the triceps surae in young male subjects.

The relationship between electrically evoked isometric and isokinetic properties of the triceps surae have been studied in 11 healthy male subjects. The results showed that the time to peak tension (TPT) and half relaxation time (1/2 RT) of the maximal twitch were 110 +/- 11 ms and 82 +/- 11 ms respectively, and the peak rates of rise of contraction (delta P50, delta P200) and relaxation (delta PR50, delta PR200) at 50 and 200 Hz were 0.36 +/- 0.07, 0.48 +/- 0.08 and 1.27 +/- 0.33, 1.25 +/- 0.27% Po ms-1 respectively. The decline in force during a fatigue test was significantly (P less than 0.02) associated with the decrease in maximal relaxation rate (r = 0.79). The TPT was significantly (P less than 0.05) and inversely related to delta P50 and delta P200. The mean angle specific torque-velocity relationship for the 11 subjects was adequately described by the empirical exponential equation of the form: V = 16.5 (e-P/30.8-e-84.3/30.8) where V = velocity (rads s-1) and P = torque (Nm). The only significant association found between the isometric and isokinetic properties of the muscle was between delta PR200 and the torque expressed at a given velocity of 4 rads s-1. This lack of association between the two variables is difficult to explain with certainty but it is suggested that it may be due to the differential effects of Ca2+ release and uptake and cross-bridge turnover rate in the two situations.

Adult↗