SEARCH · PubMed Health

Results for “Isometric Contraction”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,315 records · Page 73Linked to original sources

PubMed1993

Effect of inotropic stimulation on the negative force-frequency relationship in the failing human heart.

BACKGROUND: In severe human heart failure, an increase in frequency of stimulation is accompanied by a reduced force of contraction in vivo and in vitro. The present study was aimed to investigate whether inotropic stimulation influences the inverse force-frequency relationship in failing human myocardium. METHODS AND RESULTS: The effects of the cAMP-independent positive inotropic agents ouabain (0.01 mumol/L) and BDF 9148 (0.1 mumol/L) as well as the beta-adrenoceptor agonist isoprenaline (0.01 mumol/L and 0.1 mumol/L) on the force-frequency relationship in electrically driven papillary muscle strips from nonfailing (brain death, n = 5) and terminally failing (NYHA class IV, heart transplants, dilated cardiomyopathy, n = 22) human myocardium were studied. For comparison, we examined the effect of elevation of the extracellular Ca2+ concentration (3.2 mmol/L and 6.2 mmol/L). In nonfailing myocardium, force of contraction, peak rate of tension rise, and peak rate of tension decay increased, whereas time to peak tension and time to half relaxation decreased following an increase of stimulation frequency. In NYHA class IV, force of contraction gradually declined followed by changes of other parameters of isometric contraction. Moderate stimulation of contractility by isoprenaline (0.01 mumol/L) partly reversed the negative force-frequency relationship in NYHA class IV and preserved the positive force-frequency relationship in nonfailing myocardium. The addition of ouabain and BDF 9148 together restored completely the force-frequency relationship in NYHA class IV. In contrast, high concentrations of isoprenaline (0.1 mumol/L) and an elevation of the extracellular Ca2+ concentration enhanced the decline in force of contraction in the presence of higher stimulation frequencies. CONCLUSIONS: It is concluded that functionally important changes occur in the intracellular Ca2+ handling, leading to the negative force-frequency relationship in terminally failing human myocardium. Interestingly, the negative force-frequency relationship can be restored by agents producing positive inotropic effects by elevation of the intracellular Na+ concentration. These findings suggest that hitherto unknown changes in the intracellular ionic homeostasis occur in the failing human heart. Even though increasing [Ca2+]i in failing heart cells may be detrimental, increasing [Na+bdi may be beneficial through a mechanism independent of an increase in [Ca2+]i.

Adult↗

PubMed2005

Decreases in motor unit firing rate during sustained maximal-effort contractions in young and older adults.

Previous studies have suggested that older adults may be more resistant to muscular fatigue than young adults. We sought to determine whether motor unit firing rate might be a factor that determines the response to fatiguing exercise in young and older subjects. Motor unit recordings and muscular forces were obtained from the tibialis anterior (TA) muscle of 11 young and 8 older individuals. Maximal voluntary force was first measured during maximal-effort dorsiflexion contractions. Each subject then performed a series of 15 maximal isometric contractions, with each contraction lasting 30s. A 10-s rest period separated the fatiguing contractions. As a result of the fatiguing exercise, both subject groups demonstrated a significant loss in maximal force. The force decline was less in the older adults (20.4%) than in the young adults (33.8%). As expected, prior to muscle fatigue, maximal firing rates in the TA muscle were greater in the young (28.1+/-5.8 imp/s) than in the older adults (22.3+/-4.8 imp/s). The decrease in motor unit firing rate with fatigue was also greater in the young adults (34.9%), than in the older adults (22.0%). These results suggest that the greater fatigue-resistance exhibited by older individuals might be explained by the fact that the decline in motor unit firing rate during fatigue is greater in young persons than it is in older adults.

Action Potentials↗

PubMed1998

Inhibition of the quadriceps muscles in elite male volleyball players.

Inhibition of the quadriceps muscles was assessed in 13 elite male volleyball players using the interpolated twitch technique. This technique involves applying an electrical stimulus to the voluntarily contracted quadriceps muscles to estimate the number of motor units not fully activated during the contraction. Knee extensor moments and muscle inhibition were measured during isometric contractions at knee angles of 30 degrees and 60 degrees from full extension. A medical history of knee joint injury and pain experienced in the knee during testing were assessed. Previous knee joint injury did not affect the knee extensor moments, but produced a difference in muscle inhibition: muscle inhibition in legs with previous injuries was significantly lower than muscle inhibition in legs with no previous injury. Moderate pain in the knee during testing did not affect muscle inhibition, but was associated with reduced knee extensor moments. We consider that the loss in knee extensor moments associated with pain might be caused by atrophy of the quadriceps muscles as a consequence of the disrupted training routine. The lower muscle inhibition in volleyball players with previous injury suggested that the intense rehabilitation programme that these athletes undergo after knee injury improves muscle activation. As a result, athletes with previous knee joint injuries were able to produce the same knee extensor moments as athletes with no previous injury, probably because of their ability to recruit the available motor units more completely. This recruitment may compensate for the possible loss in muscle mass encountered during the period of injury and detraining.

Adult↗

PubMed1991

Skeletal muscle bioenergetics during frequency-dependent fatigue.

The bioenergetic correlates of skeletal muscle fatigue were assessed in vivo with phosphorus-31 nuclear magnetic resonance (31P-NMR) spectroscopy. After surgical construction of latissimus dorsi muscle ventricles, seven beagles underwent 31P-NMR spectroscopy during 12-min exercise protocols at 25- and 85-Hz stimulation frequencies and during both isovolumetric and dynamic contractions. Exercise at 85 Hz was associated with significantly greater fatigue than exercise at 25 Hz. At both frequencies, the onset of exercise was associated with a marked increase in inorganic phosphate (Pi) and a decrease in phosphocreatine (PCr). As the muscle fatigued at 85 Hz but not at 25 Hz, the phosphorus spectra returned to near baseline with a decrease in Pi and increase in PCr. For a given amount of force generated, the Pi-to-PCr ratio was higher for dynamic contractions than for isovolumetric contractions. This study indicates that high-frequency fatigue is unlikely to result from the direct effects of high-energy phosphate metabolism and that contractions producing external work consume more metabolic energy than equally forceful isometric contractions.

Animals↗

PubMed2004

Brunkow exercises and low back pain.

Brunkow exercises starting with dynamic contraction of hands and feet with fixed point on the wrist or/and heal. Dynamic contraction from the beginning, transferring through kinetic chain, leads to isometric contraction of the group of muscles, which has to be included in the exercise. Starting positions determine the group of muscles to be trained. The purpose of this study is to investigate influence of Brunkow exercises on spinal motion improvement and pain relief and to evaluate use of Brunkow exercises, as a routine method for lower back pain in Physical Medicine and Rehabilitation Centres. Thirty-four patients with symptoms of low back pain were included in study. Patients received a mean of 14.9 treatments with standard deviation of 8.96. All patients were assessed before and after the treatment for spinal mobility and flexibility as well as pain intensity. All parameters for spinal movements showed statistically significant improvement in patients with low back pain who practiced Brunkow exercise program at the end of treatment in relations to pre-treatment values, with significant difference of p<0.01 for all motions. Pain was reduced on VAS for X=1.,7 with S.D. 1.97. Difference Test was t=6.020 with significant difference p<0.01. Flexibility of spine increased, so average difference in values before and after treatment for Shober test was 0.5 cm with SD 0.65. Difference test was t=3.794 with significant difference p<0.01. Brunkow exercises for low back pain are beneficial treatment for increasing flexibility and mobility of spine and improving the pain.

Adult↗

PubMed1984

Short-term memory in the in situ canine myocardium.

We studied the effect of intracycle (short-term) mechanical history on canine myocardial performance. Intracycle muscle force and/or length history was varied, and the resultant changes in end-systolic force-length relationship were analyzed. Antecedent isotonic shortening impaired, whereas isometric force development enhanced end-systolic myocardial performance. A history of shortening concurrent with force development produced an intermediate effect. We conclude that decreasing force or length impairs whereas increasing length or force enhances performance in the same cycle. Different combinations of antecedent force and length changes affect end-systolic performance by algebraic summation (superposition) of their disparate effects. Time measurements established that 1) total systolic time varied little with altered history, 2) isotonic shortening took longer than isometric contraction in reaching a point P in the force-length plane, and 3) less time was therefore available for contraction after P with antecedent isotonic shortening than with antecedent isometric force development. This history-dependent time differential accounts for the corresponding differential in performance.

Animals↗

PubMed1996

Myoelectric and mechanical manifestations of muscle fatigue in voluntary contractions.

Endurance is a clinically relevant muscle parameter. It would be desirable to be able to estimate it without the need for a contraction sustained to exhaustion. The purpose of this work was to investigate the capability of the initial rate of spectral compression of the surface electromyographic (EMG) signal to predict mechanical endurance during sustained voluntary contractions of the human tibialis anterior muscle. Six healthy subjects performed voluntary isometric contractions of the tibialis anterior at 80, 70, 60, and 50% of the maximal voluntary contraction level. The contractions were sustained for 90, 120, 150, and 170 seconds, respectively. These intervals exceed the normal endurance time for this muscle and allow for a decrease of torque output. The slope of the median frequency, computed over the first 30 seconds of the contraction, was used to describe the initial spectral compression of the EMG signal. Significant correlations were found: 1) between contraction level and endurance time (p < 0.05 for each subject) and 2) between median (or mean) frequency slope and endurance time (p < 0.0001 for all subjects pooled together). The regression between median frequency slope and endurance time showed intersubject variations possibly related to the tibialis anterior muscle fiber type content. It is concluded that clinical use of the EMG spectral technique in assessing muscle fatigue may enable the clinician to estimate the endurance time without having the subject sustain a contraction until the point of contractile failure. This could be an advantage for some patient populations, such as the severely disabled, arthritic, or frail elderly, that might not be able to tolerate long duration contractions.

Adult↗

PubMed1995

Distinct molecular processes associated with isometric force generation and rapid tension recovery after quick release.

It was proposed by Huxley and Simmons (Nature 1971, 233:533-538) that force-generating cross-bridges are attached to actin in several stable positions. In this concept, isometric force is generated by the same mechanism as the quick tension recovery after an abrupt release of length; i.e., when crossbridges proceed from the first postulated stable position to the second and/or subsequent positions, resulting in straining of the elastic elements within the cross-bridges. Therefore, isometric force is generated by cross-bridges in the second or even subsequent stable positions. However, through mechanical measurements of skinned rabbit psoas muscle fibers, we found that during isometric contraction only the first stable state is significantly occupied; i.e., isometric force is generated by cross-bridges in the first of the stable states. Thus, isometric force and the quick tension recovery appear to result from two distinctly different molecular processes. We propose that isometric force results from a structural change in the actomyosin complex associated with the transition from a weakly bound configuration to a strongly bound configuration before the reaction steps in the Huxley-Simmons model, whereas a major component of quick tension recovery originates from transitions among the subsequent strongly bound states. Mechanical, biochemical, and structural evidence for the two distinct processes is summarized and reviewed.

Actins↗

PubMed2002

Motor unit recruitment strategies investigated by surface EMG variables.

During isometric contractions of increasing strength, motor units (MUs) are recruited by the central nervous system in an orderly manner starting with the smallest, with muscle fibers that usually show the lowest conduction velocity (CV). Theory predicts that the higher the velocity of propagation of the action potential, the higher the power at high frequencies of the detected surface signal. These considerations suggest that the power spectral density of the surface detected electromyogram (EMG) signal may give indications about the MU recruitment process. The purpose of this paper is to investigate the potential and limitations of spectral analysis of the surface EMG signal as a technique for the investigation of muscle force control. The study is based on a simulation approach and on an experimental investigation of the properties of surface EMG signals detected from the biceps brachii during isometric linearly increasing torque contractions. Both simulation and experimental data indicate that volume conductor properties play an important role as confounding factors that may mask any relation between EMG spectral variables and estimated CV as a size principle parameter during ramp contractions. The correlation between spectral variables and CV is thus significantly lower when the MU pool is not stable than during constant-torque isometric contractions. Our results do not support the establishment of a general relationship between spectral EMG variables and torque or recruitment strategy.

Adult↗

PubMed1996

Excitatory drive to the alpha-motoneuron pool during a fatiguing submaximal contraction in man.

1. This study was undertaken to examine changes of excitatory drive to the triceps surae alpha-motoneuron pool during fatiguing submaximal isometric contractions in man. Eight healthy subjects maintained isometric plantar flexions at 30 percent of maximum voluntary contraction (MVC) until the limit of endurance (range, 6-9 min). 2. Excitability of the alpha-motoneuron pool to Ia afferent stimulation (H reflex), electromyograms (EMG) and maximum compound motor unit action potentials (Mmax) from the lateral (LG) and medial heads (MG) of the gastrocnemius as well as from the soleus muscle (Sol) were recorded throughout the contraction. Superimposed maximum twitch torques (twitch occlusion) and isometric torque fluctuations (tremor) were also recorded as indirect measures of excitatory drive. 3. H reflexes were studied at different levels of underlying voluntary contraction to assess the relationship between H reflex amplitude and excitatory drive. With increasing levels of underlying contraction up to MVC, superimposed H reflex amplitude increased for LG in six subjects, for MG in all eight and for Sol in five. In the remaining cases, H reflex amplitude first increased and then plateaued between 30-50% of MVC. 4. H/Mmax ratios increased during fatigue in those muscles that showed an H reflex amplitude increase with high levels of underlying contraction. In these cases, LG and MG H/Mmax increased significantly after about 50 and 20% of endurance time onward, respectively, whereas Sol H/Mmax demonstrated a significant increase up to 40% of endurance time. 5. EMG root mean square (r.m.s.) increased linearly throughout the contraction for all three muscles, while tremor r.m.s. increased in a non-linear way, with a steeper increase from 60% of endurance time onward. Superimposed twitch amplitude decreased significantly from 25% of endurance time onward. 6. It is concluded that during fatiguing isometric contractions at 30% of MVC, the excitatory drive to the triceps surae alpha-motoneuron pool increases. This is thought to be a compensatory mechanism to facilitate recruitment of new, unfatigued motor units (MUs), and/or to increase MU firing rates. The facts that the twitch is not abolished at endurance limit and that the EMG does not attain its unfatigued MVC level are strong indications that central fatigue occurred during the sustained submaximal contraction.

Action Potentials↗

PubMed2001

The reversal of antagonists facilitates the peak rate of tension development.

OBJECTIVE: The present study was designed to test the effects of the reversal of antagonists on the peak rate of tension development (dF/dt(max)) of the elbow extensors. DESIGN: Experimental, with matched controls. SETTING: A biomechanics research laboratory. PARTICIPANTS: Twenty-six healthy women without a history of upper extremity injury or neurologic disorder, randomly assigned to experimental (n = 13) or control (n = 13) groups. INTERVENTIONS: Two groups of healthy subjects followed identical exercise protocols, except that the control group performed maximal isometric contractions of the elbow extensors and the experimental group executed a maximal isometric elbow flexion contraction immediately before a maximal elbow extension contraction. Both groups performed 5 cycles of a 2-second contraction with 22-second rest periods between agonist muscle contractions and were evaluated at 4 test sessions spaced 2 weeks apart. MAIN OUTCOME MEASURES: All measurements were done with the shoulder and elbow at 90 degrees of flexion in the sagittal plane to ensure reproducibility. A load cell was used to measure elbow extension moment and to calculate the peak rate of tension development (dF/dt(max)). Biceps and triceps brachii surface electromyographic activity was monitored concurrently. The electromyographic measures were mean spike (peak-to-peak) amplitude and mean spike frequency of the biceps and triceps brachii activity. RESULTS: Intraclass dF/dt(max) and electromyographic reliability was good (r > or = .72) in both groups. Because biceps electromyographic measures were considerably less reliable (r < or = .53), they were not included in our analysis. While dF/dt(max) increased quadratically in both groups (p <.05), the experimental group was on average 36.1Nm. s(-1) (63%) greater across sessions 2 to 4 (p <.05). In contrast, triceps electromyographic activity did not differ significantly between groups (p >.05). The means averaged across groups exhibited a quadratic increase from session 1 to session 4: 91microV or 48% for mean spike amplitude (p <.05) and 7Hz or 16% for mean spike frequency (p <.05). CONCLUSIONS: The greater dF/dt(max) for the experimental group was not associated with increased electromyographic activity. The experimental group appeared to use the biomechanic properties of the pretensioned extensor muscle-tendon complex, rather than neurologic biasing, to accomplish its power gains.

Analysis of Variance↗

PubMed1993

Fatigue and EMG changes in the masseter and temporalis muscles during sustained contractions.

Local muscle fatigue was investigated in the anterior temporal and masseter muscles during sustained isometric contractions. The volunteers' subjective perception of fatigue or pain was recorded at intervals during the sustained contractions, and this level was then compared to objective measures of fatigue. These objective measures included shift in median frequency of the power spectrum of the surface-detected EMG signal, and change in signal amplitude. The relationship was investigated while closing on a bite force meter placed between the second premolars and first molars unilaterally, and while simply clenching with the teeth together without the bite force meter. It was found that the subjective perception of fatigue, as measured on a visual analogue scale (VAS), had a nearly linear relationship with time, and that the relationship between the VAS score and median frequency shift was rather closer than the relationship with amplitude change.

Analysis of Variance↗

PubMed1984

Exogenous coenzyme Q attenuates the tension prolongation phenomenon.

In order to determine whether exogenous coenzyme Q can modify the course of mechanical changes in the myocardium during hypoxia and reoxygenation, we studied isolated rat heart muscles contracting isometrically at Lmax, 12 times per minutes at 28 degrees C. The prehypoxic mechanical parameters of 6 control muscles and 6 muscles treated with coenzyme Q10 (40 m micrograms/ml in the bath) did not differ, nor did the changes in developed tension and resting tension during 15 min of hypoxia and 45 min of reoxygenation. However, the time it took for the tension to fall to 50% of the peak value (RT1/2), the time from the onset of contraction to peak isometric tension (TPT) and TPT + RT1/2 on early reoxygenation were significantly diminished in coenzyme Q treated muscles. Thus, the tension prolongation phenomenon was attenuated in preparations of coenzyme Q treated rat heart muscle. It is concluded from this study that exogenous coenzyme Q does not modify the mechanical performance of isolated rat heart muscle during hypoxia but significantly attenuates the tension prolongation phenomenon. Since oxygen free radicals are known to induce mitochondrial dysfunction and impairment of calcium sequestration in the sarcoplasmic reticulum, the mechanism by which coenzyme Q attenuates the tension prolongation phenomenon is probably attributable to its function as an antioxidant.

Animals↗

PubMed1999

Influence of nitric oxide on vascular resistance and muscle mechanics during tetanic contractions in situ.

Studies of the effect of nitric oxide (NO) synthesis inhibition were performed in the isometrically contracting blood-perfused canine gastrocnemius-plantaris muscle group. Muscle blood flow (Q) was controlled with a pump during continuous NO blockade produced with either 1 mM L-argininosuccinic acid (L-ArgSA) or N(G)-nitro-L-arginine methyl ester (L-NAME) during repetitive tetanic contractions (50-Hz trains, 200-ms duration, 1/s). Pump Q was set to match maximal spontaneous Q (1.3-1.4 ml. min(-1). g(-1)) measured in prior, brief (3-5 min) control contraction trials in each muscle. Active tension and oxygen uptake were 500-600 g/g and 200-230 microl. min(-1). g(-1), respectively, under these conditions. Within 3 min of L-ArgSA infusion, vascular resistance across the muscle (R(v)) increased significantly (from approximately 100 to 300 peripheral resistance units; P < 0.05), whereas R(v) increased to a lesser extent with L-NAME (from approximately 100 to 175 peripheral resistance units; P < 0.05). The increase in R(v) with L-ArgSA was unchanged by simultaneous infusion of 0.5-10 mM L-arginine but was reduced with 1-3 microg/ml sodium nitroprusside (41-54%). The increase in R(v) with L-NAME was reversed with 1 mM of L-arginine. Increased fatigue occurred with infusion of L-ArgSA; active tension and intramuscular pressure decreased by 62 and 66%, whereas passive tension and baseline intramuscular pressure increased by 80 and 30%, respectively. These data indicate a possible role for NO in the control of R(v) and contractility within the canine gastrocnemius-plantaris muscle during repetitive tetanic contractions.

Animals↗

PubMed2005

Influence of ionic strength on the time course of force development and phosphate release by dogfish muscle fibres.

We measured the effects of ionic strength (IS), 200 (standard) and 400 mmol l(-1) (high), on force and ATP hydrolysis during isometric contractions of permeabilized white fibres from dogfish myotomal muscle at their physiological temperature, 12 degrees C. One goal was to test the validity of our kinetic scheme that accounts for energy release, work production and ATP hydrolysis. Fibres were activated by flash photolysis of the P(3)-1-(2 nitrophenyl) ethyl ester of ATP (NPE-caged ATP), and time-resolved phosphate (P(i)) release was detected with the fluorescent protein MDCC-PBP, N-(2[1-maleimidyl]ethyl)-7-diethylamino-coumarin-3-carboxamide phosphate binding protein. High IS slowed the transition from rest to contraction, but as the fibres approached the isometric force plateau they showed little IS sensitivity. By 0.5 s of contraction, the force and the rate of P(i) release at standard and high IS values were not significantly different. A five-step reaction mechanism was used to account for the observed time courses of force and P(i) release in all conditions explored here. Only the rate constants for reactions of ATP, ADP and P(i) with the contractile proteins varied with IS, thus suggesting that the actin-myosin interactions are largely non-ionic. Our reaction scheme also fits previous results for intact fibres.

Actomyosin↗

PubMed1990

The descending limb of the force-sarcomere length relation of the frog revisited.

1. We studied the descending limb of the force-sarcomere length relation in single frog muscle fibres using sarcomere isometric contractions. 2. Sarcomere length was measured simultaneously with two independent methods: a laser diffraction method and a segment length method that detects the distance between two markers attached to the surface of the fibre, about 800 microns apart. Both methods were used to keep sarcomeres at constant length during contraction. 3. Fibres were selected for low resting tension since it was known from previous experiments that for such fibres the force developed by fixed-end tetani is much higher than that predicted by the degree of filament overlap. 4. With fixed-end tetani, force decline with increase of sarcomere length was small between 2.0 and 3.0 microns. At a sarcomere length of 3.0 microns, force was about 90% of maximal. 5. With sarcomere isometric tetani, force was considerably lower than with fixed-end tetani. Force was maximal at about 2.1 microns and decreased to zero at about 3.6 microns. At intermediate lengths the descending limb was within 80 nm of the values predicted from filament overlap. 6. We investigated why force of fixed-end contractions was much higher than that generated by sarcomere isometric contractions. 7. During the force plateau of fixed-end tetani at sarcomere lengths longer than about 2.0-2.2 microns, sarcomeres in the fibres mid-region were not isometric, but instead stretched slowly. By measuring the force-velocity relation it was shown that this slow stretch elevates active force well beyond sarcomere isometric force. 8. Stretch of the central region was also observed during the tetanic force rise. This was shown to result in an increase of passive force that grew larger at longer sarcomere lengths. At about 3.6 microns the increase of passive force was similar to the total force generated by fixed-end contractions at this length. 9. Laser diffraction and segment length methods gave the same results, diminishing the chance that any systematic artifact underlies our findings. 10. While earlier experiments from this laboratory carried out on fibres held at constant length during contraction did not reveal a linear descending limb, the present results support the linear descending limb as a characteristic feature of isometrically contracting sarcomeres.

Animals↗

PubMed1998

Voluntary activation of human elbow flexor muscles during maximal concentric contractions.

1. To measure voluntary activation of human elbow flexor muscles during maximal concentric contractions, the twitch interpolation method was modified to enable detection of torque increments evoked by single stimuli during contractions of up to 300 deg s-1. Subjects flexed the elbow to rotate a loaded beam 'as fast as possible' (load typically 23-58 N m) from 70 deg below to 70 deg above the horizontal. Electrical stimuli were delivered to biceps brachii when the beam passed through the horizontal. Voluntary activation was estimated from the amplitude of the interpolated twitch, which was expressed as a percentage of the twitch produced by relaxed muscles shortening at the same velocity. 2. In eleven subjects, the level of voluntary activation during repeated maximal concentric contractions (median 99.4%) did not differ significantly from that during maximal isometric contractions (98.0%). Voluntary activation during maximal contractions did not depend on shortening velocity and was the same when tested at two angles 30 deg apart. 3. To induce fatigue, five subjects repeatedly lifted and lowered a heavy load at about 30 deg s-1, and continued for ten to twelve contractions after they needed assistance to continue lifting. All maintained the capacity to attain maximal levels of activation. 4. It is concluded that voluntary drive to elbow flexor muscles during maximal concentric contractions is usually maximal or near-maximal, and that this level of drive can be maintained during development of peripheral fatigue.

Adult↗

PubMed1998

The influence of stimulation frequency on force-velocity characteristics of in situ rat medial gastrocnemius muscle.

Force (and power)-velocity characteristics were determined at different stimulation frequencies in th situ rat muscle with nerve stimulation at 36 degrees C. In isometric contractions (duration, 150 ms), maximal force is generated at approximately 120 Hz. In contrast, in the high velocity (250 mm s-1) shortening contractions, frequencies of approximately 400 Hz were needed to obtain maximal dynamic force, while 120 Hz elicited only approximately 26% of the maximum. At the highest velocity measured, power production was significantly different (P < 0.05) among frequencies of 80, 120, 200 and 400 Hz, suggesting that maximal shortening velocity should be assessed using very high stimulation frequencies. However, the results further indicate that lower frequencies may be adequate in exercise studies that investigate fatigue and changes in power output during series of repetitive contractions.

Animals↗