Intramuscular and surface electromyogram changes during muscle fatigue

Twelve male subjects were tested to determine the effects of motor unit (MU) recruitment and firing frequency on the surface electromyogram (EMG) frequency power spectra during sustained maximal voluntary contraction (MVC) and 50% MVC of the biceps brachii muscle. Both the intramuscular MU spikes and surface EMG were recorded simultaneously and analyzed by means of a computer-aided intramuscular spike amplitude-frequency histogram and frequency power spectral analysis, respectively. Results indicated that both mean power frequency (MPF) and amplitude (rmsEMG) of the surface EMG fell significantly (P less than 0.001) together with a progressive reduction in MU spike amplitude and firing frequency during sustained MVC. During 50% MVC there was a significant decline in MPF (P less than 0.001), but this decline was accompanied by a significant increase in rmsEMG (P less than 0.001) and a progressive MU recruitment as evidenced by an increased number of MUs with relatively large spike amplitude. Our data suggest that the surface EMG amplitude could better represent the underlying MU activity during muscle fatigue and the frequency powers spectral shift may or may not reflect changes in MU recruitment and rate-coding patterns.

Potential for gross muscle hypertrophy in older men

The time course of strength gain with respect to the contributions of neural factors and hypertrophy was studied in five young men and five older men during the course of 8 weeks progressive strength training. Young and old men showed similar and significant percentage increases in strength. However, the neurophysiological adaptations in response to the training were quite different. Increases in maximal muscle activation (neural factors) played a dominant role throughout the training for old subjects, while young subjects showed strength gains due to neural factors only at the initial stage, with hypertrophy becoming the dominant factor after some 4 weeks of training. Our data suggest that the effect of muscle training in the old may entirely rest on the neural factors presumably acting of various levels of the nervous system which could result in increasing the maximal muscle activation level in the absence of significant hypertrophy.

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71 +/- 31.3 to 505 +/- 188 microV, p less than 0.01) and mean power frequency (89 +/- 13.3 to 123 +/- 23.5 Hz, p less than 0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike amplitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412 +/- 79 to 972 +/- 117 microV, p less than 0.01) and mean firing frequency (17.8 +/- 5.4 to 24.7 +/- 4.1 Hz, p less than 0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.

Oxygen availability and motor unit activity in humans

Six men were studied to determine the interrelationships among blood supply, motor unit (MU) activity and lactate concentrations during intermittent isometric contractions of the hand grip muscles. The subjects performed repeated contractions at 20% of maximal voluntary contraction (MVC) for 2 s followed by 2-s rest for 4 min with either unhindered blood circulation or arterial occlusion given between the 1st and 2nd min. The simultaneously recorded intramuscular MU spikes and surface electromyogram (EMG) data indicated that mean MU spike amplitude, firing frequency and the parameters of surface EMG power spectra (mean power frequency and root mean square amplitude) remained constant during the experiment with unhindered circulation, providing no electrophysiological signs of muscle fatigue. Significant increases in mean MU spike amplitude and frequency were, however, evident during the contractions with arterial occlusion. Similar patterns of significant changes in the surface EMG spectra parameters and venous lactate concentration were also observed, while the integrated force-time curves remained constant. These data would suggest that the metabolic state of the active muscles may have played an important role in the regulation of MU recruitment and rate coding patterns during exercise.

Spontaneous regression of localized neuroblastoma detected by mass screening

PURPOSE

To clarify whether and when neuroblastomas identified through screening do regress, and to ascertain how to treat them appropriately, we observed screened patients who had localized tumors, without any therapeutic intervention.

PATIENTS AND METHODS

The criteria for the observation program were as follows: disease stage I or II; tumor less than 5 cm in diameter; no invasion to the intraspinal canal or growth to the great vessels; urinary vanillylmandelic acid (VMA) and homovanillic acid (HVA) less than 50 microg/mg creatinine; and informed consent. Of 25 patients identified through screening for 6-month-old infants in Saitama Prefecture, Japan between April 1994 and March 1996, 11 patients who met the criteria and one other patient with stage III tumor were enrolled onto the program. They were examined by abdominal ultrasonography (US) and their urinary VMA and HVA levels were assessed approximately once per month. The observation periods ranged from 4 to 27 months.

RESULTS

The 11 tumors decreased in size, although one of these 11 tumors initially enlarged until the patient was 12 months of age and decreased in size thereafter. One other tumor slightly increased in size. Urinary VMA levels decreased in all patients. None of the tumors had completely disappeared by the last observation day.

CONCLUSION

Our results suggest that regression of screened neuroblastoma is not a rare phenomenon. At present, it seems reasonable to adopt a wait-and-see strategy, with careful observation, for selected stage I or II tumors identified in infants screened at 6 months of age.

Assessment of lower-back muscle fatigue using electromyography, mechanomyography, and near-infrared spectroscopy

We have investigated the etiology of lower-back muscle fatigue using simultaneous recordings of electromyography (EMG), mechanomyography (MMG), and near-infrared spectroscopy (NIRS) in an attempt to shed some light on the electrophysiological, mechanical, and metabolic characteristics, respectively. Eight male subjects performed isometric back extensions at an angle of 15 degrees with reference to the horizontal plane, for a period of 60 s. Surface EMG, MMG and NIRS signals were recorded simultaneously from the center of the erector spinae at the level of L3. NIRS was measured to determine the level of muscle blood volume (BV) and oxygenation (Oxy-Hb). The root mean square amplitude value (RMS) of the EMG signal was significantly increased at the initial phase of contraction and then fell significantly, while mean power frequency (MPF) of the EMG signal decreased significantly and progressively as a function of time. There were also significant initial increases in RMS-MMG that were followed by progressive decreases at the end of fatiguing contractions. MPF-MMG remained unchanged. Muscle BV and Oxy-Hb decreased dramatically at the onset of the contraction and then remained almost constant throughout the rest of the contraction. These results, obtained by simultaneous recordings of EMG, MMG, and NIRS, demonstrate that the restriction of blood flow due to high intramuscular mechanical pressure is one of the most important factors in muscle fatigue in the lower-back muscles. In addition, the simultaneous recording system described here can be used to obtain more reliable information regarding the mechanism(s) of lower-back muscle fatigue.

Clockwise whirlpool sign at color Doppler US: an objective and definite sign of midgut volvulus

PURPOSE

To evaluate the clockwise whirlpool sign at color Doppler ultrasound (US) in the diagnosis of midgut volvulus.

MATERIALS AND METHODS

Pediatric patients (160 boys, 76 girls; age range, 0 day to 14 years) with possible midgut volvulus underwent abdominal gray-scale US and color Doppler US. Midgut volvulus was diagnosed by recognition of the whirlpool sign (wrapping of the superior mesenteric vein and the mesentery around the superior mesenteric artery). If the whirlpool rotated clockwise with caudal movement of the transducer, the direction of the whirlpool was defined as clockwise.

RESULTS

Color Doppler US showed a whirlpool sign that was clockwise in 12 of 13 patients with surgically confirmed midgut volvulus and counterclockwise in three patients without midgut volvulus. Sensitivity, specificity, and positive predictive value of clockwise whirlpool sign for midgut volvulus were 92%, 100%, and 100%.

CONCLUSION

The clockwise whirlpool sign is diagnostic of midgut volvulus. Color Doppler US should be performed as an initial imaging study in children suspected of having midgut volvulus.

Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer

The purpose of this investigation was to test whether the concept of critical power used in previous studies could be applied to the field of competitive swimming as critical swimming velocity (vcrit). The vcrit, defined as the swimming velocity over a very long period of time without exhaustion, was expressed as the slope of a straight line between swimming distance (dlim) at each speed (with six predetermined speeds) and the duration (tlim). Nine trained college swimmers underwent tests in a swimming flume to measure vcrit at those velocities until the onset of fatigue. A regression analysis of dlim on tlim calculated for each swimmer showed linear relationships (r2 greater than 0.998, P less than 0.01), and the slope coefficient signifying vcrit ranged from 1.062 to 1.262 m.s-1 with a mean of 1.166 (SD 0.052) m.s-1. Maximal oxygen consumption (VO2max), oxygen consumption (VO2) at anaerobic threshold, and the swimming also velocity at the onset of blood lactate accumulation (vOBLA) were also determined during the incremental swimming test. The vcrit showed significant positive correlations with VO2 at anaerobic threshold (r = 0.818, P less than 0.01), vOBLA (r = 0.949, P less than 0.01) and mean velocity of 400 m freestyle (r = 0.864, P less than 0.01). These data suggested that vcrit could be adopted as an index of endurance performance in competitive swimmers.

Identification and characterization of genes encoding polycyclic aromatic hydrocarbon dioxygenase and polycyclic aromatic hydrocarbon dihydrodiol dehydrogenase in Pseudomonas putida OUS82

Naphthalene and phenanthrene are transformed by enzymes encoded by the pah gene cluster of Pseudomonas putida OUS82. The pahA and pahB genes, which encode the first and second enzymes, dioxygenase and cis-dihydrodiol dehydrogenase, respectively, were identified and sequenced. The DNA sequences showed that pahA and pahB were clustered and that pahA consisted of four cistrons, pahAa, pahAb, pahAc, and pahAd, which encode ferredoxin reductase, ferredoxin, and two subunits of the iron-sulfur protein, respectively.

Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels

Motor unit activation patterns were studied during four different force levels of concentric and eccentric actions. Eight male subjects performed concentric and eccentric forearm flexions with the movement range from 100 degrees to 60 degrees in concentric and from 100 degrees to 140 degrees elbow angle in eccentric actions. The movements were started either from zero preactivation or with isometric preactivation of the force levels of 20, 40, 60 and 80% MVC. The subjects were then instructed to maintain the corresponding relative force levels during the dynamic actions. Intramuscular and surface EMG was recorded from biceps brachii muscle. Altogether 28 motoneuron pools were analyzed using the intramuscular spike-amplitude frequency (ISAF) analysis technique of Moritani et al. The mean spike amplitude was lower and the mean spike frequency higher in the isometric preactivation phase than in the consequent concentric and eccentric actions. When the movements started with isometric preactivation the mean spike amplitude increased significantly (P<0.001) up to 80% in isometric and concentric actions but in eccentric actions the increase continued only up to 60% (P<0.01). The mean spike frequency in isometric preactivation and in concentric action with preactivation was lower only at the 20% force level (P<0.01) as compared to the other force levels while in eccentric action with preactivation the increase between the force levels was significant (P<0.01) up to 60%. When the movement was started without preactivation the mean spike amplitude at 20% and at 40% force level was higher (P<0.01) in eccentric action than in concentric actions. It was concluded that the recruitment threshold may be lower in dynamic as compared to isometric actions. The recruitment of fast motor units may continue to higher force levels in isometric and in concentric as in eccentric actions which, on the other hand, seems to achieve the higher forces by increasing the firing rate of the active units. At the lower force levels mean spike amplitude was higher in eccentric than in concentric actions which might indicate selective activation of fast motor units. This was, however, the case only when the movements were started without isometric preactivation.

Neuromuscular adaptations during the acquisition of muscle strength, power and motor tasks

Neuromuscular performance is determined not only by the size of the involved muscles, but also by the ability of the nervous system to appropriately activate the muscles. Adaptive changes in the nervous system in response to training are referred to as neural adaptation. This article briefly reviews current evidence regarding the neural adaptations during the acquisition of muscle strength power and motor tasks and will be organized under four main topics, namely: (i) muscle strength gain: neural factors versus hypertrophy, (ii) neural adaptations during power training, (iii) neuromuscular adaptations during the acquisition of a motor task, and (iv) neuromuscular adaptations during a ballistic movement.

Neuromuscular, metabolic, and kinetic adaptations for skilled pedaling performance in cyclists

PURPOSE

The purpose of this study was to clarify the reason for the difference in the preferred cadence between cyclists and noncyclists.

METHODS

Male cyclists and noncyclists were evaluated in terms of pedal force, neuromuscular activity for lower extremities, and oxygen consumption among the cadence manipulation (45, 60, 75, 90, and 105 rpm) during pedaling at 150 and 200 W. Noncyclists having the same levels of aerobic and anaerobic capacity as cyclists were chosen from athletes of different sports to avoid any confounding effect from similar kinetic properties of cyclists for lower extremities (i.e., high speed contraction and high repetitions in prolonged exercise) on both pedaling performance and preferred cadence.

RESULTS

The peak pedal force significantly decreased with increasing of cadence in both groups, and the value for noncyclists was significantly higher than that for cyclists at each cadence despite the same power output. The normalized iEMG for vastus lateralis and vastus medialis muscles increased in noncyclists with rising cadence; however, cyclists did not show such a significant increase of the normalized iEMG for the muscles. On the other hand, the normalized iEMG for biceps femoris muscle showed a significant increase in cyclists while there was no increase for noncyclists. Oxygen consumption for cyclists was significantly lower than that for noncyclists at 105 rpm for 150 W work and at 75, 90, and 105 rpm for 200 W work.

CONCLUSIONS

We conclude that cyclists have a certain pedaling skill regarding the positive utilization for knee flexors up to the higher cadences, which would contribute to a decrease in peak pedal force and which would alleviate muscle activity for the knee extensors. We speculated that pedaling skills that decrease muscle stress influence the preferred cadence selection, contributing to recruitment of ST muscle fibers with fatigue resistance and high mechanical efficiency despite increased oxygen consumption caused by increased repetitions of leg movements.

Autonomic responsiveness to acute cold exposure in obese and non-obese young women

OBJECTIVE

Conflicting results have emerged over the nature of autonomic nervous system abnormalities in human obesity. This present study was designed to investigate the sympatho-vagal activities and their responsiveness to acute cold exposure in age- and height-matched obese and non-obese young women.

SUBJECTS

Twenty-four age- and height-matched obese (Weight: 68.1 +/- 2.64 kg, BMI: 26.3 +/- 0.74 kg/m2, %Fat: 39.9 +/- 1.23%) and non-obese young women (Weight: 46.9 +/- 0.77 kg, BMI: 18.5 +/- 0.18 kg/m2, %Fat: 22.9 +/- 0.8%).

MEASUREMENTS

Plasma leptin, insulin, glucose and lipid concentrations were measured at rest. The sympathetic (SNS) and parasympathetic (PNS) nervous system activities were assessed by means of power spectral analysis of heart rate variability (HRV) for 15 min under control (25 degrees C) or acute cold exposure (10 degrees C) conditions. The very low (VLO) frequency component, and SNS (low/high power), and PNS (high/total power) indexes were used to evaluate thermoregulatory sympathetic function, and cardiac sympathetic and parasympathetic nervous activities, respectively.

RESULTS

Plasma leptin concentration was significantly greater in the obese than in the control group (47.3 +/- 7.00 vs 12.1 +/- 1.22 ng.ml-1, P < 0.001). There was a highly positive correlation between plasma leptin concentration and percent of body fat (r = 0.863, P < 0.001). During the resting condition, there was no significant difference in any of the parameters of the HRV between the obese and control groups. Upon acute cold exposure, the VLO frequency component associated with thermoregulation (309 +/- 49.9 vs 578 +/- 142.2 ms2, P < 0.05) as well as its responsiveness (25-10 degrees C delta changes: 17 +/- 82.9 vs 326 +/- 138.2 ms2, P < 0.05) were significantly lower in the obese than in the control group.

CONCLUSION

Our data indicate that a reduced autonomic, especially sympathetic responsiveness associated with thermoregulation and possibly leptin resistance might be aetiological factors of obesity in young women.

Optimal pedaling rate estimated from neuromuscular fatigue for cyclists

This study was designed to examine the optimal pedaling rate for pedaling exercise at a given work intensity for cyclists. Six college-aged cyclists each performed six sessions of heavy pedaling exercise at individually selected work rates based on their aerobic capacity. The optimal pedaling rate was evaluated on the basis of minimal neuromuscular fatigue as evidenced by the integrated electromyogram (iEMG) slope defined by the changes in iEMG as a function of time. The means of the iEMG slope demonstrated a quadratic curve versus pedaling rate. The mean values at 80 rpm (0.53 (SD 0.20) microV.min-1) and 90 rpm (0.67 (SD 0.23) microV.min-1) were significantly smaller than those values at any other pedaling rate. On the other hand, the mean value of oxygen uptake (VO2) expressed as a percent of the subject's maximal VO2 (% VO2max) at each pedaling rate also showed a quadratic curve with minimal values at about 60 or 70 rpm. VO2 at 70 rpm (84.0 (SD 5.0) % VO2max) was significantly smaller than those values at 80 rpm (86.3 (SD 3.5) % VO2max), 90 rpm (87.4 (SD 3.8) % VO2max), and 100 rpm (90.1 (SD 3.8) % VO2max). These data strongly suggest that the optimal pedaling rate estimated from neuromuscular fatigue in working muscles is not coincident with the pedaling rate at which the smallest VO2 was obtained, but with the preferred pedaling rate of the subjects. Our findings also suggest that the reason that cyclists prefer a higher pedaling rate is closely related to the development of neuromuscular fatigue in the working muscles.

The muscle sound properties of different muscle fiber types during voluntary and electrically induced contractions

Soundmyogram (SMG) and electromyogram signals were recorded simultaneously from the relatively fast medial gastrocnemius (MG) and slow soleus (SOL) during voluntary and electrically induced contractions. Using a spike-triggered averaging technique, the averaged elementary sound and corresponding MU spikes were also obtained from about 35 different MUs identified. The rms-SMG of MG increased as a function of force (P < 0.01). On the contrary, these values for SOL increased up to 60% MVC (P < 0.01), but decreased at 80% MVC. The relationship between the peak to peak amplitude of SMG and MU spike indicated significant positive correlations (r = 0.631 to approximately 0.657, P < 0.01). During electrical stimulation at 5 Hz, the SMG power spectral peak frequency (PF) was matched with stimulation frequency in both muscles. At higher stimulation frequencies, e.g., > 15 Hz, only in the MG was SMG-PF synchronized with stimulation frequency; the slow SOL did not show such synchronization. Our data suggest that the SMG frequency components might reflect active motor unit firing rates, and that the SMG amplitude depends upon mechanical properties of contraction, muscle fiber composition, and firing rate during voluntary and electrically induced contractions.

Exercise training and autonomic nervous system activity in obese individuals

PURPOSE

This study was designed to investigate the effects of 12 wk of exercise training on autonomic nervous system (ANS) in 18 obese middle-aged men (N = 9) and women (N = 9) (age: 41.6 +/- 1.2 yr; BMI: 27.3 +/- 0.4 kg x m(-2); %fat: 29.6 +/- 1.3%, mean +/- SE).

METHODS

Each subject participated in an aerobic exercise training at anaerobic threshold (AT), consisting of 30 min/session, 3 times/wk, for 12 consecutive weeks. The ANS activities were assessed by means of power spectral analysis of heart rate variability (HRV) at resting condition before, at 5 wk, and after the exercise program.

RESULTS

The exercise training resulted in a significant decrease in body mass, BMI, and % fat (P < 0.01) but not in lean body mass (P > 0.05) together with a significant increase in the AT VO2 (P < 0.01). Our power spectral data indicated that there were significant increases in the low-frequency component associated with the sympathovagal activity (0.03--0.15 Hz, 348.5 +/- 66.8 vs 694.7 +/- 91.5 ms(2), P < 0.01), the high-frequency vagal component (0.15--0.4 Hz, 146.3 +/- 30.4 vs 347.7 +/- 96.5 ms(2), P < 0.05), and the overall autonomic activity as evaluated by total power (0.03--0.4 Hz, 494.8 +/- 88.5 vs 1042.4 +/- 180.9 ms(2), P < 0.01) of HRV after the training.

CONCLUSIONS

Twelve weeks of exercise training has significantly improved both the sympathetic and parasympathetic nervous activities of the obese individuals with markedly reduced ANS activity, suggesting a possible reversal effect of human ANS functions. These favorable changes may also have an influence on the thermoregulatory control over the obesity.

Determination of maximal power output at neuromuscular fatigue threshold

The purpose of this study was to determine the maximal power output at the neuromuscular fatigue threshold (EMGFT), as estimated from electromyographic (EMG) data from representative leg muscles during cycling. The rate of rise in integrated EMG activity as a function of time (iEMG slope) was calculated at each of four constant-power-output ergometer bouts for 20 subjects. The iEMG slopes were plotted against work rates that were well described as linear functions (0.84 < R2 < 0.99). This iEMG slope vs. work rate relationship was extrapolated to zero slope to give an intercept on the power axis that was interpreted as the highest work rate sustainable without EMG evidence of neuromuscular fatigue (EMGFT). Each individual EMGFT was then expressed in terms of an O2 output (VO2) equivalent on the basis of the individual delta efficiency calculated during a linearly increasing maximal exercise test on the same bicycle ergometer. Results indicated a highly significant correlation (r = 0.92, P < 0.01) between EMGFT VO2 and anaerobic threshold VO2, as determined by conventional gas exchange criteria. The mean EMGFT VO2 (1.84 +/- 0.55 l/min) was, however, significantly greater (P < 0.01) than the anaerobic threshold VO2 (1.72 +/- 0.54 l/min). It was suggested that the EMGFT may provide an attractive alternative to the measurement of the highest work rate that can be sustained without evidence of neuromuscular fatigue.

Comparison of thermogenic sympathetic response to food intake between obese and non-obese young women

OBJECTIVE

Sympathetic nervous system abnormality in humans is still a matter of debate. The present study was designed to examine diet-induced autonomic nervous system activity and metabolic change in obese and non-obese young women.

RESEARCH METHODS AND PROCEDURES

Sixteen age- and height-matched obese and non-obese young women participated in this study. Sympathovagal activities were assessed by means of our newly developed spectral analysis procedure of heart-rate variability during the resting condition and after mixed-food ingestion (480 kcal). Energy expenditure was also measured under these two conditions.

RESULTS

There was no significant difference in any of the parameters of the heart-rate variability between the obese group and control group during the resting condition. In the control group, both absolute values (221.5 +/- 54.5 vs. 363.8 +/- 43.7 ms2, p < 0.05) and relative values (0.23 +/- 0.03 to 0.36 +/- 0.02, p < 0.05) of a very-low-frequency component and global sympathetic nervous system index (1.46 +/- 0.19 vs. 3.26 +/- 0.61, p < 0.05) were significantly increased after mixed-food ingestion compared with the values obtained after resting condition. However, no such sympathetic response was found in the obese group. Energy expenditure increased in the two groups after the meal, but the magnitude of the increase above the preprandial resting condition was significantly greater in the control group than in the obese group (11.2 +/- 2.3 vs. 6.7 +/- 0.8%, p < 0.05).

DISCUSSION

Our data suggest that despite identical sympathovagal activities at the resting condition, obese young women may possess a reduced sympathetic response to physiological perturbation such as mixed food intake, which might be related to lowered capacity of thermogenesis and the state of obesity.

Does critical swimming velocity represent exercise intensity at maximal lactate steady state?

The purpose of this investigation was to determine whether the critical swimming velocity (vcrit), which is employed in competitive swimming, corresponds to the exercise intensity at maximal lactate steady state. vcrit is defined as the swimming velocity which could theoretically be maintained forever without exhaustion and expression as the slope of a regression line between swimming distances covered and the corresponding times. A total of eight swimmers were instructed to swim two different distances (200 m and 400 m) at maximal effort and the time taken to swim each distance was measured. In the present study, vcrit is calculated as the slope of the line connecting the two times required to swim 200 m and 400 m. vcrit determined by this new simple method was correlated significantly with swimming velocity at 4 mmol.l-1 of blood lactate concentration (r = 0.914, P < 0.01) and mean velocity in the 400 m freestyle (r = 0.977, P < 0.01). In the maximal lactate steady-state test, the subjects were instructed to swim 1600 m (4 x 400 m) freestyle at three constant velocities (98%, 100% and 102% of vcrit). At 100% vcrit blood lactate concentration showed a steady-state level of approximately 3.2 mmol.l-1 from the first to the third stage and at 98% of vcrit lactate concentration had a tendency to decrease significantly at the fourth stage. On the other hand, at 102% of vcrit, blood lactate concentration increased progressively and those of the third and fourth stages were significantly higher than those at 100% of vcrit (P < 0.05). These data suggest that vcrit, which can be calculated by performing two timed, maximal effort swimming tests, may correspond to the exercise intensity at maximal lactate steady state.

Effect of physical activity on autonomic nervous system function in lean and obese children

OBJECTIVE

The autonomic nervous system (ANS) is a key factor in the regulation of energy balance and body fat storage;however, to what extent the physical activity during the childhood years contributes to variations in ANS function is still unclear. The present study was designed to investigate the ANS activity in lean and obese children, focusing on the differences in physical activity levels.

SUBJECTS

This study was performed on 1080 school children initially recruited to the present study. In all, 24 physically active and 24 inactive obese children (> or =120% of the standard body weight) were chosen as samples. Then, 24 lean-active and 24 lean-inactive children, who were matched individually in age, gender, height, and the amount of sports activity, were carefully selected from the remaining children.

MEASUREMENTS

Physical activity was classified as the frequency of participation in after-school sports activities (active; > or =3 times per week, inactive; nothing). The ANS activities were measured during the resting condition by means of heart rate (HR) variability power spectral analysis, which enables us to identify separate frequency components, that is, low frequency (LF; 0.03-0.15 Hz), reflecting mixed sympathetic (SNS) and parasympathetic nervous system (PNS) activity, high frequency (HF; 0.15-0.5 Hz), mainly associated with PNS activity, and total power (TP; 0.03-0.5 Hz), evaluating the overall ANS activity. The spectral powers were log transformed for statistical testing.

RESULTS

The lean-active group demonstrated lower resting HR as well as significantly higher TP, LF, and HF powers compared to the remaining groups. In contrast, the obese-inactive group showed significantly lower TP (P<0.05 vs the remaining groups), LF (P<0.05 vs the lean groups), and HF power (P<0.05 vs the lean groups), respectively. The obese-active and lean-inactive groups were nearly identical in all spectral parameters. The correlation analysis revealed that TP among 48 inactive children was significantly and negatively associated with the percentage of body fat (r=-0.53, P<0.001); however, such correlation among 48 active children was modest (r=-0.33, P=0.02).

CONCLUSION

Our data suggest that obese children possess reduced sympathetic as well as parasympathetic nervous activities as compared to lean children who have similar physical activity levels. Such autonomic reduction, associated with the amount of body fat in inactive state, might be an etiological factor of onset or development of childhood obesity. On the other hand, regular physical activities could contribute to enhance the overall ANS activity in both lean and obese children. These findings further imply that regular physical activity might be effective in preventing and treating obesity beginning in the childhood.

Effects of capsaicin-containing yellow curry sauce on sympathetic nervous system activity and diet-induced thermogenesis in lean and obese young women

The present study was designed to investigate the effect of capsaicin, a pungent component of red pepper, on the sympathetic nervous system (SNS) activity and energy metabolism in 16 age- and height-matched lean and obese young women. The sympatho-vagal activities were assessed by means of spectral analysis of heart rate variability (HRV) during the resting condition and after the meal (2,016 kJ) with capsaicin (3 mg). Energy expenditure was also measured under the two conditions. There was no significant difference in any of the parameters of the HRV between the obese and control groups at rest. After the capsaicin diet, however, the very low frequency component (0.007-0.035 Hz) associated with thermogenesis (315.8+/-78.0 vs. 814.8+/-211.7 ms2 x Hz(-1), p<0.05) as well as its responsiveness (delta changes: 14.6+/-104.4 vs. 369.2+/-121.7 ms2 x Hz(-1), p<0.05) were significantly lower in the obese than the control group. Energy expenditure was significantly increased in the control group after the meal (5,574.7+/-221.2 to 6,114.7+/-239.0 kJ day(-1); p<0.01), but no such significant thermogenic response was detected in the obese group despite nearly identical lean body mass of the control group. Our data indicate that regardless of the resting level of sympatho-vagal activities, the reduced sympathetic responsiveness to physiological perturbation such as a capsaicin diet, which may cause impaired diet-induced thermogenesis and further weight gain, could be an important etiological factor leading to obesity in young women.

Maximal voluntary force of bilateral and unilateral leg extension

The aims were: (1) to investigate whether the 10-20% lower force during bilateral (BL) as compared to unilateral (UL) leg extension could be due to a general inability to activate fully a large number of muscles simultaneously, (2) to analyse the EMG signal of the quadriceps femoris during leg extensions, (3) to study the BL/UL force ratio in extension of the knee, and (4) to study the BL/UL leg extension force ratio in untrained and trained subjects. A 10% lower maximal voluntary isometric force was demonstrated during BL as compared to UL leg extension. This force discrepancy did not change when a total arm load of 250 N was applied simultaneously. Nor did the absolute force levels change, which indicates that the lower BL leg extension force is not due to a general mechanism of reduced activation with an increased number of muscles recruited in maximal voluntary contractions. Integrated EMG activity, mean power frequency and root mean square value of the EMG amplitude did not differ between UL and BL leg extensions. The knee extension force was slightly greater (4%) during BL than UL contractions. These findings are arguments against a reduced activation of the knee extensor muscles being the cause of the lower bilateral leg extension force. No differences in BL/UL force ratio were noted between groups of untrained and trained subjects despite the fact that several of the trained groups do different forms of BL leg extensions regularly. Thus, it does not appear that training readily affects the BL/UL leg extension force ratio.