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.

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.

MATSUMOTO, TAMAKI, CHIEMI MIYAWAKI, HIDETOSHI UE, TOMO KANDA, YASUHIDE YOSHITAKE, AND TOSHIO MORITANI. Comparison of thermogenic sympathetic response to food intake between obese and non-obese young women. Obes Res. 2001;9:78 -85. 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 Ϯ 54.5 vs. 363.8 Ϯ 43.7 ms 2 , p Ͻ 0.05) and relative values (0.23 Ϯ 0.03 to 0.36 Ϯ 0.02, p Ͻ 0.05) of a very-low-frequency component and global sympathetic nervous system index (1.46 Ϯ 0.19 vs. 3.26 Ϯ 0.61, p Ͻ 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 Ϯ 2.3 vs. 6.7 Ϯ 0.8%, p Ͻ 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.

NAGAI, NARUMI, TAMAKI MATSUMOTO, HIROKO KITA, AND TOSHIO MORITANI. Autonomic nervous system activity and the state and development of obesity in Japanese school children. Obes Res. 2003;11:25-32. Objective: The autonomic nervous system (ANS) plays an important role in regulating energy expenditure and body fat content; however, the extent to which the ANS contributes to pediatric obesity remains inconclusive. The aim of this study was to evaluate whether sympathetic and/or the parasympathetic nerve activities were altered in an obese pediatric population. We further examined a physiological association between the duration of obesity and the sympatho-vagal activities to scrutinize the nature of ANS alteration as a possible etiologic factor of childhood obesity. Research Methods and Procedures: Forty-two obese and 42 non-obese healthy sedentary school children were carefully selected from 1080 participants initially recruited to this study. The two groups were matched in age, gender, and height. The clinical records of physical characteristics and development of the obese children were retrospectively reviewed to investigate the onset and progression of obesity. The ANS activities were assessed during a resting condition by means of heart rate variability power spectral analysis, which enables us to identify separate frequency components, i.e., total power (TP), low-frequency (LF) power, and high-frequency (HF) power. The spectral powers were then logarithmically transformed for statistical testing. Results:The obese children demonstrated a significantly lower TP (6.77 Ϯ 0.12 vs. 7.11 Ϯ 0.04 ln ms 2 , p Ͻ 0.05), LF power (6.16 Ϯ 0.12 vs. 6.42 Ϯ 0.05 ln ms 2 , p Ͻ 0.05), and HF power (5.84 Ϯ 0.15 vs. 6.34 Ϯ 0.07 ln ms 2 , p Ͻ 0.01) compared with the non-obese children. A partial correlation analysis revealed that the LF and HF powers among 42 obese children were negatively associated with the duration of obesity independent of age (LF: partial r ϭ Ϫ0.55, p Ͻ 0.001; HF: partial r ϭ Ϫ0.40, p Ͻ 0.01). The obese children were further subdivided into two groups based on the length of their obesity. All three spectral powers were significantly reduced in the obese group with obesity of Ͼ3 years (n ϭ 18) compared to the group with obesity of Ͻ3 years. Discussion: Our data indicate that obese children possess reduced sympathetic as well as parasympathetic nerve activities. Such autonomic depression, which is associated with the duration of obesity, could be a physiological factor promoting the state and development of obesity. These findings further imply that preventing and treating obesity beginning in the childhood years could be an urgent and crucial pediatric public health issue.