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PubMed1999

Extensibility and symmetry of actin filaments in contracting muscles.

When isometrically contracting muscles are subjected to a quick release followed by a shortening ramp of appropriate speed (V(o)), tension decays from its value at the isometric plateau (P(o)) to <0. 05 P(o) with the same time course as the quick part of the release; thereafter, tension remains at a negligible level for the duration of the shortening ramp. X-ray diffraction data obtained under these conditions provide evidence that 1) at V(o) very few heads form an actomyosin complex, while the number of heads doing so at P(o) is significant; 2) relative to rest the actin filament at V(o) is approximately 0.12% shorter and more twisted, while it is approximately 0.3% longer and less twisted at P(o); and 3) the myosin heads attaching to actin during force development do so against a thin filament compliance of at least 0.646 +/- 0.046% nm per P(o).

Actins↗
PubMed1976

Circulatory effects of isometric muscle contractions, performed separately and in combination with dynamic exercise.

Studies on central circulation and regional blood flow were performed in healthy male volunteers at rest, during sustained isometric forearm contraction at 20% of MVC, during dynamic leg exercise (100 W) and during combined isometric and dynamic exercise. In 10 subjects pulmonary oxygen uptake, arterio-venous oxygen difference, heart rate, leg blood flow and blood pressures in the pulmonary and subclavian arteries and in the right atrium were measured. In 4 of these subjects the temperature was measured in mixed venous blood and in subcutaneous tissue in an attempt to further analyse the blood flow distribution through central versus peripheral parts of the circulatory system. In 5 other subjects the splanchnic blood flow was estimated by hepatic vein catheterization and dye dilution technique at rest and during isometric forearm contraction. It was found that cardiac output, oxygen uptake, heart rate and arterial blood pressure all increased in response to isometric contraction. Quantitatively the changes in heart rate and cardiac output induced by a sustained contraction were more marked when the contraction was performed separately than when it was added to dynamic exercise. In spite of the increased arterial pressure, the leg blood flow did not increase significantly. Neither did the splanchnic blood flow increase in response to hand-grip contrmic exercise. Isometric exercise, however, caused a blood temperature fall and a rise in subcutaneous temperature indicating an increased blood flow through the skin. It is concluded that during sustained isometric muscle contraction 1. the blood flow increase is mainly distributed to peripheral circulatory areas, 2. a concomitant dynamic exercixe interferes with the circulatory adaptation only to a small extent.

Abdomen↗
PubMed1990

Reflex responses of renal nerve activity during isometric muscle contraction in cats.

Renal sympathetic nerve activity (RSNA), arterial blood pressure (AP), and heart rate (HR) were measured during isometric muscle contraction of a hindlimb in chloralose-anesthetized cats. In 14 cats RSNA, AP, and HR increased during a 1-min contraction by 45%, 39 mmHg, and 11 beats/min, respectively; however, in three cats there was a brief initial decrease in RSNA followed by an increase. In 11 cats isometric contraction was maintained for 5 min by alternate stimulation of the L7 and S1 ventral roots. In the first 1 min of sustained contraction, there was a positive correlation (gamma = 0.58, P less than 0.005) between RSNA and tension development. Thereafter RSNA remained elevated despite a tension decrease, and there was no significant correlation between these changes. The RSNA response to contraction of both hindlimbs was greater than that to contraction of either hindlimb alone. Passive stretch of the hindlimb muscle significantly increased RSNA. Thus the initial increase in RSNA during sustained contraction is likely due to activation of muscle mechanoreceptors, whereas the later increase is probably caused by activation of the muscle metaboreceptors.

Animals↗
PubMed1980

Epitrochlearis muscle. II. Metabolic effects of contraction and catecholamines.

Effects of isometric contraction and catecholamines on glucose and glycogen metabolism in skeletal muscle were investigated with the in vitro rat epitrochlearis preparation. Mechanical performance and glycogenolysis exhibited two phases. During the initial 30 min, muscle work was 30% greater and glycogenolysis 8- to 10-fold faster than the steady-state values in the subsequent 3-4 h of contraction. Glucose uptake was increased by contraction and remained relatively constant during stimulation. Epinephrine (10(-9) to 10(-6) M) and norepinephrine (10(-7) to 10(-5) M) produced inotropic and glycogenolytic effects blocked by propranolol but not phentolamine. Chemical sympathectomy and propranolol blocked the initial glycogenolytic and inotropic effects produced by isometric contraction, suggesting that they were caused by the release of endogenous catecholamines. Net lactate production in resting muscles accounted for > 50% of total glucosyl units utilized. During contraction net lactate production accounted < 10-15% of total glycosyl flux indicating that rat fast-twitch pale muscle is capable of significant rates of aerobic glucose oxidation. Oleate and caprylate did not affect mechanical performance, glycogen, or glucose metabolism in resting or contracting muscles.

Animals↗
PubMed2004

The isolated and combined effects of menstrual cycle phase and time-of-day on muscle strength of eumenorrheic females.

Diurnal variation in muscle performance has been well documented in the past few years, but almost exclusively in the male population. The possible effects of the menstrual cycle on human circadian rhythms have remained equivocal, particularly in the context of muscle strength. The purpose of the study was to analyze the isolated and combined effects of circamensal variation and diurnal changes on muscle strength. Eight eumenorrheic females (age 30 +/- 5 yrs, height 1.63 +/- 0.06m and body mass 66.26 +/- 4.6kg: mean +/- SD) participated in this investigation. Isokinetic peak torque of knee extensors and flexors of the dominant leg were measured at 1.05, 3.14rad.s(-1) (through 90 degrees ROM) at two times-of-day (06:00, 18:00 h) and five time points of the menstrual cycle (menses, mid-follicular, ovulation, mid-luteal, late luteal). In addition, maximum voluntary isometric contraction of knee extensors and flexors and electrically stimulated isometric contraction of the knee extensors were measured at 60 degrees of knee flexion. Rectal temperature was measured during 30min before the tests. There was a significant time-of-day effect on peak torque values for isometric contraction of knee extensors under electrical stimulation (P< 0.05). At 18:00 h, muscle force was 2.6% greater than at 06:00 h. The time-of-day effect was not significant when the tests were performed voluntarily without stimulation: effect size calculations indicated small differences between morning and evening for maximal voluntary isometric contraction and peak torque (at 1.05rad.s(-1) for the knee extensors. A circamensal variation was observed for peak torque of knee flexors at 1.05rad.s(-1), extensors at 3.14rad.s(-1), and also isometric contraction of knee flexors, values being greatest at the ovulation phase. Interaction effects between time-of-day and menstrual cycle phase were not observed in any of the indices of muscle strength studied. The phase of the menstrual cycle seemed to have a greater effect than did the time-of-day on female muscle strength in this group of subjects. The present results suggest that peripheral rather than central mechanisms (e.g., motivation) are implicated in the diurnal variation of maximal isometric strength of women.

Adult↗
PubMed2006

Active hyperemia and vascular conductance differ between men and women for an isometric fatiguing contraction.

To understand the role of muscle perfusion in the sex differences of muscle fatigue, we compared the time to task failure, postcontraction (active) hyperemia, and vascular conductance for an isometric fatiguing contraction performed by young men and women with the handgrip muscles at 20% of maximal voluntary contraction (MVC) force. In study 1, the men (n = 16) were stronger than the women (n = 18), and study 2, the men (n = 7) and women (n = 7) were matched for strength. Isometric contractions were sustained during two sessions: 1) until the target force could no longer be achieved or 2) for 4 min. For both studies, blood flow and vascular conductance were similar for the men and women at rest and after 10 min of occlusion, and at task failure for the fatiguing contraction estimated using forearm venous occlusion plethysmography. In study 1, the time to task failure was longer for the women (11.4 +/- 2.8 min) than for the men (8.4 +/- 2.4 min; P = 0.003). However, at the end of the 4-min contraction, active hyperemia and vascular conductance were greater for the men than the women (99 vs. 70% peak blood flow; P < 0.001). In study 2, the men and women had similar strength and a similar time to failure (8.4 +/- 1.6 vs. 8.6 +/- 2.3 min). Active hyperemia was greater for the men than the women (86 vs. 64% peak flow; P = 0.038) after the 4-min contraction, as was vascular conductance (80 vs. 57% peak conductance; P = 0.02). Thus the briefer time to failure of men than women for an isometric fatiguing contraction is a function of the greater strength of men but is not dependent on differences in the active hyperemia and vascular conductance.

Adult↗
PubMed1978

[Characteristics of the isometric myocardial contractions during its activation by the slow response (author's transl)].

The characteristics of the isometric contractions of rabbit's left atria were studied during its activation by the slow response of the cardiac action potential. When compared to the conditions where activation occurs with normal action potentials it verifies that contraction amplitude decreases, becoming slowly and larger. Positive inotropic effects that enhance calcium entrance through the membrane (Ca and IPA), during action potential, increase contraction amplitude. Negative inotropic effects that depress the slow response (Ach and verapamil) also depress the contractions. Staircase becomes reversed, with contraction strength decreasing with increase in rate of beating. It was seen that adequate conduction is an important factor for the normality of contractions, and the importance of the fast component of the cardiac action potential, for this occurrence, was remembered.

Acetylcholine↗
PubMed2002

Differences in strength and surface electromyogram characteristics between pre-pubertal gymnasts and untrained boys during brief and maintained maximal isometric voluntary contractions.

The present study was aimed at investigating differences of maximal strength (F(max)) of the elbow flexors and characteristics of the surface electromyogram (EMG) between six gymnasts (G) and six untrained (UT) 10-year-old boys during brief and maintained maximal voluntary isometric contraction (MVC). The F(max) was estimated during 5 s MVC (maximal test, MT) and normalized to the cross sectional area (CSA) of the arm. The EMG signal of the biceps brachii was recorded during MT and during a 25 s maintained MVC (fatigue test). Values were calculated for root-mean-square (rms(MT)) and mean power frequency (MPF(MT)) of the EMG signal for the duration of the MT. For the fatigue test, MPF were normalized to the initial value (MPF(n)) and kinetics were expressed by the slope coefficient of linear regression. Although F(max) and F(max)/CSA tended to be higher for G than UT, the differences did not reach significance. The MPF(MT) was significantly higher for G [mean (SD)][136 (8) Hz] than for UT [125 (9) Hz]. The MPF(n) slope coefficients were significantly greater for G than for UT [-1.0 (0.2) and -0.5 (0.3), respectively]. When all the children were considered, F(max) was significantly correlated to MPF(MT) (r = 0.61). These results showed that gymnasts tend to have higher F(max) and F(max)/CSA accompanied by a significantly higher MPF(MT) and a steeper MPF downshift. Moreover, children with greater strength tended to have higher MPF(MT). It is suggested that spatial and/or temporal recruitment of more fatigable fast motor units could have been enhanced in G and more generally, that it could be a mechanism that would explains, in part, the level of force production in children.

Child↗
PubMed1989

Relation between electromyogram and torque of isometric reflex contractions in man.

Human subjects maintained isometric plantar or dorsal flexions of the ankle in a matching task. H-reflexes of different sizes were superimposed on the steady activity. The peak-to-peak amplitude of the reflexes was measured on the electromyogram (EMG) of the soleus muscle. The size of the corresponding muscle contractions was determined on the isometric torque signal in relation to the maintained flexion force. The EMG-torque relation which was defined as the reflex muscle contraction as a function of the EMG reflex signal approximated a square root function for a given steady contraction level. It was not modulated by steady dorsal flexions, but it decreased continuously with stronger plantar steady torques. This dependence was caused by the silent period following the reflex discharge. Since the reflex discharge and the silent period were near in time to the duration of the contraction, the silent period had a direct effect on the reflex contraction amplitude.

Electromyography↗
PubMed1995

The metabolic costs of different types of contractile activity of the human adductor pollicis muscle.

1. The metabolic costs and physiological consequences of shortening contractions of a human muscle working in situ have been compared with those of the muscle maintaining a continuous isometric contraction and when performing repeated brief isometric contractions. 2. After a total of 10 s stimulation, the shortening and intermittent brief isometric protocols had very similar effects, causing a 30% loss of force and a threefold increase in the half-time of relaxation. This was in contrast to the continuous isometric contraction protocol where there was less than 10% loss of force or slowing of relaxation. 3. The ATP cost over the first 5 s of the continuous isometric protocol was 27 mmol (l intracellular water)-1 while for the shortening and repeated brief isometric protocols the costs were 48 and 46 mmol (l intracellular water)-1, respectively. 4. The results show that shortening and repeated brief isometric contractions are considerably more energetically demanding, and hence more fatiguing, than sustained isometric contractions.

Adenosine Triphosphate↗
PubMed2001

Role of protein phosphatases in regulation of cardiac inotropy and relaxation.

We studied the effects of the protein phosphatase (PP) inhibitor cantharidin (Cant) on time parameters and force of contraction (FOC) in isometrically contracting electrically driven guinea pig papillary muscles. We correlated the mechanical parameters of contractility with phosphorylation of the inhibitory subunit of troponin (TnI-P) and with the site-specific phosphorylation of phospholamban (PLB) at serine-16 (PLB-Ser-16) and threonine-17 (PLB-Thr-17). Cant (after 30 min) started to increase FOC (112 +/- 4% of control, n = 10) and TnI-P and PLB-Thr-17 (120 +/- 5 and 128 +/- 7% of control) without any alteration of relaxation time. Cant (10 microM) started to increase PLB-Ser-16, but the relaxation was shortened at only 100 microM (from 140 +/- 9 to 116 +/- 12 ms, n = 9). Moreover, 100 microM Cant, 3 min after application, started to increase PLB-Thr-17, TnI-P, and FOC. Cant (100 microM) began to increase PLB-Ser-16 after 20 min. This was accompanied by shortening of relaxation time. Differences in protein kinase activation or different substrate specificities of PP may explain the difference in Cant-induced site-specific phosphorylation of PLB in isometrically contracting papillary muscles. Moreover, PLB-Thr-17 may be important for inotropy, whereas PLB-Ser-16 could be a major determinant of relaxation time.

Animals↗
PubMed1982

Metabolic heat production in isometric ischaemic contractions of human adductor pollicis.

The rate of intramuscular temperature rise (dT/dt) has been measured in ischaemic isometric contractions of adductor pollicis in man as an index of the rate of energy turnover. Experiments were designed so as to compare dT/dt in contractions made at a constant force but at differing intramuscular temperatures or degrees of ischaemic fatigue. In the same experiments relaxation rate was measured. As intramuscular temperature was raised from 25 to 37 degrees C, dT/dt increased with a Q10 of 1 . 8 and was closely correlated with relaxation rate. During constant force submaximal ischaemic contractions dT/dt fell, failed to recover with ischaemic rest but did so within 10 min of restoration of the circulation. During the fatiguing contraction and recovery thereafter dT/dt was linearly correlated with relaxation rate. The results show that the rate of energy turnover at a constant force is susceptible to change in association with alteration in the contractile speed of the muscle. It is suggested that relaxation rate may be used as an index of the capacity of the muscle to liberate energy.

Adult↗
PubMed1993

Intravasal use of pliable K(+)-selective electrodes in the femoral vein of humans during exercise.

Continuous recording of plasma K+ concentration ([K+]) during exercise would be valuable in determining K+ fluxes associated with muscle activation. Pliable polyvinyl chloride electrodes were constructed by incorporation of valinomycin into a polyvinyl chloride membrane attached to the end of a catheter with a 1 mm outer diameter. Through an outer catheter the electrode was inserted into the femoral vein of human subjects. The setup allowed easy in vivo calibration, and rapid changes of femoral venous [K+] of < 0.1 mmol/l could easily be detected. Drift over 1 h amounted to < 3 mV and was corrected for by analysis of blood samples. Rapid changes in femoral venous [K+] occurred during and after dynamic exercise, short and prolonged isometric contractions, and repetitive isometric contractions of the quadriceps muscle. Combined with arterial blood sampling and flow measurements, the electrodes will allow good approximation of 1) cellular K+ efflux rate associated with increased electrical activity, 2) rate of intramuscular reuptake of K+ mediated by the Na(+)-K+ pump, and 3) rate of K+ loss to or uptake from the circulation by the muscle.

Exercise↗
PubMed1961

Ultrastructure of the resting and contracted striated muscle fiber at different degrees of stretch.

Passive stretch, isometric contraction, and shortening were studied in electron micrographs of striated, non-glycerinated frog muscle fibers. The artifacts due to the different steps of preparation were evaluated by comparing sarcomere length and fiber diameter before, during, and after fixation and after sectioning. Tension and length were recorded in the resting and contracted fiber before and during fixation. The I filaments could be traced to enter the A band between the A filaments on both sides of the I band, creating a zone of overlap which decreased linearly with stretch and increased with shortening. This is consistent with a sliding filament model. The decrease in the length of the A and I filaments during isometric contraction and the finding that fibers stretched to a sarcomere length of 3.7 micro still developed 30 per cent of the maximum tetanic tension could not be explained in terms of the sliding filament model. Shortening of the sarcomeres near the myotendinous junctions which still have overlap could account for only one-sixth of this tension, indicating that even those sarcomeres stretched to such a degree that there is a gap between A and I filaments are activated during isometric contraction (increase in stiffness). Shortening, too, was associated with changes in filament length. The diameter of A filaments remained unaltered with stretch and with isometric contraction. Shortening of 50 per cent was associated with a 13 per cent increase in A filament diameter. The area occupied by the fibrils and by the interfibrillar space increased with shortening, indicating a 20 per cent reduction in the volume of the fibrils when shortening amounted to 40 per cent.

Animals↗
PubMed2004

Prospective study of the relationship between musculoskeletal and psychological complaints and electromyographic activity during isometric muscular contractions in a working population.

OBJECTIVES: This study focused on determining whether musculoskeletal and psychological complaints reported monthly over a 4-month period predicted muscular activity during and immediately after standardized worktasks and whether muscular activity during and immediately after these tasks predicted changes in complaint severity in the following 12 months. METHODS: Surface electromyography (EMG) was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles of 45 postal workers (30 women) during sustained submaximal (25% of peak force) isometric contractions (wrist extension and shoulder abduction). Self-reported health complaints were recorded monthly. Musculoskeletal and psychological complaint-severity indices (MSI and PI, respectively) were computed from complaint-severity scores (intensity score x duration score). The history of complaints over the previous 4 months was included in adjusted regression models to predict muscular activity during and immediately after submaximal contractions. Muscular activity was included in adjusted models to predict changes in the complaint severity over the subsequent 12-month period. RESULTS: A higher MSI predicted a lower EMG level in the trapezius muscle during submaximal contractions (P<0.014), whereas the PI did not predict the level of EMG in any of the muscles studied (P > 0.194). The EMG activity did not predict changes in the complaint severity over the subsequent 12 months. CONCLUSIONS: These findings may support the hypotheses of pain adaptation or the dysfunction of synergistic muscular control in relation to musculoskeletal complaints. However, the findings did not indicate that increased or decreased muscular activity is a risk factor for heightened levels of complaints in the subsequent 12 months.

Adult↗
PubMed1987

X-ray study of myosin heads in contracting frog skeletal muscle.

Using synchrotron radiation, the behaviour of the diffuse X-ray scatter was investigated in the relaxed and active phases of auxotonic and isometric contractions. Muscles were stimulated tetanically for 0.75 of a second, leaving intervals of three minutes between successive contractions. In isometric contractions the scatter is very asymmetric, which means that the myosin heads have a strongly preferred orientation. During tension rise the scatter expands in the meridional direction and contracts in the equatorial direction, the maximal local intensity change being about 20%. The shape change indicates that on average the myosin heads become oriented more perpendicularly to the fibre axis. The distribution of orientations at peak tension is quite different from that we found previously in X-ray scattering data from rigor muscles. In auxotonic contractions where muscles shorten against an increasing tension the scatter is practically circularly symmetrical. This suggests that during shortening the myosin heads go evenly through a wide range of orientations. It is concluded that the results from both the auxotonic and isometric experiments provide strong support for the rotating myosin head model. In isometric contractions the transition between the relaxed phase and peak tension is accompanied by an overall increase in scattering intensity of about 10%: this corresponds to a relative increase in the fraction of disordered myosin heads by almost 30%.

Animals↗