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PubMed1986

The time course of ATP cleavage by contracting amphibian and mammalian skeletal muscles.

The pattern of energy liberation by tetanically stimulated muscle is reasonably well defined. The energy liberation and ATPase rate are affected by a variety of mechanical factors which, aside from the case of shortening mammalian muscles, are well documented. The amount of ATP consumed by processes associated with the calcium release and sequestration is not trivial, amounting to 30-40% of the energy liberation in an isometric contraction. The energy liberated by isometric and shortening muscles is ultimately accounted for by known chemical reactions. However, there are two instances in which the time course of energy liberation does not correspond to the time course of high energy phosphate utilization. In an isometric tetanus, 30-40 mJ of energy per gram of muscle is produced by reactions probably associated with intracellular circulation of calcium, but not immediately involving ATP hydrolysis. Second, in rapidly shortening muscles, an unknown reaction can produce 6-7 mJ of energy per gram of muscle which is not immediately associated with high energy phosphate splitting. However, immediately after the cessation of shortening, this reaction is reversed by an ATP hydrolysis. Finally, a technique is now available which permits one to perform transient kinetic studies on skinned muscle fibers which are shortening and/or developing force. This development should enable the design of experiments in which the effect of mechanical conditions upon specific steps of the ATPase mechanism can be examined.

Adenosine Triphosphate↗

PubMed1981

Blood flow in "red" and "'white" calf muscles in cats during isometric and isotonic exercise.

Muscle blood flow was measured with the radioactive microspheres technique in plantar flexors of seven cats. Right leg performed rhythmic isotonic and left leg sustained isometric contractions. Flows were determined at rest, during stimulation periods, and immediately after 90 sec of high frequency stimulation. During isotonic contraction, flow to m. soleus (a "red muscle") increased progressively as stimulation increased: 0.06 +/- 0.02, 0.21 +/- 0.05, 0.42 +/- 0.12, and 0.26+/- 0.06 ml/min/g during rest, 10 Hz, 50 Hz, and after 50 Hz stimulation frequencies, respectively. Corresponding values for gastrocnemius medialis were 0.06 +/- 0.01, 0.38 +/- 0.05, 0.27 +/- 0.03, and 0.28 +/- 0.06 (x +/- S.E.). These last values were characteristic also for the other "'white" muscles investigated: gastrocnemius lateralis, and plantaris. During sustained isometric exercise at 5 and 25 Hz stimulation frequency the patterns were similar, except that higher flows were observed, and during recovery flows actually increased again in the "white" muscles. Blood pressure did not change. Resting flows were similar in innervated and denervated muscles. These data demonstrate that during isotonic as well as isometric exercise there is an increased resistance to flow with increasing muscle tension in "white" but not in "red" muscles. This supports the notion that a condition of relative ischaemia exists in "white" muscles during both isometric and isotonic contractions which is more pronounced during isometric exercise. The results corroborate previous studies on isometric exercise and suggest that the pattern during isotonic exercise is similar.

Animals↗

PubMed1992

Investigations of the subtype of alpha 2-adrenoceptor mediating contractions of the human saphenous vein.

1. We have examined the effects of a series of alpha 2-adrenoceptor antagonists against isometric contractions to noradrenaline in human saphenous vein, and correlated these potencies with affinities for the alpha 2A-ligand binding site of human platelet and the alpha 2B-ligand binding site of rat kidney. 2. The alpha 2B-selective adrenoceptor antagonists, prazosin, ARC 239 and HV 723 showed high, and the alpha 2A-selective antagonist BRL 44408 showed low, potency in human saphenous vein. 3. Potency in human saphenous vein correlated better with affinity for the alpha 2B-ligand binding site (r = 0.71, n = 12, P less than 0.01) than with affinity for the alpha 2A-ligand binding site (r = 0.56, n = 12, non significant). 4. It is concluded that the postjunctional alpha 2-adrenoceptor of human saphenous vein resembles an alpha 2B-ligand binding site more than an alpha 2A-ligand binding site.

Adrenergic alpha-Antagonists↗

PubMed2006

Exertion dependent alternations in force fluctuation and limb acceleration during sustained fatiguing contraction.

The study was conducted to contrast exertion-dependent changes in electromyography (EMG), force fluctuation (FF), and limb acceleration (LA) during isometric contraction to attain a versatile picture of muscle fatigue. Fifteen volunteers performed sustained index abduction at 25 and 75% maximal voluntary contractions (MVC) until failure at the tasks; meanwhile, changes in temporal/spectral features of force, muscle activity of the first dorsal interosseous (FDI), and acceleration of the index and hand were monitored. The results showed a manifest increase in all recorded signals for the 25% MVC paradigm, especially for LA, which demonstrated the largest increment in amplitude. In addition to progressive enhancement of the mechanical coupling of the metacarpophalangeal (MCP) joint, the 25% MVC paradigm added to EMG-FF and EMG-LA coherences (CohEMG-FF and CohEMG-LA) at 8-12 Hz and the shift of the spectral peak of the LA to higher frequencies. In contrast, the 75% MVC paradigm did not modulate significantly the spectral peak of LA. Also, CohEMG-FF, CohEMG-LA at 8-12 Hz, and the mechanical coupling of the MCP joint, were conversely undermined consequent to the high exertion paradigm. The present study suggests that LA was most susceptible to muscle fatigue following sustained contraction at a lower exertion level, and diverging alternations among various physiological signals ascribed to exertion-dependent contributions of central and peripheral origins to muscle fatigue.

Acceleration↗

PubMed2000

Changes in electromyogram during upper limb muscle contraction induced by resistive loaded breathing in humans.

Expiratory only resistive loaded breathing (RL) reduces high energy electromyogram (EMG) power (EH) during an isometric contraction of a leg extensor muscle, but not an arm flexor. An interaction between afferent activity during expiratory RL and inspiratory non-loaded phases of breathing, which the contraction spanned, could have accounted for the reduced EH in these long contractions. Therefore this study tested the hypothesis that brief arm extensor muscle contractions (70% of maximal force), performed during a single phase of expiratory RL, would also exhibit reduced EH. Surprisingly, EH in triceps, but not biceps brachii was reduced significantly when the contraction was performed during inspiratory RL rather than expiratory RL. The results suggest that either (a) short and prolonged contractions or (b) motor drive to arm and leg extensors are affected differently by RL.

Adult↗

PubMed1989

Temporal relationship between blood flow changes and release of ions and metabolites from muscles upon single weak contractions.

The temporal changes in muscular blood flow and in the release of ions and metabolites have been studied during and after short-lasting isometric contractions. Blood velocities in the human femoral artery were measured using pulsed bidirectional Doppler ultrasound equipment during single contractions of the quadriceps muscle group. Contractions of 5 s and 30 s duration and at a tension of 10% MVC (maximal voluntary contraction) were investigated. Even the contractions of 5 s duration caused conspicuous post-contraction increases in femoral arterial flow. The following substances were analysed in the femoral vein during and following the contractions: potassium, lactate, inorganic phosphate, calcium, sodium, oxygen and haemoglobin. Following contractions of 5 s duration, an increase in the venous potassium concentration (by 0.9 mM) was found, but there was no change in concentration of any of the other substances analysed. Following contractions of 30 s duration a more marked increase in the venous potassium concentration (by 1.5 mM) was found, and in addition substantial changes in venous lactate concentration and oxygen saturation. There were small but statistically significant changes in the concentrations of inorganic phosphate and calcium, but no change in the sodium concentration. The time-course and magnitude of the changes in venous potassium concentration fit well with the idea that potassium is important in the initiation and regulation of the functional hyperaemia in contracting muscles.

Adult↗

PubMed1999

The measurement of force/velocity relationships of fresh and fatigued human adductor pollicis muscle.

The purpose of the study was to obtain force/velocity relationships for electrically stimulated (80 Hz) human adductor pollicis muscle (n = 6) and to quantify the effects of fatigue. There are two major problems of studying human muscle in situ; the first is the contribution of the series elastic component, and the second is a loss of force consequent upon the extent of loaded shortening. These problems were tackled in two ways. Records obtained from isokinetic releases from maximal isometric tetani showed a late linear phase of force decline, and this was extrapolated back to the time of release to obtain measures of instantaneous force. This method gave usable data up to velocities of shortening equivalent to approximately one-third of maximal velocity. An alternative procedure (short activation, SA) allowed the muscle to begin shortening when isometric force reached a value that could be sustained during shortening (essentially an isotonic protocol). At low velocities both protocols gave very similar data (r2 = 0.96), but for high velocities only the SA procedure could be used. Results obtained using the SA protocol in fresh muscle were compared to those for muscle that had been fatigued by 25 s of ischaemic isometric contractions, induced by electrical stimulation at the ulnar nerve. Fatigue resulted in a decrease of isometric force [to 69 (3)%], an increase in half-relaxation time [to 431 (10)%], and decreases in maximal shortening velocity [to 77 (8)%] and power [to 42 (5)%]. These are the first data for human skeletal muscle to show convincingly that during acute fatigue, power is reduced as a consequence of both the loss of force and slowing of the contractile speed.

Adult↗

PubMed2005

Strength training: isometric training at a range of joint angles versus dynamic training.

Strength training with isometric contractions produces large but highly angle-specific adaptations. To contrast the contractile mode of isometric versus dynamic training, but diminish the strong angle specificity effect, we compared the strength gains produced by isometric training at four joint angles with conventional dynamic training. Thirty-three recreationally active healthy males aged 18 - 30 years completed 9 weeks of strength training of the quadriceps muscle group three times per week. An intra-individual design was adopted: one leg performed purely isometric training at each of four joint angles (isometrically trained leg); the other leg performed conventional dynamic training, lifting and lowering (dynamically trained leg). Both legs trained at similar relative loads for the same duration. The quadriceps strength of each leg was measured isometrically (at four angles) and isokinetically (at three velocities) pre and post training. After 9 weeks of training, the increase in isokinetic strength was similar in both legs (pooled data from three velocities: dynamically trained leg, 10.7%; isometrically trained leg, 10.5%). Isometric strength increases were significantly greater for the isometrically trained leg (pooled data from four angles: dynamically trained leg, 13.1%; isometrically trained leg, 18.0%). This may have been due to the greater absolute torque involved with isometric training or a residual angle specificity effect despite the isometric training being divided over four angles.

Adaptation, Physiological↗

PubMed2026

Acute pre-exercise oral intake of cacao polyphenols attenuates central fatigue development during sustained low-intensity ankle dorsiflexion exercise in healthy young male adults.

This double-blind, randomized, crossover study investigated whether acute cacao polyphenol (CP) ingestion attenuates neuromuscular fatigue in 19 healthy young male adults. After consuming CP or placebo, participants performed sustained isometric ankle dorsiflexion at 25% of maximal voluntary isometric contraction (MVIC) torque until task failure. Neuromuscular function (MVIC torque, voluntary activation [VA%], doublet torque) and electromyographic activity were assessed before, immediately after, and during recovery (1 and 5 min). Salivary chromogranin A and α-amylase were measured at baseline and 1 min post-task. Only VA% and chromogranin A showed significant condition × time interactions (p = 0.027-0.039). VA% significantly decreased from before to immediately after the task and 1 min post-task in both conditions (p ≤ 0.002), with significantly higher values in the CP condition than in placebo immediately after and 1 min post-task (p ≤ 0.037). Chromogranin A significantly increased from baseline to 1 min post-task in the placebo condition (p < 0.001) but not in the CP condition (p = 0.083). These findings suggest that acute CP ingestion attenuates the decline in central motor drive and autonomic stress responses following strenuous exercise, without altering performance or peripheral recovery, supporting its potential as a nutritional strategy for modulating central fatigue.

Humans↗

PubMed2001

Involvement of cyclic GMP in nitric-oxide-induced gastric relaxation Comparison of the actions of cyclic GMP and cyclic AMP.

BACKGROUND: Smooth muscle relaxation induced by various agents that increase the cellular levels of cyclic nucleotides (cAMP and cGMP) is accompanied by a decrease in intracellular Ca2+ concentration. However, little is known about the differences between the inhibitory effects of cAMP and cGMP on the contraction of smooth muscle. OBJECTIVE: To compare the effects and underlying mechanisms of cAMP and cGMP on the inhibition of gastric smooth muscle contraction, cyclic nucleotide promoting agents, as well as cell membrane permeable cyclic nucleotides were used. METHODS: Isometric contraction was measured from circular muscle strips prepared from the fundus of cat stomach in a cylinder-shaped chamber filled with Krebs-Ringer solution (pH 7.4, temperature 36 degrees C) bubbled with 5% CO2 in O2. The level of inositol phosphates (IPs) was measured. RESULTS: Forskolin and sodium nitroprusside significantly inhibited acetylcholine (ACh)-induced gastric smooth muscle contraction and increased the cellular levels of cAMP and cGMP, respectively. Direct application of 8-Br-cAMP and 8-Br-cGMP also significantly inhibited ACh-induced contraction. Both verapamil and TMB-8 inhibited ACh-induced contraction. The combined inhibitory effect of verapamil and TMB-8 was significantly greater than the effect of either one, separately. Forskolin or sodium nitroprusside similarly augmented the effect of verapamil. However, the inhibitory effect of TMB-8 was augmented only by 8-Br-cGMP or sodium nitroprusside but not by 8-BrcAMP or forskolin. Forskolin and 8-Br-cAMP significantly inhibited the formation of inositol phosphates stimulated by ACh. CONCLUSIONS: cAMP inhibits the contraction mechanism associated with intracellular Ca2+ mobilization as well as extracellular Ca2+ influx, while cGMP inhibits contraction by inhibiting the mechanism associated with extracellular Ca2+ influx.

Acetylcholine↗

PubMed1987

The effects of beta-blockade on electrically stimulated contraction in fatigued human triceps surae muscle.

The contractile characteristics of the triceps surae muscle group were examined before and after repeated isometric contractions in two groups of eight healthy young males. Single twitches and trains of stimuli at 10, 20, 50 and 100 Hz were delivered to the muscle using supramaximal voltages. Subjects were treated with beta-blockade (2 X 80 mg oral propranolol, beta-b) or matched placebo in a double-blind crossover design. Four different exercise conditions were studied: (I) maximal voluntary contraction (MVC); (II) MVC during circulatory occlusion; (III) electrical stimulation at 20 Hz using 50% of voltage required for maximal torque production; and (IV) electrical stimulation with occlusion. Each contraction was for 5 s with 5 s recovery. Total duration of exercise was 10 min for non-occluded contractions and to a 50% decline in torque output with occlusion. At rest prior to exercise, maximal voluntary contraction was significantly reduced (5.7%) by beta-b during 40 observations in 16 subjects. Following exercise without occlusion (I and III), the reduction in torque output of the muscle at 10 and 20 Hz stimulation was generally greater during beta-b than placebo. This low frequency fatigue was longer-lasting with beta-b. The shorter lasting reduction in torque at 50 and 100 Hz was generally not different between beta-b placebo. After exercise with occlusion (II and IV), the torque output at all stimulation frequencies was reduced to a similar extent in both placebo and beta-b at most comparison points. Twitch responses after exercise with occlusion showed decreases in peak tension and time to peak tension and a lengthening of one-half relaxation time in both placebo and beta-b. It was concluded that the greater reduction in torque output of the triceps surae muscle group at low frequencies during beta-b was probably a consequence of a reduction in blood flow relative to the placebo treatment. This relative low frequency fatigue could be responsible for the increased perception of effort in patients exercising during beta-blocker therapy.

Adult↗

PubMed2005

Can mechanical myotonometry or electromyography be used for the prediction of intramuscular pressure?

The aim of the study was to characterize the electromechanical properties of skeletal muscle during isometric loading as well as to assess the potential of estimating intramuscular pressure by electrical and mechanical methods. Simultaneous electromyography (EMG), mechanical myotonometry (MYO, frequency and decrement of decay) and intramuscular pressure (IMP) measurements were conducted at rest and during short-term and long-term isometric contractions in patients with chronic pain in the anterior leg or dorsal forearm. The EMG amplitude and MYO(freq) accounted significantly (24-73%, p < 0.0001) for the variations in the IMP under short-term isometric loading. The IMP, EMG and MYO(freq) increased linearly with the relative muscle load (r = 0.868-0.993, p < 0.05). Mean values of EMG amplitudes at the contraction levels of 75% and 100% maximum voluntary contraction (MVC) and MYO(freq) values at all contraction levels (0-100% MVC) were higher for subjects with pathological values of IMP than for those with IMP values in the normal range. Total changes in IMP and EMG amplitude during 1 min isometric contraction were linearly interrelated (r = 0.747, p < 0.0001). We conclude that both surface electromyography and myotonometry parameters are indicative of intramuscular pressure, but neither of these methods can be used alone to diagnose non-invasively chronic compartment syndrome with acceptable accuracy.

Adolescent↗

PubMed1985

Energetic aspects of skeletal muscle contraction: implications of fiber types.

In this chapter fundamental energetic properties of skeletal muscles as elucidated from isolated muscle preparations are described. Implications of these intrinsic properties for the energetic characterization of different fiber types and for the understanding of locomotion have been considered. Emphasis was placed on the myriad of physical and chemical techniques that can be employed to understand muscle energetics and on the interrelationship of results from different techniques. The anaerobic initial processes which liberate energy during contraction and relaxation are discussed in detail. The high-energy phosphate (approximately P) utilized during contraction and relaxation can be distributed between actomyosin ATPase or cross-bridge cycling (70%) and the Ca2+ ATPase of the sacroplasmic reticulum (30%). Muscle shortening increases the rate of approximately P hydrolysis, and stretching a muscle during contraction suppresses the rate of approximately P hydrolysis. The economy of an isometric contraction is defined as the ratio of isometric mechanical response to energetic cost and is shown to be a fundamental intrinsic parameter describing muscle energetics. Economy of contraction varies across the animal kingdom by over three orders of magnitude and is different in different mammalian fiber types. In mammalian skeletal muscles differences in economy of contraction can be attributed mainly to differences in the specific actomyosin and Ca2+ ATPase of muscles. Furthermore, there is an inverse relationship between economy of contraction and maximum velocity of muscle shortening (Vmax) and maximum power output. This is a fundamental relationship. Muscles cannot be economical at developing and maintaining force and also exhibit rapid shortening. Interestingly, there appears to be a subtle system of unknown nature that modulates the Vmax and economy of contraction. Efficiency of a work-producing contraction is defined and contrasted to the economy of contraction. Unlike economy, maximum efficiency of work production varies little across the animal kingdom. There are difficulties associated with the measurement of maximum efficiency of contraction, and it has yet to be determined unequivocally if the maximum efficiency of contraction varies in different fiber types. The intrinsic properties of force per cross-sectional area, economy, and Vmax determine the basic energetic properties of skeletal muscles. Nonetheless, the mechanics and energetics of skeletal muscles in the body are profoundly influenced by muscle architecture, attachment of muscles to the skeleton, and motor unit organization.(ABSTRACT TRUNCATED AT 400 WORDS)

Actomyosin↗

PubMed2004

Roles of motor-unit recruitment in producing force variability of simulated muscle contractions.

The purpose of this study was to examine the effect of motor-unit recruitment on force variability by using computer simulated isometric contractions of a hand muscle (i.e., first dorsal interosseus). The force was simulated at 10 levels of excitation, ranging from 10 to 100% of maximum. Two recruitment conditions were simulated to compare the relative effect of motor-unit recruitment (MUR) on the relationship of force variability and level of force. One condition (40%MUR) recruited all motor units at 40% of the maximum excitation level, and the other (50%MUR) recruited all motor units at 50% of the maximum. The 40%MUR condition had a greater number of motor units than the 50%MUR group before the excitation level reached 50% of the maximum. The results showed that force variability increased at a faster rate before the completion of motor-unit recruitment and, thereafter, increased at a slower rate. In addition, the 40%MUR group showed greater force variability than the 50%MUR group. These data suggest that motor-unit recruitment is an important factor in causing force variability.

Biomechanical Phenomena↗

PubMed1995

Differential tachykinin receptor subtype activation in capsaicin and KCl contractions of guinea pig trachealis.

We assessed the role of endogenously secreted tachykinins in mediating contraction caused by potassium chloride (KCl) in guinea pig tracheal smooth muscle (TSM) strips in vitro. Maximal isometric contraction was elicited with approximately 45 mM KCl and was 196 +/- 8% of the response to electrical field stimulation (% EFS) in the same tissues. Muscarinic receptor blockade with atropine modestly attenuated this contraction caused by KCl to 175 +/- 9 %EFS (P < 0.05), and treatment with a selective neurokinin subtype 1 (NK1) receptor antagonist, LY-297911, caused even greater inhibition of KCl-elicited contraction to 124 +/- 8 %EFS (P < 0.001). By contrast, SR-48968, a selective NK2 antagonist, had no effect on contraction caused by KCl (183 +/- 9 %EFS; P = NS vs. KCl alone). However, given together at the same concentration, SR-48968 augmented the inhibition of contraction caused by LY-297911 to 93 +/- 15 %EFS (P < 0.05 vs. LY-297911 alone). In contrast to the effect on KCl-induced contraction, LY-297911 caused only moderate inhibition of the contraction caused by capsaicin to 81 +/- 13 %EFS (P < 0.05 vs. control, 114 +/- 15 %EFS), whereas SR-48968 caused substantial attenuation of contraction caused by capsaicin to 23 +/- 5 %EFS (P < 0.005 vs. LY-297911). We demonstrate that a significant portion of the contraction caused by KCl, in addition to capsaicin, is elicited in guinea pig TSM through neurokinin secretion. However, NK1 receptors predominantly mediate contraction caused by KCl, and NK2 receptors predominantly mediate contraction elicited by capsaicin in guinea pig airway smooth muscle.

Animals↗

PubMed2000

Quadriceps activation and perceived exertion during a high intensity, steady state contraction to failure.

The ability to sustain a high-intensity, steady-state muscle contraction may have differential effects on neuromuscular activation and perceived exertion. The purpose of this study was to examine changes in neuromuscular activation and perceived exertion at a near-maximal steady-state contraction of the quadriceps in healthy men. Seventeen healthy, college-aged male volunteers were studied during isometric contractions equivalent to 80% of the maximum voluntary contraction (MVC). Perceived exertion was measured with a modified category-ratio scale (CR-10). The CR-10 scale was anchored with one high anchor at 100% MVC and one low anchor at 10% MVC. Subjects then performed an 80% MVC for as long as they could sustain it. Subjects were asked to rate the feelings in their quadriceps every 5 s during the contraction. The results demonstrated significant increases in neuromuscular activation of the vastus medialis and vastus lateralis muscles (P < 0.05) during the 80% MVC, but there were no significant muscle by time interactions. The results also demonstrated a significant increase (P < 0.05) in perceived exertion during the 80% MVC. Neuromuscular activation of both muscles, and perceived exertion, were found to increase in linear (P < 0.05) and quadratic (P < 0.05) trends. Alterations in motor unit discharge properties or impairments in muscle fiber membrane excitability may account for nonlinear increases in vastii muscle activation and perceived exertion.

Adult↗

PubMed1998

Trunk muscle co-contraction increases during fatiguing, isometric, lateral bend exertions. Possible implications for spine stability.

STUDY DESIGN: Surface electromyographic activities were recorded from bilateral trunk muscles as test participants maintained a 50% maximum, voluntary, isometric, lateral bend contraction to volitional exhaustion. OBJECTIVES: To challenge the trunk with a prolonged lateral bend task and observe the responses of the agonist and antagonist muscles to the resulting fatigue. SUMMARY OF BACKGROUND DATA: Lateral bend exertions of the trunk have been identified as a risk factor for injury to spine tissues. However, little is known about the response of spine muscles to fatigue and the subsequent implications for spine stability, joint loading, and tissue injury. Surface electromyography provides a window on muscle loading and progressive fatigue. METHODS: Eleven male participants performed a set of maximum lateral bend exertions at the start of the trial, then maintained an upright standing posture while resisting 50% of the maximum moment until volitional exhaustion, then performed another set of maximum contractions. All contractions were isometric. Measurements were made, throughout each contraction, of the lateral bend moment and abdominal and trunk extensor electromyographic activities at six bilateral surface electromyography sites. Electromyographic amplitude and mean power frequency were calculated with 500-millisecond segments recorded serially every 800 milliseconds. Mean values were calculated with data from the first and last 5% of the trial durations. Statistics determined if fatigue had a significant effect on maximum moment and the electromyographic statistics for each muscle site. RESULTS: Fatigue resulted in a significant decrease in maximum lateral bend moment and increase in moment variability. Decreases in mean power frequency, indicating fatigue, were observed in three agonists (the trunk extensors) and one antagonist muscle. Unfatigued agonist electromyographic amplitudes correlated well with the mechanical advantage of muscles to generate lateral bend moments. Unfatigued antagonist activities were low except for the thoracic erector spinae. The agonists and antagonists demonstrated average increases of 17% and 8%, respectively, when pooled across muscles. Much of this change was predicted to have been associated with increases in co-contracting muscle forces. CONCLUSIONS: The trunk responded to a prolonged, lateral bend contraction by increasing co-contraction as agonist trunk muscles fatigued. It was proposed that the fatigue compromised neural coordination and that the co-contraction served to maintain spine stability.

Adult↗

PubMed1986

Effects of fatigue and recovery on electromyographic and isometric force- and relaxation-time characteristics of human skeletal muscle.

Effects of fatigue produced by a maintained 60% isometric loading on electromyographic and isometric force-time and relaxation-time characteristics of human skeletal muscle were studied in 21 males accustomed to strength training. Fatigue loading resulted in a slight but not significant change in the maximal integrated EMG of a maximal isometric contraction, and a large decrease (20.4 +/- 6.3%, p less than 0.001) in maximal force. Fatigue loading increased (p less than 0.05-0.01) neural activation of the muscles during rapidly produced submaximal isometric forces, but had a considerable adverse effect (p less than 0.001) on the corresponding force-time characteristics. Correlations between the relative changes after fatigue in the IEMG/force ratio at the maximal force level, and in the IEMG/force ratios of the early phases of the force-time curve were not significant, but gradually became significant (p less than 0.01) at higher force levels. The average IEMG of the muscles in the relaxation phase of contraction remained unaltered by fatigue, while a marked deleterious change in the relaxation-time variables (p less than 0.001) occurred concomitantly. During the subsequent 3 min rest period considerable (12.1 +/- 7.0%, p less than 0.001) recovery was noted in the maximal force, with smaller (insignificant or p less than 0.05-0.01) changes in the force-time and relaxation-time variables, while the average IEMG of force production decreased (p less than 0.01-0.001). The present findings suggest that fatigue leading to a worsening in force-time, in maximal force and in the relaxation-time parts of a maximal isometric contraction might take place primarily in the contractile processes.

Adult↗