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PubMedDate not supplied

Experimental hyperthyroidism IV. Myocardial muscle mechanics and oxygen consumption in euand hyperthyroidism.

Myocardial mechanics and oxygen consumption were studied in right ventricular papillary muscles taken from cats pretreated with cristalline L-thyroxine (1 mg/kg/day, i.p.) 8-18 days prior to the examination. Isotonic afterloaded and isometric contractions were employed. Oxygen consumption was determined polarographically. Data obtained were compared with control studies on papillary muscles taken from euthyroid cats. In isotonic afterloaded contractions the extent of shortening was nearly identical in both groups. However, maximum rate of isometric tension development and velocity of isotonic shortening were considerably increased in hyperthyroid myocardium. Myocardial oxygen consumption was significantly increased in hyperthyroidism, primarily due to an increased maximum rate of isometric tension development and-to a lesser extent-to increased isotonic contraction velocity. In isometric contractions maximum tension development (preload near Lmax) was similar in both groups. However, maximum rate of isometric tension development was markedly increased in hyperthyroidism. A close and linear relationship was found between maximum rate of isometric tension development (isometric contraction) and myocardial oxygen consumption. The results demonstrate increases of velocity factors of myocardial performance in experimental hyperthyroidism. Myocardial oxygen consumption is significantly increased. This increase in oxygen consumption quantitatively has its mechanical equivalent in increased isometric contraction velocity and, to a small amount, in increased isotonic contraction velocity.

Animals↗

PubMed2003

Strength training reduces force fluctuations during anisometric contractions of the quadriceps femoris muscles in old adults.

The greater fluctuations in motor output that are often exhibited by old adults can be reduced with strength training. The purpose of the study was to determine the effect of strength and steadiness training by old adults on fluctuations in force and position during voluntary contractions with the quadriceps femoris muscle. Healthy old adults (65-80 yr) completed 16 wk of heavy-load (80% of maximum, n = 11) strength training, heavy-load steadiness training (n = 6), or no training (n = 9). Steadiness training required subjects to match the angular displacement about the knee joint to a constant-velocity template. The Heavy-Load group experienced a 5.5% increase in muscle volume, a 25% increase in maximal voluntary contraction force, and a 26% increase in the one-repetition (1-RM) load. The Heavy-Load Steady group experienced increases of 11.5, 31, and 36%, respectively. The maximal electromyogram signal of quadriceps femoris increased by 51% in the two training groups. The coefficient of variation (CV) for force during submaximal isometric contractions did not change with training for any group. Although both training groups also experienced a reduction in CV for force during anisometric contractions with a 50% 1-RM load, the standard deviation of position did not change with time for any group. The Heavy-Load Steady group also experienced a reduction in CV for force during the training contractions performed with the 80% 1-RM load. Thus strength training reduced the force fluctuations of the quadriceps femoris muscles during anisometric contractions but not during isometric contractions.

Aged↗

PubMed2004

Preceding muscle activity influences motor unit discharge and rate of torque development during ballistic contractions in humans.

To investigate the effect of initial conditions on the modulation of motor unit discharge during fast voluntary contractions, we compared ballistic isometric contractions of the ankle dorsiflexor muscles that were produced from either a resting state or superimposed on a sustained contraction. The torque of the dorsiflexors and the surface and intramuscular EMGs from the tibialis anterior were recorded. The results showed that the performance of a ballistic contraction from a sustained contraction ( approximately 25% maximal voluntary contraction (MVC)) had a negative effect on the maximal rate of torque development. Although the electromechanical delay was shortened, the EMG activity during the ballistic contraction was less synchronized. These observations were associated with a significant decline in the average discharge rate of single motor units (89.8 +/- 3.8 versus 115 +/- 5.8 Hz) and in the percentage of units (6.2 versus 15.5% of the whole sample) that exhibited double discharges at brief intervals (= 5 ms). High-threshold units that were not recruited during the sustained contraction displayed the same activation pattern, which indicates that the mechanisms responsible for the decline in discharge rate were not restricted to previously activated units, but appear to influence the entire motor unit pool. When a premotor silent period (SP) was observed at the transition from the sustained muscular activity to the ballistic contraction (19% of the trials), these adjustments in motor unit activity were not present, and the ballistic contractions were similar to those performed from a resting state. Together, these results indicate that initial conditions can influence the capacity for motor unit discharge rate and hence the performance of a fast voluntary contraction.

Adult↗

PubMed1997

Intracellular mechanisms underlying prostaglandin F2alpha-stimulated phasic myometrial contractions.

These studies sought to test the hypothesis that prostaglandin F2alpha (PGF2alpha)-stimulated phasic myometrial contractions are characterized by the activation of the phosphatidylinositol-signaling pathway resulting in the generation of cytosolic calcium oscillations. For the experiments described in this report rat myometrial tissue was used, after the tissue was loaded with fura 2, to perform cytosolic calcium imaging studies and to perform computer-digitalized in vitro isometric contraction studies. Consistent with the above hypothesis, the cytosolic calcium-imaging studies demonstrated PGF2alpha-stimulated cytosolic calcium oscillations occurring simultaneously with phasic contractions. The in vitro isometric contraction studies confirmed that previously reported inhibitors of the phosphatidylinositol-signaling pathway and cytosolic calcium oscillation mechanisms resulted in significant inhibition of PGF2alpha-stimulated phasic myometrial contractions. In summary, these studies have provided substantial support for the hypothesis that PGF2alpha-stimulated phasic myometrial contractions are generated by intracellular signaling mechanisms involving activation of the phosphatidylinositol-signaling pathway and the production of cytosolic calcium oscillation-like phenomena.

Animals↗

PubMed1991

Acoustic myography as an indicator of force during sustained contractions of a small hand muscle.

To test the hypothesis that muscle sound amplitudes would remain constant during sustained submaximal isometric contractions, we recorded acoustic myograms from the abductor digiti minimi muscle in 12 subjects at 15, 25, 50, and 75% of a maximum voluntary contraction (MVC). Muscle sounds were detected with an omni-directional electret microphone encased in closed-cell foam and attached to the skin over the muscle. Acoustic amplitudes from the middle and end of the sustained contractions were compared with the amplitudes from the beginning of contractions to determine whether acoustic amplitudes varied in magnitude as force remained constant. Physiological tremor was eliminated from the acoustic signal by use of a Fourier truncation at 14 Hz. The amplitudes of the acoustic signal at a contraction intensity of 75% MVC remained constant, reflecting force production over time. At 50% MVC, the root-mean-square amplitude decreased from the beginning to the end of the contraction (P less than 0.05). Acoustic amplitudes increased over time at 15 and 25% MVC and were significantly higher at the end of the contractions than at the beginning (P less than 0.05). Alterations in the acoustic amplitude, which reflect changes in the lateral vibrations of the muscle, may be indicative of the different recruitment strategies used to maintain force during sustained isometric contractions.

Acoustics↗

PubMed2005

Force enhancement and relaxation rates after stretch of activated muscle fibres.

The residual force enhancement following muscle stretch might be associated with an increase in the proportion of attached cross-bridges, as supported by stiffness measurements. In this case, it could be caused by an increase in the attachment or a decrease in the detachment rate of cross-bridges, or a combination of the two. The purpose of this study was to investigate if the stretch-induced force enhancement is related to cross-bridge attachment/detachment kinetics. Single muscle fibres dissected from the lumbrical muscle of frog were place at a length approximately 20% longer than the plateau of the force-length relationship; they were maximally activated, and after full isometric force was reached, ramp stretches were imposed with amplitudes of 5 and 10% fibre length, at a speed of 40% fibre length s(-1). Experiments were performed in Ringer's solution, and with the addition of 2, 5 and 10 nM of 2,3-butanedione monoxime (BDM), a drug that places cross-bridges in a pre-power-stroke, state, inhibiting force production. The total force following stretch was higher than the corresponding force measured after isometric contraction at the corresponding length. This residual force enhancement was accompanied by an increase relaxation time. BDM, which decreases force production during isometric contractions, considerably increased the relative levels of force enhancement. BDM also increased relaxation times after stretch, beyond the levels observed during reference contractions in Ringer's solution, and beyond isometric control tests at the corresponding BDM concentrations. Together, these results support the idea that force enhancement is caused, at least in part, by a decrease in cross-bridge detachment rates, as manifested by the increased relaxation times following fibre stretch.

Analysis of Variance↗

PubMed1983

Variation of amount of muscle discharges during ballistic isometric voluntary contraction in man.

The effect of force velocity on the relation between voluntary force exertion and amount of muscle discharge was investigated. Surface e.m.g. from the adductor pollicis muscle and voluntary force curve were recorded simultaneously. Examined forces were selected from 500 to 3500 g at the peak force (about 30% of MVC) with time to peak of force curve between 60 ms and 500 ms. The amount of discharge during slow ramp contraction increased with the force increment. In ballistic contractions there was a wide variation in both the amount and rate of discharge. It is suggested that the modes of motor unit activities are different between force exertion with time to peak of less than 150 ms and force exertion with time to peak of over 150 ms.

Adult↗

PubMed1994

Differential effects of voluntary and involuntary activation on contractile characteristics of two human muscles.

To compare the maximal rate of rise of torque (MRRT) of quadriceps femoris and adductor pollicis during voluntary and involuntary contractions, subjects performed voluntary isometric contractions as rapidly as possible over the full range of force-producing capacity. Involuntary contractions were evoked with single shocks and with trains of 10 pulses at 100 Hz at increasing voltages applied directly to the femoral and ulnar nerves. There were linear relationships between MRRT and absolute torque in both muscles during involuntary and voluntary contractions. At the same absolute torque, quadriceps femoris had a higher MRRT than adductor pollicis when both were voluntarily activated (P < 0.05). However, there was no difference in MRRT between these muscles during stimulated contractions. Compared with involuntary tetanic contractions, MRRT during voluntary contractions was the same in quadriceps femoris and was less in adductor pollicis (P < 0.05). These observations suggest that in activating some muscles, such as adductor pollicis, the central nervous system may adopt the strategy of a more gradual excitation to make the contractions more task appropriate.

Adult↗

PubMed2004

Muscle-fiber conduction velocity estimated from surface EMG signals during explosive dynamic contractions.

Muscle-fiber conduction velocity (CV) was estimated from surface electromyographic (EMG) signals during isometric contractions and during short (150-200 ms), explosive, dynamic exercises. Surface EMG signals were recorded with four linear adhesive arrays from the vastus lateralis and medialis muscles of 12 healthy subjects. Isometric contractions were at linearly increasing force from 0% to 100% of the maximum. The dynamic contractions consisted of explosive efforts of the lower limb on a sledge ergometer. For the explosive contractions, muscle-fiber CV was estimated in seven time-windows located along the ascending time interval of the force. There was a significant correlation between CV values during the isometric ramp and explosive contractions (R = 0.75). Moreover, CV estimates increased significantly from (mean +/- SD) 4.32 +/- 0.46 m/s to 4.97 +/- 0.45 m/s during the increasing-force explosive task. It was concluded that CV can be estimated reliably during dynamic tasks involving fast limb movements and that, in these contractions, it may provide important information on motor-unit control properties.

Adult↗

PubMed1992

High-voltage pulsed galvanic stimulation: effects of frequency of current on blood flow in the human calf muscle.

1. Twelve healthy subjects received high-voltage pulsed galvanic stimulation (115-475 V d.c.) delivered in separate treatments of 2, 32 and 128 pulses/s for 10 min at the subject's maximum tolerable voltage while calf muscle blood flow was measured by non-invasive Whitney strain-gauge venous occlusion plethysmography. 2. The high-voltage pulsed galvanic stimulation was administered with negative polarity by an intermittent mode of 30 s on, 30 s off. Measurements of calf muscle blood flow were made during each 30 s period when the stimulus was off. The effect of one 30 s maximum isometric contraction of the calf muscles on blood flow was used as a standard for evaluating the effectiveness of high-voltage pulsed galvanic stimulation on calf muscle blood flow. 3. Significant (paired t-tests; P less than 0.05) increases in calf muscle blood flow over the preceding baseline levels occurred for the isometric contraction (322%) and for frequencies of 2 pulses/s (33.5%) and 128 pulses/s (13.36%), but not for a frequency of 32 pulses at which calf muscle blood flow increased in only six of 12 subjects. The mean increases in calf muscle blood flow at 2 and 128 pulses/s represented 11.63% and 4.0%, respectively, of that resulting from the isometric contraction. 4. A clear positive correlation between voltage level and the magnitude of increase in calf muscle blood flow was demonstrated but differed for each frequency used.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

Blood flow in the human Achilles tendon assessed by laser Doppler flowmetry.

This study evaluated microvascular perfusion in the human Achilles tendon by laser Doppler flowmetry (LDF) at rest, during vascular occlusion, and during passive stretch and isometric contraction of the triceps surae. In 40 healthy volunteers, an intratendinous needle probe was introduced 5 mm above the distal insertion, at the midportion, and at the musculotendinous junction of both legs. Values were obtained at rest and during temporary vascular occlusion. Twenty-eight of the subjects also were assessed during physical provocation by passive stretch and isometric contraction of the triceps surae. Blood flow was significantly lower near the calcaneal insertion but otherwise was distributed evenly in the tendon. The output signal showed a pulsatile variation synchronous with heart activity, and temporary vascular occlusion always caused a pronounced reduction in LDF values. Passive stretch and isometric contraction induced a progressive decline in LDF values as tension increased. Hyperemia often appeared after contraction. Tendon blood flow was higher in women and decreased with increasing age.

Achilles Tendon↗

PubMed1996

Influence on isometric muscle contraction during shoulder abduction by changing occlusal situation.

Many studies have been performed on the mandibular orthopedic repositioning appliance (MORA) as an athletic performance enhancer since the late 1970s. The concept that changing the mandibular position would increase the strength and improve the performance of subjects has been a source of controversy among various researchers. The strength of shoulder abduction and electronic activities of six muscle groups in the upper body were tested on seven subjects in this study. The normalized data for four different situations: rest position, MVC (Maximum Voluntary Contraction) in an intercuspal position, MVC with placebo, and MVC with MORA in supported rest position were analysed. Statistical analysis demonstrated that the MVC with MORA was significantly stronger than the rest position for upper appendage strength. The electronic activities of all muscle groups were significantly greater with MORA than those in rest position, and some of them were significantly greater with MORA than with either MVC or placebo situation. THe results suggested that the muscle activities of the upper appendage were increased by biting MORA in the supported rest position during the shoulder abduction performance.

Adult↗

PubMed2005

Impact of stretch-shortening cycle rest interval on in vivo muscle performance.

INTRODUCTION: Overuse and overtraining models have implicated both metabolic and mechanical disturbances as contributors to muscle damage and performance decrement but have produced equivocal results. The purpose of the present study was to investigate the impact of rest interval between sets of stretch-shortening cycles (SSC) on static and dynamic muscle performance METHODS: Animals were randomly assigned to groups (N = 8 per group) of 10-s, 1-min, or 5-min rest between sets of isometric contractions (10-s, 1-min, or 5-min CON), or SSC (10-s, 1-min, or 5-min INJ). The dorsiflexor muscles were exposed in vivo to either seven sets of 10 SSC (500 degrees . s) or seven sets of isometric contractions. Performance was characterized by isometric exertions and positive, negative, and net work, at pretest, during the sets of SSC, and 48 h postexposure RESULTS: The isometric force at 48 h after the 10-s and 5-min INJ groups were statistically different from the 1-min group (P < 0.05), whereas there was no difference in the CON groups. Negative work of the INJ groups were statistically lower at 48 h than pretest values (P < 0.05), whereas there was no change in positive work. Of the real-time parameters, there was a difference in minimum force and positive work (P < 0.05) with treatment with the 10-s INJ group being most affected. CONCLUSION: SSC conducted at shorter work-rest cycles resulted in a more profound isometric force decrement 48 h postexposure, and in real-time changes in isometric prestretch force and positive work. These results indicate that short rest intervals between athletic or vocational tasks of heightened physical exertion (i.e., high intensity) may adversely affect performance and increase injury susceptibility.

Animals↗

PubMed1993

[Determining normal ultrasound values of the supra- and infraspinatus muscles].

Sonographic investigations concerning thickness of tendons (in longitudinal sections) and muscles (in resting position and isometric contraction) were performed on 60 volunteers without shoulder complaints. Increase in muscle thickness within isometric contraction did not depend on thickness in resting position. There was no side difference in tendon thickness. Since the observed values (median 4-5 mm) were far below measurements cited in the greater part of literature, we tried to verify our data by sonographic and anatomic examination of shoulders of corpses. We conclude that, by standardising, thickness measurement of tendons and muscles might become more important in diagnosis of shoulder complaints.

Adult↗

PubMed1974

The effect of the performance of work on total energy output and metabolism during muscular contraction.

1. The production of heat (h) and work (w) and the changes in phosphocreatine (PCr) and ATP have been measured on tetanized isolated frog muscles (unpoisoned and in oxygen at 0 degrees C) during shortening at constant velocity and during isometric contraction (both without relaxation). The former type of contraction was designed to maximize the fraction w/(h + w); the latter to minimize it.2. The duration of the isometric contraction was made considerably longer than that of the isovelocity contraction so that the (h + w) productions during the two contractions were approximately equal.3. The PCr break-down during the working contraction was considerably greater than that during the isometric contraction.4. No detectable ATP changes occurred.5. The break-down of PCr is sufficient to account for the work evolved: there is no reason to suppose that the work comes from an unidentified source.6. In both types of contraction extra energy is evolved that cannot be accounted for by concurrent splitting of PCr. The time course of evolution of this extra energy is similar in all types of contraction, suggesting that it may arise from a process other than cross-bridge interaction.7. The results are discussed in terms of current cross-bridge theory and muscle kinetics. The mean cycle times of a cross-bridge during working and isometric contractions are 0.12 sec and 0.34 sec respectively. During the working contraction cross-bridges spend about one quarter of the time attached to actin filaments.

Adenosine Triphosphate↗

PubMed1985

Storage and release of mechanical energy by active muscle: a non-elastic mechanism?

In frog muscle fibres, tetanically stimulated at a sarcomere length of about 2 micron, stretched at a velocity of 1 lengths-1 and released against a force equal to the maximum isometric, P0, a phase of rapid isotonic shortening takes place after release. As the amplitude of the stretch is increased from 1.5 to 9% of the initial length: (1) the amount of rapid isotonic shortening increases up to 9-10 nm per half sarcomere and (2) the stiffness of the fibre (an indication of the number of bridges attached) decreases to a value about equal to that measured during an isometric contraction. If a 5-10 ms delay is left between the end of stretch and release, the amount of rapid isotonic shortening increases to about 12 nm hs-1. A 300-500 ms delay, however, results in a decrease in rapid isotonic shortening to about 5 nm hs-1 and also results in a velocity transients against P0 that are similar to those described during release from a state of isometric contraction. It is concluded that the force attained after large, fast stretches is due to a greater force developed by each bridge and not to a greater number of bridges. After the elastic recoil (when the force is suddenly reduced to P0), these strained bridges are able to shorten by about 12 nm hs-1, suggesting that, during and immediately after stretching, they are charged to levels of potential energy greater than those attained in an isometric contraction.

Animals↗

PubMed1986

Potassium and sodium shifts during in vitro isometric muscle contraction, and the time course of the ion-gradient recovery.

Intracellular potassium ([K+]i), interstitial potassium ([K+]inter), intracellular sodium ([Na+]i), and resting membrane potential (RMP) were measured before and after repetitive stimulation of mouse soleus and EDL (extensor digitorum longus) muscles. At rest, RMP was -69.8 mV for soleus and -74.9 mV for EDL (37 degrees C). [K+]i was 168 mM and 182 mM, respectively. In soleus, free [Na+]i was 12.7 mM. After repetitive stimulation (960 stimuli) RMP had decreased by 11.9 mV for soleus and by 18.2 mV for EDL. [K+]i was reduced by 32 mM and 48 mM, respectively, whereas [K+]inter was doubled. In soleus [Na+]i had increased by 10.6 mM, demonstrating that the [K+]i-decrease is three times higher than the [Na+]i-increase. It is concluded that this difference reflects different activity induced movements of Na and K, and that the difference is not due to the Na/K pumping ratio. The possible involvement of the potassium loss in muscle fatigue is discussed. After stimulation RMP recovered with a time constant of 0.9 min for soleus and 1.5 min for EDL. Within the first minutes after stimulation the intracellular potassium concentration increased by 20.4 mM/min for soleus and 21.7 mM/min for EDL. Free [Na+]i decreased with less than 10 mM/min. The mechanisms underlying the different rate of changes are discussed.

Animals↗

PubMed2006

Reliability of normalisation methods for EMG analysis of neck muscles.

Acceptable reliability of normalisation contractions in electromyography (EMG) is paramount for testing conducted over a number of days or if normal laboratory strength testing equipment is unavailable. This study examined the reliability of maximal voluntary isometric contractions (MVIC) and sub-maximal (60%) isometric contractions for use in neck muscle EMG studies. Surface EMG was recorded bilaterally from eight sites around the neck at C4/5 level from five healthy male subjects. Subjects performed MVIC and sub-maximal normalisation contractions using an isokinetic dynamometer (ID) and a portable cable dynamometer with attached strain gauge (PCD) in addition to a MVIC against a manual resistance (MR). Subjects were tested in flexion, extension, left and right lateral bending and were retested by the same tester within a two-week period. Intra class correlation co-efficients (ICC) were calculated for each testing method and contraction direction and a mean ICC was calculated across all contraction directions. All normalisation methods produced excellent within-day reliability (mean ICC >0.80) but only the MVICs using the ID and PCD had acceptable reliability when assessed between-days. This study confirmed the validity of using MVICs elicited using the ID and PCD as reliable reference contractions for the normalisation of neck EMG.

Adult↗