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PubMed2005

Neuromuscular control of the knee during a resisted single-limb squat exercise.

BACKGROUND: Closed kinetic chain exercises such as single-limb squats are preferred for knee rehabilitation. A complete understanding of the neuromuscular control of the knee during the single-limb squat is essential to increase the efficiency of rehabilitation programs. HYPOTHESIS: Performing a controlled single-limb squat with resistance to knee flexion and extension will increase the coactivation of the hamstring muscle group, thus reducing the quadriceps/hamstrings ratio. STUDY DESIGN: Descriptive laboratory study. METHODS: A total of 15 healthy human subjects (7 women, 8 men) performed controlled single-limb squats in a custom mechanical device that provided resistance to both flexion and extension. Subjects performed the task at 3 levels of resistance, set as a percentage of body weight. Surface electromyographic recordings from 7 muscles (gluteus medius, rectus femoris, vastus medialis oblique, vastus lateralis, biceps femoris, semitendinosus, and medial gastrocnemius) were collected during the task. RESULTS: Biceps femoris activity during knee flexion increased from approximately 12% maximum voluntary isometric contractions during low resistance (0% body weight) to approximately 27% maximum voluntary isometric contractions during high resistance (8% body weight). Although the quadriceps had greater activity than the hamstrings at all levels of resistance, the quadriceps/hamstrings ratio declined significantly with resistance (F2,27 = 29.05; P = .012) from 3.0 at low resistance to 2.32 at the highest resistance. CONCLUSIONS: Performing controlled resisted single-limb squats may help to simultaneously strengthen the quadriceps and facilitate coactivation of the hamstrings, thus reducing anterior tibial shear forces. The coactivation may also increase the dynamic control of the knee joint. CLINICAL RELEVANCE: The typical single-limb squat exercise performed in the clinic does not usually control for bidirectional resistance and knee joint excursion. As seen in this study, controlled single-limb squats at increased levels of resistance help to increase the coactivation of the hamstring muscles, which is essential to optimize neuromuscular control of the knee.

Adult↗

PubMedDate not supplied

Post-contraction variations in motor pool excitability.

Previous evidence has shown a marked increase in motoneuronal excitability during muscle contraction and decreased post-contraction excitability. This post-contraction inhibition has been shown to persist up to 800 msec following the end of contraction (Gottlieb and Agarwal, 1973) and apparently continues with gradual recovery up to one minute (Enoka, Hutton and Eldred, 1980). These effects have been attributed in part to influences of muscle receptors on the homonymous motor pool. The purpose of the present study was to observe the time course of short term post-contraction effects, following a maximum isometric contraction, on excitability of the homonymous motoneuronal pool as measured by the Hoffmann reflex. Each of five subjects lay supine on a table with shoulder blocks to prevent movement during resisted plantar flexion. The subject's preferred foot was placed in a stirrup attached to a force gauge which measured tension. Recording electrodes were applied over the soleus muscle, and M- and H-responses were evoked by percutaneous electrical stimulation of the tibial nerve at the popliteal fossa. The stimulus strength was set to elicit a control H-reflex at rest of half the magnitude of the maximum H-reflex value. For each trial the subject performed a 6 s maximum contraction of the triceps surae muscle group, beginning on one visual signal and ending on a second visual signal. The H-reflex was elicited at semi-random intervals prior to and after the visual signal to relax. Increased H-reflex excitability was observed during muscle contraction followed by a marked decrease in evoked amplitudes with gradual recovery following the end of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMedDate not supplied

Daily changes in neural activation, force-time and relaxation-time characteristics in athletes during very intense training for one week.

Daily changes in neural activation, force-time and relaxation characteristics of the leg extensor muscles were examined in 8 elite strength athletes during a very intense strength training period with two daily sessions for one week. The maximal neural activation (integrated EMG) of the muscles and their maximal isometric extension force remained statistically unaltered during the entire experimental period. Average integrated EMG of the muscles during the rapid relaxation phase of the isometric contraction and the relaxation-time curve remained statistically unaltered as well. However, considerable decreases (p less than 0.05-0.001) occurred both in the neural activation and in the force production in the early phases of the rapid isometric contraction during the course of the one-week period. After one full day of rest only slight and statistically nonsignificant changes took place in the mean values of the neuromuscular performance variables examined. The present findings indicate that the high overall daily volume of heavy resistance strength training stimuli may have a considerable role in leading to temporary worsening specifically in the rapid neural activation and correspondingly in rapid force production of the neuromuscular performance capacity even during a very short training period. In athletic training the volume of these types of training stimuli should therefore be carefully considered and matched with the specific purpose of the training season and with the specific requirements of various athletic activities.

Adult↗

PubMed2006

Muscle fatigue decreases short-interval intracortical inhibition after exhaustive intermittent tasks.

OBJECTIVE: Central fatigue is the inability of central commands to recruit maximum evocable muscle force during voluntary contraction. Here, we investigate how fatigue affects the inhibitory circuits of the motor cortex. METHODS: MEPs, short interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were evaluated using a paired pulse transcranial magnetic stimulation (TMS) paradigm before, during and after a series of 5 isometric contractions of the FDI muscle to 50% maximal voluntary contraction (MVC). Each contraction lasted 2 min and was separated from the next by a pause of 2 min 40 s. Twelve male healthy subjects (range from 22 to 51 years) participated in experiment 1, in which the intensity of test stimulus was constant throughout the experiment. Eight of the same subjects (range from 26 to 51 years) participated in experiment 2, in which the intensity of test stimulus was adjusted so that the amplitude of the test MEP was kept constant throughout the measurement. RESULTS: As expected, test MEPs gradually decreased with progressive fatigue and recovered to control values with 5-10 min of rest. Because of the change in MEP amplitude, changes in percent SICI (reduced inhibition) and percent ICF (increased facilitation) in experiment 1 are difficult to interpret. When the test MEP was maintained at a constant size in experiment 2 there was no change in percent ICF, but the reduction in SICI was still present although it recovered to control values within the first 5-10 min of rest. CONCLUSIONS: SICI in FDI decreases transiently after a series of fatiguing isometric contractions. This decrease may compensate to some extent for reduced cortical excitability after muscle fatigue.

Adult↗

PubMed1993

Investigation of the subtypes of alpha 1-adrenoceptor mediating contractions of rat aorta, vas deferens and spleen.

1. The subtypes of alpha 1-adrenoceptor mediating contractions to exogenous noradrenaline (NA) or phenylephrine in rat vas deferens, spleen and aorta, and mediating contractions to endogenous NA in rat vas deferens have been examined. 2. In rat vas deferens, the competitive antagonists prazosin, WB 4101, benoxathian and 5-methyl-urapidil inhibited contractions to NA with pA2 values of 9.26, 9.54, 9.02 and 8.43, respectively. The irreversible antagonist chloroethylclonidine (CEC) (100 microM) failed to affect contractions to NA. 3. In rat vas deferens in the presence of nifedipine (10 microM), contractions to NA were significantly attenuated and under these conditions, CEC (100 microM) significantly reduced the maximum response to NA. 4. In rat spleen, the competitive antagonists prazosin, WB 4101 and benoxathian inhibited contractions to phenylephrine with pA2 values of 9.56, 8.85 and 7.60, respectively, and 5-methyl-urapidil had a KB of 6.62. CEC (100 microM) significantly reduced the maximum contraction to phenylephrine. 5. In rat aorta, the competitive antagonists, prazosin, WB 4101, benoxathian and 5-methyl-urapidil inhibited contractions to NA with pA2 values of 9.45, 9.21, 8.55 and 8.12, respectively. CEC (100 microM) produced an approximately parallel shift in the potency of NA, without significantly reducing the maximum response. 6. In epididymal portions of rat vas deferens in the presence of nifedipine (10 microM), the isometric contraction to a single electrical pulse was significantly reduced by CEC (100 microM), and by the competitive antagonists prazosin, WB 4101, benoxathian and 5-methyl-urapidil at concentrations of 1 nM. 7. In prostatic portions of rat vas deferens, the alpha l-adrenoceptor agonist, amidephrine, produced concentration-dependent increases in the isometric contraction to a single electrical stimulus and the maximum increase in the evoked response produced by amidephrine was unaffected by CEC (100 microM).8. Contractions of rat vas deferens produced by NA (and amidephrine) are mediated predominantly by alpha lA-adrenoceptors as shown by the high potency of alpha lA-adrenoceptor selective antagonists and the lack of effect of CEC. A small CEC-sensitive response, particularly in epididymal portions, was revealed in the presence of nifedipine. Contractions of rat spleen are mediated by alpha lB-adrenoceptors since alpha 1A selective antagonists showed low potency and CEC significantly reduced the maximum contraction to phenylephrine. Contractions of rat aorta to NA are mediated by non-alpha lA, non-alpha lB-adrenoceptors, due to the high potency of the aMA-selective antagonists and sensitivity to CEC.9. The noradrenergic contraction of epididymal portions of rat vas deferens in the presence of nifedipine is CEC-sensitive, but the alpha 1 A-selective antagonists showed high potency, suggesting that this response is mediated by non-alpha lA, non-alpha 1B-adrenoceptors.10. In conclusion, at least three subtypes of functional alpha 1-adrenoceptors have been demonstrated in these studies.

Adrenergic alpha-Agonists↗

PubMed2004

Influence of aging on sex differences in muscle fatigability.

The purpose of this study was to compare time to task failure for a sustained isometric contraction performed at a submaximal intensity with elbow flexor muscles by young and old men and women. Twenty-seven young (14 men and 13 women, 18-35 yr) and 18 old (10 men and 8 women, 65-80 yr) adults sustained an isometric contraction at 20% of maximal voluntary contraction torque until target torque could no longer be achieved for > or = 5 s. Young adults were stronger than old adults (66.8 +/- 17.9 vs. 47.7 +/- 18.1 N x m, P < 0.05), and men were stronger than women (69.8 +/- 17.9 vs. 47.1 +/- 15.3 N x m, P < 0.05), with no interaction between age and sex (P > 0.05). Time to task failure was longer for old than for young adults (22.8 +/- 9.1 vs. 14.4 +/- 7.6 min, P < 0.05) and for young women than for young men (18.3 +/- 8.0 vs. 10.8 +/- 5.2, P < 0.05), but there was no difference between old women and men (21.3 +/- 10.7 and 24.1 +/- 8.0 min, respectively, P > 0.05) or between young women and old adults (P > 0.05). Mean arterial pressure, heart rate, average electromyographic (EMG) activity, and torque fluctuations of elbow flexor muscles increased during the fatiguing contraction (P < 0.05) for all subjects. Rates of increase in mean arterial pressure, heart rate, and torque fluctuations were greater for young men and old adults, with no differences between old men and women (P > 0.05). Similarly, the rate of increase in EMG activity was greater for young men than for the other three groups. EMG bursts were less frequent for old adults (P < 0.05) at the end of the fatiguing contraction, and this was accompanied by reduced fluctuations in torque. Consequently, time to task failure was related to target torque for young, but not old, adults, and differences in task duration were accompanied by parallel changes in the pressor response.

Adult↗

PubMed1989

Reproducibility of estimation of blood flow in the human masseter muscle from measurements of 133Xe clearance.

The reproducibility of estimations of the masseter intramuscular blood flow (IMBF) was assessed bilaterally within and between clinical sessions. The 133Xe clearance in nine normal individuals was measured before, during, immediately after, and after endurance of isometric contraction at an attempted level of 50% of maximum voluntary clenching contraction. An overall low reproducibility of the estimations was found. This result was probably caused by uncertainties about the exact site of intramuscular 133Xe deposition, errors in assessment of the plots of clearance, and variabilities in the relative contraction levels sustained and, especially, in the overall muscle effort. In agreement with previous reports concerning other skeletal muscles, the 133Xe clearance method provided inconsistent estimates of absolute values of IMBF also in this clinical setting. Although there was a high intra-individual variation in the relative level of isometric contraction sustained, the endurance test induced distinct changes in IMBF, among which the estimate of post-endurance hyperemia was the most consistent for each individual. Therefore, measurements of 133Xe clearance seem to be useful to detect intra-individual changes in masseter IMBF resulting from isometric work.

Electromyography↗

PubMed1982

Dynamic and steady state characteristics of the rabbit gastrocnemius muscle determined in series measurements.

1. Within the range of the given conditions of measuring static and dynamic properties of the rabbit gastrocnemius muscle the following results were obtained: a) the dependence of the maxima of isotonic shortening upon the relative length of the muscle at constant load is linear; b) the parameters of the non-linear dependence of the passive elastic force of the muscle upon its relative length (measured in series) were identified using asymptotic regression; c) the time course of isotonic contractions (at an interval from 0 to 0.3 s after the beginning of stimulation) could be satisfactorily approximated by responses of a linear system to a step-function; d) the time course of isometric contractions (at an interval from 0 to 0.3 s after the beginning of stimulation) could be closely approximated by responses of a linear system to a step-function. 2. The time constants of isotonic and isometric contractions were determined as the parameters of the corresponding linear systems. 3. The maximum rates of the isometric and isotonic contractions were determined as maxima of the first derivatives of the responses of the corresponding models. 4. The experimental set-up made it possible to compare the values of the parameters concomitantly followed at various muscle lengths and at various loads.

Animals↗

PubMed1989

Fatigue induced by static work.

Despite its low energy cost, isometric contraction can result in the onset of local muscle fatigue. The onset of fatigue occurs more rapidly when the relative force exerted is greater than 15-20% of the maximum voluntary contraction (MVC) of the muscle considered, and when the contraction time is increased. The maximum maintenance time (limit-time) and the corresponding relative force are linked by a hyperbolic relation. Ischaemia promotes accumulation of acid metabolites produced during contraction, and hinders their elimination, thus constituting the main causal factor in the onset of local muscle fatigue. The introduction of rest periods of sufficient duration to ensure restoration of normal blood flow through the muscle is an effective way of delaying, or even preventing, the onset of muscle fatigue. Other factors may also be taken into account, such as the position in which the static work is performed, and the nature and number of muscles used simultaneously, etc. Numerous laboratory and field studies have allowed the development of various models that take into account the conditions relating to isometric contractions during static work.

Fatigue↗

PubMed2002

Differing effects of inotropic agents on length change deactivation of isolated rat myocardium.

BACKGROUND: A rapid change in length of cardiac muscle during isometric contraction is followed by developed force that is less than appropriate for the new length because of deactivation of the contractile system. Length change deactivation may have favorable or unfavorable effects on cardiac function, depending on the circumstances under which it is produced. METHODS: Left ventricular papillary muscles from male Sprague-Dawley rats were arranged for recording of isometric force. After each control or reference isometric contraction, a quick release-quick stretch V-step was applied to the following contraction. For each repetition of control and experimental contractions, the time of application of V-steps was increased by 20 ms until peak force was reached. Effects of these V-steps were assessed from ratios of peak redeveloped force to peak force in an isometric reference contraction. Slopes of plots of these ratios versus time after the onset of the contraction were used to quantify the effects of inotropic agents on deactivation. RESULTS: Increasing calcium from 2.5 to 5.0 or 7.5 mM increased force by 12+/-4% (mean+/-SEM), did not change time to peak, and did not significantly alter the deactivation slope. Adding 5 mM epinephrine increased force by 16+/-5%, decreased time to peak by 34+/-3%, and increased the deactivation slope by 106+/-9% (P<0.001). Caffeine had variable effects on peak force, increased time to peak by 47+/-4%, and decreased the deactivation slope by 71+/-5% (P<0.001). CONCLUSIONS: The quantitatively different effects of the three agents on length change deactivation slopes and time to peak force suggest a common mechanism, probably involving thin-filament cooperativity.

Animals↗

PubMed1991

Shortening of muscle fibres during stretch of the active cat medial gastrocnemius muscle: the role of tendon compliance.

1. The length of muscle fibres in the medial gastrocnemius (MG) muscle of the anaesthetized cat was measured using ultrasound techniques. During the course of 'isometric' contractions, the muscle fibres shortened by stretching the compliant tendons, until the muscle fibres could no longer produce enough force to stretch the tendons further. At optimal muscle length (Lo) the maximal shortening of muscle fibres was 28%. 2. At muscle lengths much longer than Lo, 'isometric' contractions produced a slow shortening of the muscle fibres as the tendons were stretched and this resulted in a slow rise in tension. This phenomenon, usually referred to as 'creep', is due to low power at long muscle fibre length. This study shows that the series compliance present in the tendons is the major contributor to 'creep' in the cat MG muscle. As the tendons stretched during the course of the contraction, the average sarcomere length became shorter providing greater filament overlap and increasing power. 3. Slow to medium speed stretches applied shortly after the onset of contraction, as occurs in cat MG during walking and trotting, were entirely taken up in the tendons and the muscle fibres actually shortened throughout the imposed muscle stretch. 4. When early stretches were applied at muscle lengths longer than Lo, stretch of the muscle resulted in a peak force that was less than if the stretch had not been applied. This was the reverse of the situation for stretches at lengths less than Lo. When stretch was applied after attaining peak force, the force was greatly enhanced and the muscle fibres were also stretched. 5. Using the same techniques in a freely walking cat, the muscle fibres shortened by 1.0 +/- 0.3 mm during the stance phase of the step-cycle when the muscle was being stretched, in 198 consecutive step-cycles. 6. The tendons act as a mechanical buffer to protect muscle fibres from damage during eccentric contractions. 7. Since stretches of the MG muscle are not faithfully imposed on the muscle fibres, studies of muscle spindle function during locomotion need to take into consideration these effects of tendon compliance. The dominant view, when the foot lands on the ground during normal locomotion, is that muscle spindles are stretched along with the muscle resulting in reflex enhancement of contractile force. This study shows that the muscle fibres do not stretch under these circumstances, except at high speeds of locomotion when the stretch rate is also high.

Animals↗

PubMed1989

Normally cycling and latch bridges in venous smooth muscle.

We have studied the mechanical properties of the crossbridges of canine saphenous venous smooth muscle during activation by electrical stimulation. A length-tension relationship study showed that at optimal length (1o) the maximum isometric tension (Po) developed was 87.9 mN/mm2. Resting tension at 1o was only 6% Po, which indicates a very compliant muscle. Active tension was observed at muscle lengths greater than 0.3 1o. The maximum shortening capacity was 0.65 1o. Force-velocity and series elastic characteristics were determined by the method of quick release to a set of load clamps applied during the course of an isometric contraction. The data were fitted by the hyperbolic Hill equation. Estimated maximum shortening velocity at zero load (Vo) an index of the crossbridge cycling rate) was 0.26 1o/s which was attained early in the contraction (5 s after the stimulation). A 11.9% decline in Vo was observed at the plateau of the contraction which occurred at 15 s. The stress (sigma)-strain (epsilon) curve for the series elastic component could be approximated by the equation sigma = B[exp(A epsilon)-1], where A = 76.51 (1/1o), B = 2.89 mN/mm2 at 5 s and A = 61.49 (1/1o), B = 1.85 mN/mm2 at 15 s. Like other smooth muscles, in general, saphenous vein develops considerable isometric tension and has a high shortening capacity but a low shortening velocity. The reduction in shortening velocity that occurs after 5 s in an isometric contraction is probably due to the development of so-called latch bridges.

Animals↗

PubMed1991

A premovement silent period does not occur prior to rapid changes of velocity during human limb movements.

Surface electromyographic activity (EMG) of the interosseous dorsalis I, the hand extensors and the biceps muscles was analysed in human young normal subjects for the occurrence of a precontraction (PCSP) or a premovement silent period (PMSP) before a sudden increase in tension or rapid change of position after a small isometric contraction or slow isotonic movement. Contrary to the results of Conrad et al. (Exp. Brain Res., 51 (1983) 310-313), in whose experiments the contraction condition changed from isometric to isotonic, a PCSP or PMSP could never be observed in our experiments, where contractions or movements first started at a low force level or with a low speed which had to be changed most rapidly after 2 s and where the recording condition remained strictly either isometric or isotonic. It is concluded that the central nervous system resets the motoneuronal pool activity when the central program is changed from the control of an isometric to the control of an isotonic contraction.

Adult↗

PubMed2004

Conflicting effects of fatigue and potentiation on voluntary force.

The objective of this study was to investigate whether a warm-up consisting of a series of maximal contractions would augment the force and activation of subsequent leg extensor contractions. Both voluntary and evoked isometric contractions were tested to determine the mechanisms underlying the response. Nine subjects were tested for twitch, tetanic, submaximal (30%), and maximal voluntary contractile (MVC) properties before and after (1, 5, 10, and 15 minutes) one to three 10-second MVCs. MVC force either did not change following 1-2 MVCs or was depressed at 10 and 15 minutes after 3 MVCs. MVC activation was decreased (4.4-6.9%) throughout recovery, whereas submaximal contractions were minimally affected. Although overall, twitches were potentiated (15.5-19.8%) posttest, 3 MVCs had significantly greater twitch potentiation than 1 or 2 MVCs at 5 and 10 minutes. Results suggest that voluntary and evoked contractions respond differently to prior 10-second MVCs. In the present study, a warm-up routine of 1-3 MVCs of a 10-second duration did not enhance subsequent voluntary performance.

Adult↗

PubMed1990

Effects of a new cardiotonic agent 1,2-dihydro-6-methyl-2-oxo-5-[imidazo (1,2-a) pyridin-6-yl]-3-pyridine carbonitrile hydrochloride monohydrate (E-1020) on contractile force and cyclic AMP metabolism in canine ventricular muscle.

E-1020, a newly synthesized imidazopyridinylpyridine or imidazopyridine derivative (structurally closely related to the bipyridine derivative milrinone) increased the force of contraction and cyclic AMP levels in a concentration-dependent manner in the isolated canine ventricular trabeculae electrically driven at 0.5 Hz at 37 degrees C. The concentration-response curve for the increase in force of contraction by E-1020 was biphasic. The maximal positive inotropic effect (PIE) of E-1020 is comparable to that of milrinone, and its potency is 3-fold less than that of milrinone, but 10-fold higher than that of amrinone. The time course of increases in force of contraction induced by E-1020 was coincident with that of cyclic AMP accumulation. The concentration-response curve for the PIE of E-1020 was superimposable to that of cyclic AMP accumulation. A beta-adrenoceptor antagonist, (+/-)-bupranolol (3 X 10(-7) mol/l), did not affect the PIE of E-1020. The increase in the force of contraction and accumulation of cyclic AMP produced by E-1020 were inhibited by a muscarinic receptor agonist, carbachol. The relationship between the force of contraction and cyclic AMP levels in the presence of E-1020 was not modified by addition of carbachol or isoproterenol. E-1020 shifted the concentration-response curve for isoproterenol to the left. E-1020 shortened the total duration of contraction and relaxation time of isometric contractions. These findings indicate that cyclic AMP is essentially involved in the PIE of E-1020 on the canine ventricular muscle, although the possible involvement of a cyclic AMP-independent mechanism can not be excluded.

Amrinone↗

PubMed2001

Muscle vibration sustains motor unit firing rate during submaximal isometric fatigue in humans.

1. In keeping with the 'muscular wisdom hypothesis', many studies have documented that the firing rate of the majority of motor units decreased during fatiguing isometric contractions. The present study investigated whether the application of periodic muscle vibration, which strongly activates muscle spindles, would alter the modulation of motor unit firing rate during submaximal fatiguing isometric contractions. 2. Thirty-three motor units from the lateral head of the triceps brachii muscle were recorded from 10 subjects during a sustained isometric 20 % maximal voluntary contraction (MVC) of the elbow extensors. Vibration was interposed on the contraction for 2 s every 10 s. Twenty-two motor units were recorded from the beginning of the fatigue task. The discharge rate of the majority of motor units remained constant (12/22) or increased (4/22) with fatigue. Six motor units demonstrated a reduction in discharge rate that later returned toward initial values; these motor units had higher initial discharge rates than the other 16 motor units. 3. In a second series of experiments, four subjects held a sustained isometric 20 % MVC for 2 min and then vibration was applied as above for the remainder of the contraction. In this case, motor units initially demonstrated a decrease in firing rate that increased after the vibration was applied. Thus muscle spindle disfacilitation of the motoneurone pool may be associated with the decline of motor unit discharge rate observed during the first 2 min of the contraction. 4. In a third set of experiments, seven subjects performed the main experiment on one occasion and repeated the fatigue task without vibration on a second occasion. Neither the endurance time of the fatiguing contraction nor the MVC torque following fatigue was affected by the application of vibration. This finding calls into question the applicability of the muscular wisdom hypothesis to submaximal contractions.

Adult↗

PubMed1976

The time course of phosphorylcreatine resynthesis during recovery of the quadriceps muscle in man.

The time course of phosphorylcreatine (PC) resynthesis in the human m. quadriceps femoris was studied during recovery from exhaustive dynamic exercise and from isometric contraction sustained to fatigue. The immediate postexercise muscle PC content after either form of exercise was 15-16% of the resting muscle content. The time course of PC resynthesis during recovery was biphasic exhibiting a fast and slow recovery component. The half-time for the fast component was 21-22s but this accounted for a smaller fraction of the total PC restored during recovery from the isometric contraction than after the dynamic exercise. The half-time for the slow component was in each case more than 170 s. After 2 and 4 min recovery the total amount of PC resynthesized after the isometric exercise were significantly lower than from the dynamic exercise. Occlusion of the circulation of the quadriceps completely abolished the resynthesis of PC. Restoration of resynthesis occurred only after release of occlusion.

Adenosine Triphosphate↗

PubMed1996

Adaptation of the precentral cortical command to elbow muscle fatigue.

The control exerted by individual motor cortical cells on their fatigued target muscles was assessed by analyzing the discharge patterns and electromyographic (EMG) postspike effects of cortical cells in monkeys making repeated forceful, but submaximal, isometric flexions of the elbow to produce fatigue. Two monkeys were trained to perform self-paced isometric contractions (for longer than 2 s) at forces greater than 35% maximal contraction, with three sets of 20 consecutive contractions; the first and last sets were at the same force level. Pairs of EMG electrodes were implanted in the biceps brachii, brachioradialis, and triceps brachii. The cortical cell discharges were modulated with the active and passive movements of the elbow and produced consistent EMG postspike effects during isometric contraction. Muscle fatigue was assessed as a statistically significant (P < 0.05) drop in the mean power frequency of the EMG power spectrum in one or both flexors in the last set of contractions. Clear signs of muscular fatigue occurred in 20 different experimental sessions. Before fatigue, cortical cells were classified as phasic-tonic (18), phasi-cramp (three), or tonic (five). Twenty cells briskly fired to passive elbow extension, and 9 also responded to passive flexion. Only 6 cells showed a decreased discharge to passive extension. A 22-30% increase in the contraction force produced a higher discharge frequency in 13 cells, and a lower frequency in 5 cells. All cells exerted EMG postspike effects in their target muscles: 20 cells facilitated the flexors, and some of these also inhibited (3 cells) or cofacilitated (5 cells) the extensor; the other 6 cells had mixed effects: 5 of them inhibited at least one flexor, and 1 cell only facilitated the extensor. Most cells (24/26) still produced EMG postspike effects in their target muscles during fatigue, and the number of facilitated muscles increased: 21 cells facilitated the flexors, and 12 of them cofacilitated the extensor. Only 3 cells still inhibited the flexors and were tonic cells. The cortical cell firing frequency increased during fatigue in 13 cells and decreased in 8 cells. Increases involved 10 cells excited by passive elbow extension. Fourteen cells showed parallel changes in firing frequency with fatigue and force, and 9 of these cells facilitated both extensors and flexors in fatigue. Increases were found in 8 cells, decreases in 5 cells and no change in 1 cell. As muscle afferents provide substantial information to cortical cells, which in turn establish functional linkages with their target muscles before and during fatigue, the changes in cell firing frequencies during fatigue demonstrate the active participation of the motor cortex in the control of compensation for the peripheral adjustments concomitant with muscle fatigue.

Adaptation, Physiological↗