Motor unit recruitment during prolonged isometric contractions

Motor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P < 0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed "motor unit rotation" and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.     

Recruitment of the bi-articular gastrocnemius muscles during maximal isometric contractions in humans

The main purpose of this study was to investigate the maximal isometric plantarflexion moment as a function of the triceps surae's length. As the gastrocnemius are bi-articular actuators, their respective lengths were modified by variations of the ankle and knee joints angles. The mechanical results agreed ones previously published in the literature. The maximal and minimal isometric moments were attained at maximal lengthening and shortening of the gastrocnemius respectively. The gastrocnemius electromyographic (EMG) activities showed paradoxal results. Their EMG activities decreased with their lengthening when this was induced by ankle joint angle variations. Inversely, their EMG activities increased with their lengthening when this was induced by knee joint angle variations. These results allowed the authors to hypothesize the existence of specific recruitment of neuro-muscular compartments of the gastrocnemius with knee extension.     

Changes in fusimotor activity during repetitive lengthening muscle contractions in decerebrate cats

The purpose of this study was to determine whether enzymatic and histochemical characteristics of human skeletal muscle are altered with aging. Tissues from the vastus lateralis (VL) and gastrocnemius were analyzed for citrate synthase (CS) activity and fiber type in 55 sedentary men (age range 18-80 yr). In this population, CS activity in the gastrocnemius was negatively related to age (r = -0. 32, P < 0.05); there was no relationship in the VL. Treadmill-determined maximal oxygen consumption was positively related (r = 0.40, P < 0.05) to CS in the gastrocnemius but not in the VL. CS activity in the gastrocnemius was 24% lower in the oldest (>/=60 yr, n = 10) vs. the youngest (相似文献   

Characteristics of the muscle mechanoreflex during quadriceps contractions in humans

We examined muscle sympathetic nerve activity (MSNA) in the nonexercising lower limb during repetitive static quadriceps contraction paradigm at 25% maximal voluntary contraction in eight men. Subjects performed 20-s contractions with 5-s rest periods for up to 12 contractions. Although the workload was constant, we found that MSNA amplitude rose as a function of contraction number [0.6 ln (amplitude/min)/contraction]; this suggests chemical sensitization of the muscle reflex response. We employed signal-averaging techniques and then integrated the data to examine the onset latency of the MSNA response as a function of the 25-s contraction-rest period. We observed an onset latency of approximately 4-6 s. Moreover, although the onset latency did not appear to vary as a function of contraction number, the rate of MSNA increase took approximately four contractions to reach a steady-state rate of rise; this suggests contraction-induced sensitization. The onset latency reported here is similar to findings in recent animal studies, but it is at odds with latencies determined in prior human handgrip contraction studies. We believe our data suggest that 1) mechanically sensitive afferents contribute importantly to the MSNA response to the paradigm employed and 2) these afferents may be sensitized by the chemical products of muscle contraction.     

Changes in ribosomal activity of Escherichia coli cells during prolonged culture in sea salts medium

The activity of ribosomes from a clinical isolate of Escherichia coli, exposed to starvation for 7 days in sea salts medium, was investigated by measuring the kinetic parameters of ribosomal peptidyltransferase, by using the puromycin reaction as a model reaction. No alterations in the extent of peptide bond formation were observed during starvation. In contrast, a 50% reduction in the kmax/Ks ratio could be seen after 24 h of starvation; an additional 6 days of starvation resulted in a progressive but less abrupt decline in the kmax/Ks value. (kmax is the apparent catalytic rate constant of peptidyl transferase, and Ks is the dissociation constant of the encounter complex between acetyl (Ac)[3H]Phe-tRNA-poly(U)-ribosome and puromycin.) Although the distribution of ribosomal particles remained constant, a substantial decrease in the number of ribosomes per starved cell and a clear decline in the ability of ribosomes to bind AcPhe-tRNA were observed, particularly during the first day of starvation. Further analysis indicated that rRNA in general, but especially 23S rRNA, was rapidly degraded during the starvation period. In addition, the L12/L7 molar ratio decreased from 1.5 to 1 during the initial phase of starvation (up to 24 h) but remained constant during the subsequent starvation period. Ribosomes isolated from 24-h-starved cells, when artificially depleted of L7/L12 protein and reconstituted with L7/L12 protein from mid-logarithmic-phase cells, regenerated an L12/L7 molar ratio of 1.5 and restored the peptidyltransferase activity to a substantial level. An analogous effect of reconstitution on the efficiency of ribosomes in binding AcPhe-tRNA was evident not only during the initial phase but throughout the starvation period.     

Daily magnesium supplementation effect on magnesium deficiency in rats during prolonged restriction of motor activity

The objective of this investigation was to evaluate the effect of magnesium (Mg) supplements in rats during prolonged restriction of motor activity (hypokinesia [HK]) and in the presence of Mg deficiency, which is characterized by increased rather than decreased plasma Mg concentration, as occurs in ambulatory conditions. The studies were performed during 98 days of HK on 100 13-week-old Sprague-Dawlay male rats weighing 360 to 390 g. They were equally divided into four groups: (1) unsupplemented control animals (UCA), (2) unsupplemented hypokinetic animals (UHA), (3) supplemented control animals (SCA), and (4) supplemented hypokinetic animals (SHA). For the simulation of the hypokinetic effect, SHA and UHA were kept for 98 days in small individual wood cages that restricted their movements in all directions without hindering food and water intake. The SCA and SHA took daily with their food an additional 0.35 mg of Mg. Before and during the hypokinetic period of 98 days, Mg in plasma, urine, and feces, balance of Mg, food intake of Mg, and body weight were determined at different intervals. In SHA and UHA, plasma Mg concentration and excretion of Mg in urine and feces increased significantly compared with SCA and UCA. Magnesium balance was negative in UHA and AHA throughout the hypokinetic period. Body weight and a food intake decreased significantly in SHA and UHA when compared with SCA and UCA. Significant losses of Mg in SHA and UHA occurred in the presence of Mg deficiency and suggest that prolonged HK induces another factor that influences Mg metabolism. We conclude that prolonged HK causes significant changes in Mg values of plasma, urine, and feces and a negative Mg balance in rats, despite Mg supplements leading to Mg deficiency.     

Daily zinc supplementation effect on zinc deficiency in rats during prolonged restriction of motor activity

The objective of this investigation was to evaluate the effect of 47 mg zinc supplementation on deficiency of zinc in rats during 98 d of restriction of motor activity (hypokinesia), which appeared by higher plasma zinc concentration. One Hundred 13-week-old Sprague-Dawley male rats weighing 360-390 g were used to perform the studies: They were equally divided into four groups: 1. Unsupplemented control animals (UCA); 2. Unsupplemented hypokinetic animals (UHA); 3. Supplemented control animals (SCA); and 4. Supplemented hypokinetic animals (SHA). For the simulation of the effect of hypokinesia (HK), the UHA and SHA were kept in small individual cages made of wood, which restricted their movements in all directions without hindering food and water intake. The SCA and SHA received daily with their food an additional amount of zinc. Before and during the experimental period of 98 d, plasma, urinary and fecal zinc, balance of zinc, food intake, and body weight were determined at different intervals. In the SHA and UHA, the concentration of zinc in plasma, and the elimination of zinc in urine and feces increased significantly when compared with the SCA and UCA, whereas the balance of zinc was negative. The body weight and food intake decreased significantly in the SHA and UHA when compared with the SCA and UCA. The increased plasma concentration of zinc in both the SHA and UHA groups was in contrast to the observed hypozincnemia during prolonged immobilization as during prolonged hospitalization. This reaction suggests that there may be some other mechanisms that are affecting the process of control and regulation of zinc metabolism during prolonged HK. It was concluded that exposure to prolonged restriction of motor activity of rats induces significant increases in plasma concentration, fecal and urinary elimination of zinc in the presence of negative zinc balance and regardless the daily intake of large amounts of zinc with their food, leading to zinc deficiency.     

Spatial and temporal features of recurrent facilitation among motoneurons innervating synergistic muscles of the cat

1. The temporal features and strength of recurrent facilitatory potentials were examined in pairs of lumbosacral motoneurons that were separated by a known distance and were identified by antidromic stimulation of muscle nerves. One motoneuron was stimulated by injecting depolarizing current pulses, and responses were recorded in the second motoneuron. The distance between motoneurons in pairs was also measured to assess the spatial distribution in strength of recurrent facilitation in motor pools. All motoneurons in these pairs innervated muscles that act as hip or ankle extensors. 2. Recurrent facilitatory potentials were found frequently among motoneurons innervating the hindlimb extensor muscles examined. Several categories of recurrent facilitatory responses were identified. One category was composed of facilitation responses that followed an inhibition response. A second category was composed of facilitation responses that were not preceded by a significant inhibition and consisted of a monophasic response. There was a considerable range of latencies in this category. 3. Responses in which recurrent facilitatory potentials were preceded by recurrent inhibitory postsynaptic potentials (RIPSPs) among close motoneuron pairs demonstrated an inverse correlation between the durations of the facilitatory and the inhibitory phases. In addition, the duration of inhibition responses without facilitation was longer on average, than the duration of inhibitory responses that were followed by facilitation. It was suggested that recurrent facilitation may restrict the time course of RIPSPs. 4. In contrast to the topographic distribution of RIPSPs described in the previous report, amplitudes of monophasic facilitations were directly correlated with the distance separating motoneurons in pairs, rather than inversely correlated as was the case for RIPSP amplitudes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bilateral experimental muscle pain changes electromyographic activity of human jaw-closing muscles during mastication

The effects of bilateral experimental muscle pain on human masticatory patterns were studied. Jaw movements and electromyographic (EMG) recordings of the jaw-closing muscles were divided into multiple single masticatory cycles and analyzed on a cycle-by-cycle basis. In ten men simultaneous bilateral injections of hypertonic saline (5%) into the masseter muscles caused strong pain (mean+/-SE: 7.5+/-0.4 on a 0-10 scale), significantly reduced EMG activity of jaw-closing muscles in the agonist phase, and significantly increased EMG activity in the antagonist phase. Nine of the subjects reported a sensation of less intense mastication during pain. Injections of isotonic saline (0.9%) did not cause pain or significant changes in masticatory patterns. The influence of higher brain centers on conscious human mastication can not be discarded but the observed phase-dependent modulation could be controlled by local neural circuits and/or a central pattern generator in the brain stem which are capable of integrating bilateral nociceptive afferent activity.     

Absence of tonic electromyographic activity during sleep in normal and spastic nonmimetic skeletal muscles in man

An electromyographic study of nonmimetic skeletal muscles was carried out in 8 normal adults and 4 patients with spastic hemiparesis during all stages of sleep for a total of 21 nights. All normal subjects showed absence of tonic electromyographic activity in all nonmimetic skeletal muscles in all stages of sleep. Also, during quiet, relaxed wakefulness, tonic muscle discharges disappeared in the normal subjects. Three patients with upper motor neuron spasticity demonstrated results during sleep similar to those obtained in the normal subjects. In the fourth patient, tonic muscle discharges persisted into stage 2 non-REM sleep, disappeared within 30 to 240 seconds following the onset of stage 2 sleep, and were absent during stages 3 and 4 sleep and REM sleep.     

Spongiform neuropathy induced in dogs by prolonged, low-level administration of 6-aminonicotinamide (6-ANA)

The objective of this study was to demonstrate the effects of prolonged exposure to 6-ANA at low dose-levels in dogs. A male and a female Beagle dog received daily oral repetitive doses of 1 mg/kg or less for 20 weeks. Both dogs showed lacrimation, conjunctivitis, reduced motility and anemia since the second week of treatment. The female dog was more affected than the male and at the end of treatment period it had tremor, hanging lower jaw, stepping gait of the hind limbs, hunched posture, and general debilitation. Post-mortem examination of the female dog revealed prominent brain edema with pressure atrophy of the dorsal cranial bones. Microscopic examination of the nervous system revealed spongiform neuropathy in both animals mainly affecting the telencephalic cortex and hippocampal fascia dentata, the substantia gelatinosa in the spinal cord and the dorsal root and autonomic ganglia. The changes were produced by vacuolation of astrocytes in the central nervous system and perineuronal satellite cells in the ganglia. Examination of the other organs revealed thymic atrophy and high hematopoietic activity of the bone marrow in both dogs. The male had severe interstitial edema and vacuolar degeneration of the testicular seminiferous tubules and the female had marked chronic pyelonephritis. This chemically induced spongiform neuropathy in dogs obviously represents a subchronic form of the "energy deprivation syndrome" induced by impaired glucose utilization. Vacuolar degeneration of the testicular seminiferous epithelium may have the same pathogenesis.     

Changes of acid phosphatase activity of fast and slow rat muscles during ontogenetic development

Total IgG, IgG subclass and IgE antibodies specific for grass pollen allergens were measured by the red cell linked antigen-antiglobulin reaction (RCLAAR) in a serum samples from nineteen patients who had undergone a course of hyposensitization. Increases in both specific IgG and IgE antibodies were seen after treatment in most patients. In the IgG subclasses the predominant response was for IgG1 and IgG4 antibodies. Attempts were made to correlate the antibody responses with the clinical response and the results are discussed with reference to the possible mechanisms of hyposensitization.     

The relationship between EMG activity and extensor moment generation in the erector spinae muscles during bending and lifting activities

The relationship between EMG activity and extensor moment generation in the erector spinae muscles was investigated under isometric and concentric conditions. The full-wave rectified and averaged EMG signal was recorded from skin-surface electrodes located over the belly of the erector spinae at the levels of T10 and L3, and compared with measurements of extensor moment. The effects of muscle length and contraction velocity were studied by measuring the overall curvature (theta) and rate of change of curvature (d theta/dt) of the lumbar spine in the sagittal plane, using the '3-Space Isotrak' system. Isometric contractions were investigated with the subjects pulling up on a load cell attached to the floor. Hand height was varied to produce different amounts of lumbar flexion, as indicated by changes in lumbar curvature. The extensor moment was found to be linearly related to EMG activity, and the 'gradient' and 'intercept' of the relationship were themselves dependent upon the lumbar curvature at the time of testing. Concentric contractions were investigated with the subjects extending from a seated toe-touching position, at various speeds, while the torque exerted on the arm of a Cybex dynamometer was continuously measured. Under these conditions the EMG signal (E) was higher than the isometric signal (E0) associated with the same torque. E and E0 were related as follows: E0 = E/(1 + A d theta/dt), where A = 0.0014 exp (0.045P) and P = percentage lumbar flexion. This equation was used to correct the EMG data for the effect of contraction velocity. The corrected data were then used, in conjunction with the results of the isometric calibrations, to calculate the extensor moment generated by the erector spinae muscles during bending and lifting activities. The extensor moment can itself be used to calculate the compressive force acting on the lumbar spine.     

Recruitment stability in masseter motor units during isometric voluntary contractions

Recruitment of single motor units (SMUs) of the masseter muscle was studied using macro representation (MacroRep) as the indicator of motor unit size. When subjects followed a slow isometric force ramp, units were usually recruited in order of MacroRep size. However, pooling the data from repeated ramps in the same subject resulted in a weak relationship between MacroRep size and force recruitment threshold, probably due to marked variations in the relative contributions of the jaw muscles, and varying levels of cocontraction, in the development of total bite force in each ramp. The force recruitment thresholds of individual SMUs showed marked variability, but recruitment threshold stability was improved when expressed as a percentage of maximum surface electromyographic (SEMG) activity in the ipsilateral masseter. Therefore the SEMG recruitment threshold was concluded to be a more stable and accurate indicator of the SMU's position in the recruitment hierarchy in a given muscle. It was concluded that SMUs in masseter are recruited according to the size principle, and that when investigating recruitment in jaw muscles, SEMG recruitment threshold should be used in preference to force recruitment threshold.     

Thyroid function during a prolonged stay in Antarctica

Adaptation of the thyroid gland to the Antarctic environment was studied in nine healthy euthyroid tropical men of the Sixth Indian Antarctic Expedition during 1 year of their residence at polar latitudes. Circulatory concentrations of thyroid hormones, total T4 (TT4), total T3 (TT3), free T4 (FT4), free T3 (FT3), reverse T3 (rT3), thyroxine binding globulin (TBG), T3 uptake and thyroid stimulating hormone (TSH) were estimated in New Delhi and during the first week of each month of the stay in Antarctica. At the end of the Austral summer in March, the TT3 concentrations were found to be significantly lower (P < 0.01) compared to values recorded in New Delhi and showed a significant increase (P < 0.05) during the Austral winter in August. The mean TT3 concentrations from May to December were found to be significantly higher than the March or April values. Plasma TT4 and rT3 concentrations tended to decline in March but remained unaltered during the entire period in Antarctica. The FT4, FT3, TBG and T3 uptake did not show any appreciable change. Though, the TT3:TT4 ratio tended to decline in March and April suggesting decreased peripheral conversion of T4 to T3 as the possible mechanism for a decline in TT3 in March. physical exertion and prolonged exposure to extreme cold appeared to be the major contributory factors. The TSH concentration in March, April, November and December were found to be significantly higher than the New Delhi values. The morning as well as evening cortisol concentrations during the Austral winter were higher than the March values suggesting that cortisol rhythmicity was well maintained in Antarctica, albeit at a higher level. These observations indicated that the subtle changes in thyroid hormones during a prolonged stay at polar latitudes are related not only to the extreme cold but also to other factors such as physical activity, polar days and polar nights.     

The activation of back muscles during locomotion in the developing rat

The development of posture during locomotion was studied in rats from the 11th day until adulthood. The EMGs were recorded and analyzed of the left and right longissimus muscles at caudal, intermediate and rostral levels as well as of the gastrocnemius, the tibialis and the vastus medialis muscles and movements were simultaneously recorded on videotape. Results indicate that from the 12th day of life, burst activity occurs in the longissimus muscles which is phase-related to the stepcycle. Until the 21st day these muscles are most strongly activated during burst activity in the gastrocnemius muscle in the contralateral hindleg but thereafter this activation coincides with bursts in the ipsilateral gastrocnemius muscle. At adult age such activation in the LL is restricted to fast walking or to accelerations. Latencies between bursts in the longissimus muscles and the gastrocnemius muscles vary around 100 ms until the 25th day, but thereafter they decrease to adult values of less than 10 ms. The large variations in these phase-relations at all ages suggest that supraspinal influences and afferent input are important factors in this coupling. The shift from a contra- to an ipsilateral coupling between bursts in the longissimus and in the gastrocnemius muscles might indicate that an ontogenetically older pattern of locomotion with the trunk muscles playing a major role in propulsion, is replaced by a newer pattern, mainly effected by extremity movements.