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Objective: 31 magnetic resonance spectroscopy (MRS) was used to document long lasting losses in muscle oxidative capacity after bouts of intense endurance exercise. Methods: The subject was a 34 year old highly fit female cyclist (Vo2MAX = 53.3 mI/kg/min). Over a five month period, she participated in three separate intense bouts of acute unaccustomed exercise. 31P MRS measurements were performed seven weeks after the first bout and every Iwo weeks for 14 more weeks. In all cases, 31P MRS measurements followed three days after each bout. Results: The subject showed a decreased ability to generate ATP from oxidative phosphorylation and an increased reliance on anaerobic ATP production during the 70% and 100% maximal voluntary contractions after the exercise bouts. Increased rates of fatigue and increased indicators of exercise difficulty also accompanied these reductions in muscle oxidative capacity. Increased oxidative and anaerobic ATP production were needed to maintain the work level during a submaximal 45% maximal voluntary contraction exercise. Conclusions: Acute increases in intensity accompanied by a change in exercise mode can influence the ability of muscle to generate ATP. The muscles were less economical and required more ATP to generate force during the submaximal exercises. During the maximal exercises, the muscle's mitochondria showed a reduced oxidative capacity. However, these reductions in oxidative capacity at the muscle level were not associated with changes in whole body maximal oxygen uptake. Finally, these reductions in muscular oxidative capacity were accompanied by increased rates of anaerobic ATP production, fatigue, and indicators of exercise difficulty. [ABSTRACT FROM AUTHOR]

OBJECTIVE:: To investigate whether skeletal muscle anaerobic metabolism, oxidative metabolism or metabolic economy during controlled sub-maximal and near-maximal exercises is altered in overweight women after diet-induced weight reduction, and whether these parameters are different between normal-weight, obesity-prone and normal-weight obesity-resistant women with similar physical fitness levels. DESIGN:: A prospective weight loss study of overweight women and their comparison with never overweight controls. SUBJECTS:: Thirty overweight, nondiabetic, premenopausal women and 28 never overweight controls were included in this analysis. All were participating in a longitudinal investigation of the role of energy metabolism in the etiology of obesity.The overweight women were recruited specifically to have a positive family history of obesity and have a body mass index (BMI) between 27 and 30 kg/m2 and were studied in the overweight state and after reduction to a normal weight. The never-overweight controls were recruited specifically to have no personal and family history of obesity and were group matched with the weight-reduced post-overweight subjects in terms of premenopausal status, age, BMI, race and sedentary lifestyle. MEASUREMENTS:: All testing was performed following one month of weight maintenance and during the follicular phase of the menstrual cycle. Hydrostatic weighing was performed to measure body composition and a whole-body maximal oxygen uptake (VO2max) test was done to measure aerobic fitness. 31P MRS was used to determine ATP production from oxidative phosphorylation (OxPhos), ‘anaerobic’ glycolysis (AnGly), and creatine kinase (CK), as well as muscle metabolic economy. The time constant of ADP (TCADP), VPCr (ie the initial rate of PCr resynthesis following exercise), and Qmax (ie the apparent maximal oxidative ATP production rate) were also calculated as additional... [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper was to determine whether differences in hemoglobin (Hb) and muscle aerobic capacity exist between African-American (AA) and Caucasian (CA) premenopausal women and to determine whether Hb and aerobic capacity of the muscle are associated with the racial differences in maximum oxygen uptake (VO2max)., Methods: 43 AA and 46 CA sedentary premenopausal women were subjects. Percent body fat was determined by four-compartment model, leg lean tissue by dual energy x-ray absorptiometry, VO2max during a graded exercise test, aerobic capacity of the calf muscle by 31P magnetic resonance spectroscopy, and serum Hb by the cyanide method., Results: AA women had reduced VO2max (AA 29.3 +/- 3.0 vs CA 33.6 +/- 5.6 mL.kg(-1) bdw(-1).min, P < 0.01), reduced muscle aerobic capacity (AA 24.3 +/- 5.8 vs CA 21.3 +/- 4.8 s, P = 0.01, where lower values indicate higher aerobic capacity), and reduced Hb (AA 11.8 +/- 1.3 vs CA 12.9 +/- 0.8 g.dL(-1), P < 0.01). The racial difference in VO2max persisted whether the values were unadjusted or adjusted for fat-free mass or leg lean tissue. Multiple regression analysis revealed that both Hb and muscle aerobic capacity were related to VO2max after adjusting for each other, race, and either fat-free mass or leg lean tissue. Being AA was associated with reduced VO2max in mL O2.kg leg lean tissue(-1).min(-1) (zero-order simple Pearson-product correlation -0.60, P < 0.01). When multiple regression was used, the correlation between race and VO2max decreased but persisted (-0.40, <0.01) after adjusting for Hb and muscle aerobic capacity., Conclusions: These data suggest that differences in Hb and aerobic capacity of muscle are related to reduced VO2max in AA women. However, Hb and aerobic capacity of the muscle can only partially explain the racial differences in VO2max.

Purpose: It is well accepted that maximum strength is related to muscle size. The primary purpose of this study was to determine whether anthropometric or dual-energy x-ray absorptiometry (DEXA) estimates of muscle size were valid predictors of plantar flexor maximum voluntary contraction (MVC) strength and could be used in lieu of more sophisticated techniques (e.g., magnetic resonance imaging (MRI)). Additionally, we compared the relationship among MVC and three MRI-determined muscle size measures; anatomical (ACSA) and physiological (PCSA) cross-sectional areas; and muscle volume (VOLm)., Methods: We measured plantar flexor MVC at 1.83 rad and various indices of muscle size: 1) body weight, 2) total body lean mass (LM) (DEXA), 3) lower leg LM (DEXA), 4) lower leg circumference, 5) estimated muscle+bone cross-sectional area (CSA) from circumference and calf skin-fold, 6) triceps surae ACSA, 7) triceps surae PCSA, and (8) triceps surae volume (VOLm), in 39 premenopausal women (mean +/- SD: 36 +/- 8 yr, 165 +/- 6 cm, and 65 +/- 9 kg)., Results: Zero-order correlations showed significant (P < 0.05) associations between MVC and total body LM (r = 0.365), lower leg LM (r = 0.381), circumference (r = 0.584), estimated muscle+bone CSA (r = 0.447), ACSA (r = 0.733), PCSA (r = 0.715), and VOLm (r = 0.649). By using the Fisher Z-transformation, ACSA and PCSA correlated significantly higher with MVC (P < 0.05) than anthropometric and DEXA indices. Further, only ACSA and PCSA regressed to the origin, indicating the ability to predict MVC was greatest with these two measures., Conclusions: The MRI-determined muscle size indices, which were specific to the triceps surae, correlated with strength better than whole limb anthropometric and DEXA indices. In this group of women, both ACSA and PCSA appeared superior to VOLm for predicting strength. PCSA was not found to be more precise than ACSA. ACSA appears to provide adequate precision for estimating plantar flexor specific tension in vivo.

Background: The prevalence of obesity is higher in black than in white women. Differences in energy economy and physical activity may contribute to this difference., Objective: The objective of this study was to compare free-living energy expenditure and physical activity in black and white women before and after weight loss., Design: Participants were 18 white and 14 black women with body mass indexes (in kg/m(2)) between 27 and 30. Diet, without exercise, was used to achieve a weight loss of >/=10 kg and a body mass index <25. After 4 wk of energy balance in overweight and normal-weight states, body composition was assessed by using a 4-compartment model, sleeping and resting energy expenditures were assessed by using a chamber calorimeter, physiologic stress of exercise and exercise economy were measured by using standardized exercise tasks, and daily energy expenditure was assessed by using doubly labeled water., Results: Weight loss averaged 12.8 kg. Sleeping and resting energy expenditures decreased in proportion to changes in body composition. Weight reduction significantly improved physiologic capacity for exercise in both groups of women, making it easier for them to be physically active. Black women had lower body composition-adjusted energy requirements than did white women-both before and after weight loss-during sleep (9% lower, 519 kJ/d; P < 0.001), at rest (14% lower, 879 kJ/d; P < 0.001), during exercise (6% lower; P < 0. 05), and as a daily total (9% lower, 862 kJ/d; P < 0.06). By contrast, free-living physical activity was similar between the groups., Conclusions: Weight-reduced women had metabolic rates appropriate for their body sizes. Black women had lower resting and nonresting energy requirements in both overweight and normal-weight states than did white women and did not compensate with greater physical activity, potentially predisposing them to greater weight regain.

We used (31)P magnetic resonance spectroscopy to measure maximal mitochondrial function in 12 obesity-prone women before and after diet-induced weight reduction and in 12 matched, never-obese, and 7 endurance-trained controls. Mitochondrial function was modeled after maximum-effort plantar flexion from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). Weight reduction was not associated with a significant change in mitochondrial function by TC(PCr), TC(ADP), or OxPhos. Mitochondrial function was not different between postobese and never-obese controls by TC(PCr) [35.1 +/- 2.5 (SE) vs. 34.6 +/- 2.5 s], TC(ADP) (22.9 +/- 1.8 vs. 21.2 +/- 1.8 s), or OxPhos (0.26 +/- 0. 03 vs. 0.25 +/- 0.03 mM ATP/s), postobese vs. never-obese, respectively. However, TC(ADP) was significantly faster (14.5 +/- 2. 3 s), and OxPhos was significantly higher (0.38 +/- 0.04 mM ATP/s) in the endurance-trained group. These results suggest that maximal mitochondrial function is not impaired in normal-weight obesity-prone women relative to their never-obese counterparts but is increased in endurance-trained women.