The metabolic syndrome

Over the last 20 years, heart rate monitors (HRMs) have become a widely used training aid for a variety of sports. The development of new HRMs has also evolved rapidly during the last two decades. In addition to heart rate (HR) responses to exercise, research has recently focused more on heart rate variability (HRV). Increased HRV has been associated with lower mortality rate and is affected by both age and sex. During graded exercise, the majority of studies show that HRV decreases progressively up to moderate intensities, after which it stabilises. There is abundant evidence from cross-sectional studies that trained individuals have higher HRV than untrained individuals. The results from longitudinal studies are equivocal, with some showing increased HRV after training but an equal number of studies showing no differences. The duration of the training programmes might be one of the factors responsible for the versatility of the results. HRMs are mainly used to determine the exercise intensity of a training session or race. Compared with other indications of exercise intensity, HR is easy to monitor, is relatively cheap and can be used in most situations. In addition, HR and HRV could potentially play a role in the prevention and detection of overtraining. The effects of overreaching on submaximal HR are controversial, with some studies showing decreased rates and others no difference. Maximal HR appears to be decreased in almost all ‘overreaching’ studies. So far, only few studies have investigated HRV changes after a period of intensified training and no firm conclusions can be drawn from these results. The relationship between HR and oxygen uptake (VO2) has been used to predict maximal oxygen uptake (VO2max). This method relies upon several assumptions and it has been shown that the results can deviate up to 20% from the true value. The HR-VO2 relationship is also used to estimate energy expenditure during field conditions. There appears to be general consensus that this method provides a satisfactory estimate of energy expenditure on a group level, but is not very accurate for individual estimations. The relationship between HR and other parameters used to predict and monitor an individual’s training status can be influenced by numerous factors. There appears to be a small day-to-day variability in HR and a steady increase during exercise has been observed in most studies. Furthermore, factors such as dehydration and ambient temperature can have a profound effect on the HR-VO2 relationship.

Heart rate monitoring: applications and limitations.

This review examines the influence on heart rate variability (HRV) indices in athletes from training status, different types of exercise training, sex and ageing, presented from both cross-sectional and longitudinal studies. The predictability of HRV in over-training, athletic condition and athletic performance is also included. Finally, some recommendations concerning the application of HRV methods in athletes are made.The cardiovascular system is mostly controlled by autonomic regulation through the activity of sympathetic and parasympathetic pathways of the autonomic nervous system. Analysis of HRV permits insight in this control mechanism. It can easily be determined from ECG recordings, resulting in time series (RR-intervals) that are usually analysed in time and frequency domains. As a first approach, it can be assumed that power in different frequency bands corresponds to activity of sympathetic (0.04-0.15 Hz) and parasympathetic (0.15-0.4 Hz) nerves. However, other mechanisms (and feedback loops) are also at work, especially in the low frequency band. During dynamic exercise, it is generally assumed that heart rate increases due to both a parasympathetic withdrawal and an augmented sympathetic activity. However, because some authors disagree with the former statement and the fact that during exercise there is also a technical problem related to the non-stationary signals, a critical look at interpretation of results is needed. It is strongly suggested that, when presenting reports on HRV studies related to exercise physiology in general or concerned with athletes, a detailed description should be provided on analysis methods, as well as concerning population, and training schedule, intensity and duration. Most studies concern relatively small numbers of study participants, diminishing the power of statistics. Therefore, multicentre studies would be preferable. In order to further develop this fascinating research field, we advocate prospective, randomised, controlled, long-term studies using validated measurement methods. Finally, there is a strong need for basic research on the nature of the control and regulating mechanism exerted by the autonomic nervous system on cardiovascular function in athletes, preferably with a multidisciplinary approach between cardiologists, exercise physiologists, pulmonary physiologists, coaches and biomedical engineers.

Heart rate variability in athletes.

Near-infrared spectroscopy (NIRS) was initiated in 1977 by Jobsis as a simple, noninvasive method for measuring the presence of oxygen in muscle and other tissues in vivo. This review honoring Jobsis highlights the progress that has been made in developing and adapting NIRS and NIR imaging (NIRI) technologies for evaluating skeletal muscle O(2) dynamics and oxidative energy metabolism. Development of NIRS/NIRI technologies has included novel approaches to quantification of the signal, as well as the addition of multiple source detector pairs for imaging. Adaptation of NIRS technology has focused on the validity and reliability of NIRS measurements. NIRS measurements have been extended to resting, ischemic, localized exercise, and whole body exercise conditions. In addition, NIRS technology has been applied to the study of a number of chronic health conditions, including patients with chronic heart failure, peripheral vascular disease, chronic obstructive pulmonary disease, varying muscle diseases, spinal cord injury, and renal failure. As NIRS technology continues to evolve, the study of skeletal muscle function with NIRS first illuminated by Jobsis continues to be bright.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-12/issue-06/062105/Near-infrared-spectroscopy-imaging-for-monitoring-muscle-oxygenation-and-oxidative/10.1117/1.2805437.pdf

Near-infrared spectroscopy/imaging for monitoring muscle oxygenation and oxidative metabolism in healthy and diseased humans

/pdf/exercise-comes-of-age-rationale-and-recommendations-for-a-1qqpzcf80j.pdf

Exercise Comes of Age Rationale and Recommendations for a Geriatric Exercise Prescription

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° 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.

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

AMANO, M., T. KANDA, H. UE, and T. MORITANI. Exercise training and autonomic nervous system activity in obese individuals. Med. Sci. Sports Exerc., Vol. 33, No. 8, 2001, pp. 1287‐1291. 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 5 9) and women (N 5 9) (age: 41.6 6 1.2 yr; BMI: 27.3 6 0.4 kg·m 22 ; %fat: 29.6 6 1.3%, mean 6 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

Exercise training and autonomic nervous system activity in obese individuals.

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°C) or acute cold exposure (10°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°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.

Autonomic responsiveness to acute cold exposure in 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.

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

The purpose of this study was to test whether surface mechanomyogram (MMG) recorded on the skin reflects the contractile properties of individual motor units in humans. Eight motor units in the medial gastrocnemius muscle were identified, and trains of stimulation at 5, 10, 15, and 20 Hz were delivered to each isolated motor unit. There was a significant positive correlation between the duration of MMG and twitch duration. MMG amplitude decreased with increasing stimulation frequency. Reductions in MMG amplitude were in parallel with the reductions in force fluctuations, and the rate of change in both was positively correlated across the motor units. Rate of change in MMG amplitude against force was negatively correlated to half relaxation time and twitch duration. Similar negative correlations were found between force fluctuations and contractile properties. These results provide evidence supporting a direct relation between MMG and contractile properties of individual motor units within the gastrocnemius muscle, indicating that surface MMG is dependent on the contractile properties of the activated motor units in humans.

https://www.physiology.org/doi/pdf/10.1152/japplphysiol.00008.2002

Hidetoshi Ue

Papers

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

Exercise training and autonomic nervous system activity in obese individuals.

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

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

Characteristics of surface mechanomyogram are dependent on development of fusion of motor units in humans.