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PubMed2005

Comparison of brain activation after sustained non-fatiguing and fatiguing muscle contraction: a positron emission tomography study.

The concept of fatigue refers to a class of acute effects that can impair motor performance, and not to a single mechanism. A great deal is known about the peripheral mechanisms underlying the process of fatigue, but our knowledge of the roles of the central structures in that process is still very limited. During fatigue, it has been shown that peripheral apparatus is capable of generating adequate force while central structures become insufficient/sub-optimal in driving them. This is known as central fatigue, and it can vary between muscles and different tasks. Fatigue induced by submaximal isometric contraction may have a greater central component than fatigue induced by prolonged maximal efforts. We studied the changes in regional cerebral blood flow (rCBF) of brain structures after sustained isometric muscle contractions of different submaximal force levels and of different durations, and compared them with the conditions observed when the sustained muscle contraction becomes fatiguing. Changes in cortical activity, as indicated by changes in rCBF, were measured using positron emission tomography (PET). Twelve subjects were studied under four conditions: (1) rest condition; (2) contraction of the m. biceps brachii at 30% of MVC, sustained for 60 s; (3) contraction at 30% of MVC, sustained for 120 s, and; (4) contraction at 50% of MVC, sustained for 120 s. The level of rCBF in the activated cortical areas gradually increased with the level and duration of muscle contraction. The fatiguing condition was associated with predominantly contralateral activation of the primary motor (MI) and the primary and secondary somatosensory areas (SI and SII), the somatosensory association area (SAA), and the temporal areas AA and AI. The supplementary motor area (SMA) and the cingula were activated bilaterally. The results show increased cortical activation, confirming that increased effort aimed at maintaining force in muscle fatigue is associated with increased activation of cortical neurons. At the same time, the activation spread to several cortical areas and probably reflects changes in both excitatory and inhibitory cortical circuits. It is suggested that further studies aimed at controlling afferent input from the muscle during fatigue may allow a more precise examination of the roles of each particular region involved in the processing of muscle fatigue.

Adult↗

PubMed1988

Effects of fatigue on activation profiles and relative torque contribution of elbow flexor synergists.

The purpose of this study was to examine changes in elbow flexion synergies as a result of fatigue. Subjects performed a series of five isometric contractions at 10% maximum voluntary contraction step intervals. Pre- and post-fatigue surface electromyograms for the muscles were rectified, integrated, normalized, a representative equation determined using the least-squares method, and the equations numerically differentiated. The differentiated curves represent the change of excitation (and therefore, change of isometric torque) with respect to the change in resultant torque (dIEMG/dT). Pre-fatigue synergy, as indicated by the dIEMG/dT, was characterized by both co-activation and trade-off patterns. The nature of the synergy was changed as a result of fatigue. This technique, focusing upon synergistic patterns, quantifies differences in excitation profiles in a manner not previously described.

Adult↗

PubMed2005

The impact of rosiglitazone on cardiovascular responses and endurance during isometric exercise in patients with Type 2 diabetes.

BACKGROUND: Vascular endothelial function is compromised in people with diabetes reducing tissue blood flow. The present investigation was a logical extension of a previous work, examining isometric exercise performance and recovery in people with diabetes and the impact of the insulin sensitizer rosiglitazone, a drug shown to improve endothelial function. MATERIAL/METHODS: Blood pressure, heart rate and limb blood flow were examined in 12 control subjects and 8 subjects with type 2 diabetes during a series of 2 fatiguing isometric contractions at a tension of 40% of the maximum strength of the handgrip muscles; 5 minutes of rest were allowed between the contractions. RESULTS: Endurance of the first and the second contraction was significantly less in subjects with diabetes (p<0.01). HR increased significantly (p<0.01) by over three fold during exercise in control subjects compared to subjects with diabetes. There was no statistical difference in the increase in either systolic or diastolic BP during isometric exercise comparing controls to subjects with Type 2 diabetes (p>0.05), however, resting and peak blood pressures were greater in subjects with type 2 diabetes (p<0.01). Forearm blood flow, which was significantly lower at rest (p<0.01) in subjects with diabetes, only increased by about half as much during the exercise in subjects compared to control subjects. These differences were partially reversed after 3 months administration of rosiglitazone. CONCLUSIONS: Microcirculatory damage in diabetes impairs exercise performance and recovery and can be partially reversed after rosiglitazone administration.

Adult↗

PubMed1987

Quantification of peripherally induced reciprocal activation during voluntary muscle contraction.

A new method was developed to compare patterns of coactivation and reciprocal activation of antagonistic muscles during different experimental conditions. Pure coactivation was defined as a muscle activity pattern where there was never a difference between the slope of the agonist EMG record and the slope of the antagonist EMG record. The degree of reciprocal activation was defined as being proportional to the average absolute value of the difference between these two slope values. Use of this method for the comparison of muscle activity patterns during isometric contractions and unexpected movements showed that peripheral input related to agonist unloading and antagonist stretch significantly increased reciprocal activity.

Adult↗

PubMed1983

Effect of antibodies to light meromyosin on glycerinated muscle fibres and on actomyosin adenosinetriphosphatases.

The conformation change of light meromyosin influences the myosin and actin interaction, the myosin ATPase activity [22]. Starting from these data the specificity of the phenomenon has been investigated. The effect of LMM1, LMM- and LMM1-antibodies was studied on the isometric tension and relaxation of glycerol extracted muscle fibres. LMM1 was found to relax the fibres isometrically contracted by ATP-Ca2+; anti-LMM and anti-LMM1 markedly accelerated the development of isometric tension and inhibited the relaxation of ATP contracted glycerinated muscle fibres; in the presence of anti-LMM the ATPase rate of glycerinated myofibrils was slightly augmented. These results seem to indicate that LMM1 governs the actin binding site function of myosin and controls its affinity to actin. It is supposed that the reaction of myofibrils with specific LMM antibodies induced a transconformation in the myosin head increasing the affinity to actin of myosin.

Actins↗

PubMed2002

Effect of age on muscle functions investigated with surface electromyography.

The purpose of this study was to investigate changes in the surface electromyographic (EMG) signal as a means of defining age-related central and peripheral mechanisms affecting muscle fatigue. Spectral and temporal variables of the surface EMG signal were studied during voluntary isometric contractions of the dominant biceps brachii muscle in a group of 8 healthy elderly men (age range 67-86 years) and a group of 10 healthy young men (age range 23-34 years). The maximal torque developed and the rate of decrease (slope) of spectral variables and conduction velocity (CV) were statistically higher in the young subjects than in the elderly subjects. Motor unit (MU) CV distribution was also estimated from the surface EMG signal and no statistical difference was observed in its variance between the two groups. These results confirm previous findings from the tibialis anterior muscle. Thus, changes in fiber type distribution and decrease in MU firing rate with aging may be factors determining the decrease in maximal voluntary contraction torque and in myoelectric manifestations of muscle fatigue.

Action Potentials↗

PubMed1999

Pelvic restraint effect on lumbar gluteal and hamstring muscle electromyographic activation.

OBJECTIVE: To evaluate the influence of pelvic restraint on electromyographic activation (neural drive) of the lumbar extensor, gluteal, and hamstring muscles during submaximal dynamic lumbar extension in an upright seated position. DESIGN: Randomized, balanced, crossover trial comparing the electromyographic activation of the lumbar, gluteal, and hamstring muscles during dynamic lumbar extension exercise with and without pelvic restraint. SETTING: Research laboratory at a private research university. SUBJECTS: Twelve apparently healthy men (ages 18 to 50 yrs). INTERVENTION: Dynamic lumbar extension exercises. MAIN OUTCOME MEASURE: Electromyographic recordings. RESULTS: Pelvic restraint did not influence the relative activation of the lumbar, gluteal, or hamstring muscle groups (p< or =.05) during submaximal dynamic lumbar extension exercise. The relative activation of the gluteal muscles was significantly lower than that of the lumbar extensor and hamstring muscle groups. CONCLUSIONS: Pelvic restraint is not a requirement to promote neural drive to the lumbar musculature during submaximal dynamic lumbar extension exercise performed in an upright seated position. Submaximal lumbar extension exercise reduces the relative contribution of the gluteal muscles compared with a maximal voluntary isometric contraction.

Adult↗

PubMedDate not supplied

Muscle strength of trunk flexion-extension in post-stroke hemiplegic patients.

This study was undertaken to determine muscle strength of trunk flexion-extension in hemiplegic patients after stroke compared with that of normal controls. The design consisted of a nonrandomized control trial in a secondary care setting (a rehabilitation unit at a hospital facility). The subjects included 25 post-stroke male hemiplegic patients and 25 male healthy controls. The maximal peak torques of trunk flexion-extension at angular velocities of 0 degrees (isometric contraction), 60 degrees, 120 degrees, and 150 degrees/s were measured by using an isokinetic dynamometer (Cybex Trunk Extension-Flexion Unit, Cybex, Ronkonkoma, NY). Peak torque of trunk flexion and extension in hemiplegic patients was significantly smaller than that of healthy controls (P < 0.05), except isometric trunk flexion (P > 0.05). The weakness of trunk flexion-extension muscles in hemiplegic patients might be accounted for by the bilateral innervation from the motor cortex, the insufficient use of high threshold motor units, and disuse atrophy.

Abdominal Muscles↗

PubMed1992

Maximum heart rate of soricine shrews: correlation with contractile properties and myosin composition.

Maximum heart rates (HR) of three soricine shrews and six other small mammals were measured in response to a single supramaximal dose of isoproterenol (Iso) under urethan anesthesia. The highest HR, 1,043 +/- 66 (SD) beats/min (n = 3), was in least shrew (Sorex minutus, mean body mass 3.02 +/- 0.81 g). Maximum HRs of common shrew (Sorex araneus, 7.16 +/- 1.54 g) and water shrew (Neomys fodiens, 12.80 +/- 1.54 g) were 938 +/- 29 (n = 7) and 887 +/- 21 (n = 6), respectively. In general, maximum HRs of soricine shrews and other small wild mammals followed the common mammalian pattern, fHmax/Iso = 443 x Mb-0.14, determined by body size. The exponent for this equation is smaller than that of resting HR (-0.25) (Stahl, J. Appl. Physiol. 22: 453-460, 1967), predicting crossover at approximately 3 g body mass. However, resting HRs of small mammals were clearly lower than expected on the basis of body mass. Lowering resting HR below the common mammalian level, with concomitant increase in stroke volume, seems to be a prerequisite for small mammals to regulate cardiac output against the ceiling of maximum HR. Electrophoretic analysis showed that the myosin of shrew ventricles is different from those of rodent species. In native conditions, shrew myosin, designated V1', migrated faster than the V3 and V1 forms of rat heart. On SDS gradient gel the single heavy chain of shrew myosin migrated slower than the alpha- or beta-chains of rat ventricle. Differences in the molecular weight of light chains were also noted between small mammals. Despite the notable differences in myosin composition, myosin-ATPase activity of the shrew hearts was similar to that of mouse and rat heart. Because duration of isometric contraction was inversely related to resting and maximum HRs, it was concluded that in the small mammals rate and duration of contraction are determined mainly by the release and uptake rate of myoplasmic Ca2+ and less by myosin-ATPase activity.

Animals↗

PubMed1990

Comparison of the electrophysiological pattern of fatigue between athletes required to perform explosive and endurance sports.

The electrophysiological behavior of an isometric contraction sustained to fatigue, was examined in 6 long distance runners and 9 athletes involved in explosive (burst) sports, by on line computer analysis of the electrical activity of vastus medialis, rectus femoris and vastus lateralis. The experiments were carried out with a counterload of 50% of the maximal strength of the muscle. The duration of spike increased and the frequency decreased in the 3 examined muscles, in both types of sport. In the burst sports the changes of value of both parameters were statistically significant in the 3 muscles. In endurance sports the variations of duration were not significant and the changes of frequency were statistically significant only in the vastus lateralis. These results could be explained by the gradual activation of motor units of more strikingly different sizes in burst sports. Thus it may be speculated that prolonged training in burst sports may result in the automatic mobilization of higher number of small motor units, for the initiation of contraction while in endurance sports the onset of contraction is more gradual and carried out by large motor units.

Adolescent↗

PubMed1986

Clinical and experimental application of the percutaneous twitch superimposition technique for the study of human muscle activation.

The use of percutaneous stimulation for twitch superimposition in the human quadriceps has been compared with stimulation via the femoral nerve. It was found that the relationship between the extra force generated by the twitch and the level of voluntary contraction was independent of the percentage of the muscle stimulated. The technique was used to study activation of the quadriceps and biceps in a group of young healthy adults, and quadriceps activation in patients with musculo-skeletal disorders. The majority of subjects could fully activate their muscles during brief isometric contractions. Inhibition was demonstrated in patients with muscle pain and joint pathology and in subjects with a previous history of knee or joint injury. The technique is easy to use and provides a simple way of detecting inhibition where it might not be anticipated, such as in the absence of pain. Where inhibition is demonstrated the technique can be used to predict approximately the maximum force available.

Adolescent↗

PubMed1991

Control of motor-unit tension by rate modulation during sustained contractions in reinnervated cat muscle.

1. The aim of this study was to describe the control of tension by rate modulation of single motor units in reinnervated muscle. 2. Single fast-twitch motor units were isolated from medial gastrocnemius (MG) muscles in two groups of anesthetized adult cats: one in which the MG nerve was left untreated and another in which that nerve was sectioned and immediately sutured together 10-33 mo before study. Together with conventional measures of isometric contractile properties, units were tested with the use of computer-controlled feedback regulation of stimulation rate to maintain tension during continuous isometric contraction at a constant submaximal level [25% of maximal tension (Pmax)]. 3. For motor units from both groups, stimulation rate began to decline after target tension was attained and then settled at lower values for variable durations before rapidly increasing, usually within the last 5% of the contraction's duration, until reaching the experimentally selected limit of 100 pulses/s (pps). 4. Measures of the declining phase in stimulation rate occurring at the beginning of sustained contraction were not significantly different in comparison of untreated versus reinnervated muscles. These measures included 1) the magnitude of the decrease in rate, 2) the minimum rate attained, and 3) the time taken to reach minimum stimulation rate expressed as a fraction of endurance time (Et, total duration of the sustained contraction). 5. Most fast-twitch units from reinnervated muscles fell within normal limits for both endurance time and the number of stimuli applied over that period.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2002

Effects of postural muscle fatigue on the relation between segmental posture and movement.

The purpose of this study was to examine whether fatigue of postural muscles might influence the coordination between segmental posture and movement. Seven healthy adults performed series of fifteen fast wrist flexions and extensions while being instructed to keep a dominant upper limb posture as constant as possible. These series of voluntary movements were performed before and after a fatiguing submaximal isometric elbow flexion, and also with or without the help of an elbow support. Surface EMG from muscles Deltoïdeus anterior, Biceps brachii, Triceps brachii, Flexor carpi ulnaris, Extensor carpi radialis were recorded simultaneously with wrist, elbow and shoulder accelerations and wrist and elbow displacements. Fatigue was evidenced by a shift of the elbow and shoulder muscles EMG spectra towards low frequencies. Kinematics of wrist movements and corresponding activations of wrist prime-movers, as well as the background of postural muscle activation before wrist movement were not modified. There were only slight changes in timing of postural muscle activations. These data indicate that postural fatigue induced by a low-level isometric contraction has no effect on voluntary movement and requires no dramatic adaptation in postural control.

Adult↗

PubMed1978

Plantar flexor strength testing using the cybex isokinetic dynamometer.

Twenty male subjects and twenty female subjects in each of three age categories, 6 to 8 years, 14 to 16 years, and 23 to 28 years, were tested on the Cybex Isokinetic Dynamometer to obtain normative strength values for the ankle plantar flexor muscle group. Values were established for both isokinetic and isometric contractions at two speeds. The results indicate that age and weight are the significant variables in determining strength scores.

Adolescent↗

PubMed1986

The effect of PNF position of the upper extremity on rapid knee extension.

Premotor time (PMT) and maximum tension (Fmax) produced by isometric contraction of the left and the right quadriceps femoris for rapid knee extension were examined in 10 normal male subjects at two positions of the left upper extremity, neutral and facilitating position for the triceps brachii. Compared to the neutral position, PMT of the both sides decreased and Fmax increased at the facilitating position, while the contraction time did not change. It is assumed that the facilitating position brings about generalized activation of the central nervous system resulting in behavioral alertness.

Adult↗

PubMed1998

[Functional changes of forelegs of rats after C6 and C8 nerve roots resection: experimental study].

OBJECTIVE: To find new nerve roots as dynamic source, we observed the effect of C(6) and C(8) resection on the function of forelegs of roots. METHOD: Considering the relationship between C(6) and biceps brachii muscle, C(8) and flexor digitorum profundus muscle, we resected C(6) and C(8) nerve roots of right side as the experimental side and compared with the left side. The position, pace, appearance of ulcer were observed on the foot and the phenomena of self-injury were seen regularly. The EMG and tensile strength of biceps brachii and flexor digitorum profundus muscles were measured immediately at two weeks and four weeks after operation. RESULT: The position of two sides of the rats was symmetrical. There were no limp, ulceration and phenomena of self-injury. Normal wave figure in EMG was noted in the experimental side. There was no significant difference in t test in EMG wave amplitude, maximum tetanized systole tensile strength and duration of isometric contraction. CONCLUSION: C(6) and C(8) nerve roots resection has no obvious influence on function of the leg.

Animals↗

PubMed2006

In vivo ATP production during free-flow and ischaemic muscle contractions in humans.

The aim of this study was to determine how ATP synthesis and contractility in vivo are altered by ischaemia in working human skeletal muscle. The hypotheses were: (1) glycolytic flux would be higher during ischaemic (ISC) compared to free-flow (FF) muscle contractions, in compensation for reduced oxidative ATP synthesis, and (2) ischaemic muscle fatigue would be related to the accumulation of inhibitory metabolic by-products rather than to the phosphorylation potential ([ATP]/[ADP][P(i)]) of the muscle. Twelve healthy adults (6 men, 6 women) performed six intermittent maximal isometric contractions of the ankle dorsiflexors (12 s contract, 12 s relax), once with intact blood flow and once with local ischaemia by thigh cuff inflation to 220 Torr. Intracellular phosphorous metabolites and pH were measured non-invasively with magnetic resonance spectroscopy, and rates of ATP synthesis through oxidative phosphorylation, anaerobic glycolysis, and the creatine kinase reaction were determined. The force-time integral declined more during ISC (66 +/- 3% initial) than FF (75 +/- 2% initial, P = 0.002), indicating greater fatigue in ISC. [ATP] was preserved in both protocols, indicating matching of ATP production and use under both conditions. Glycolytic flux (mm s(-1)) was similar during FF and ISC (P = 0.16). Total ATP synthesis rate was lower during ISC, despite adjustment for the greater muscle fatigue in this condition (P < 0.001). Fatigue was linearly associated with diprotonated inorganic phosphate (FF r = 0.94 +/- 0.01, ISC r = 0.92 +/- 0.02), but not phosphorylation potential. These data provide novel evidence that ATP supply and demand in vivo are balanced in human skeletal muscle during ischaemic work, not through higher glycolytic flux, but rather through increased metabolic economy and decreased rates of ATP consumption as fatigue ensues.

Adenosine Triphosphate↗

PubMed1996

Predictability of jaw muscle pains from surface electromyograms.

In seven (88%) of eight healthy subjects, weak to moderate pains were elicited in the masseter muscles through the isometric contractions of maximum voluntary teeth clenching. Integrated surface electromyograms of the right and left masseter muscles were used to quantify the absolute and relative contractile activities of the two muscles. The risk (relative probability) of inducing pain onset in the single masseter muscle generating the larger amount of isometric activity was 2.5 times the risk of eliciting pain onset in the single masseter muscle generating the lesser amount of isometric activity. However, as an aid in the diagnosis of pain onset, the method of masseteric surface electromyography had a false diagnostic ratio of 0.67.

Adult↗