Influence of graded hypercapnia on endurance exercise performance in healthy humans.

Military and/or emergency services personnel may be required to perform high-intensity physical activity during exposure to elevated inspired carbon dioxide (CO2). Although many of the physiological consequences of hypercapnia are well characterized, the effects of graded increases in inspired CO2 on self-paced endurance performance have not been determined. The aim of this study was to compare the effects of 0%, 2%, and 4% inspired CO2 on 2-mile run performance, as well as physiological and perceptual responses during time trial exercise. Twelve physically active volunteers (peak oxygen uptake = 49 ± 5 mL·kg-1·min-1; 3 women) performed three experimental trials in a randomized, single-blind, crossover manner, breathing 21% oxygen with either 0%, 2%, or 4% CO2. During each trial, participants completed 10 min of walking at ∼40% peak oxygen uptake followed by a self-paced 2-mile treadmill time trial. One participant was unable to complete the 4% CO2 trial due to lightheadedness during the run. Compared with the 0% CO2 trial, run performance was 5 ± 3% and 7 ± 3% slower in the 2% and 4% CO2 trials, respectively (both P < 0.001). Run performance was significantly slower with 4% versus 2% CO2 (P = 0.046). The dose-dependent performance impairments were accompanied by stepwise increases in mean ventilation, despite significant reductions in running speed. Dyspnea and headache were significantly elevated during the 4% CO2 trial compared with both the 0% and 2% trials. Overall, our findings show that graded increases in inspired CO2 impair endurance performance in a stepwise manner in healthy humans.

Black and White race differences in aerobic capacity, muscle fiber type, and their influence on metabolic processes.

Obesity is a known risk factor for cardiometabolic disease. Increasing aerobic capacity (VO2max) reduces adiposity, maintains weight, and reduces the risk of developing obesity and cardiometabolic disease. Two major determinants of aerobic capacity are the metabolic properties specific to a particular muscle fiber type and the capacity of the cardiorespiratory system to deliver nutrient-rich content to the muscle. Recent research suggests that some race/ethnic groups, particularly non-Hispanic Black subjects, are predisposed to a reduced VO2max by way of muscle fiber type. Combined with insufficient physical activity, these characteristics place non-Hispanic Black subjects at an increased risk for obesity and other adverse health outcomes when compared with other race/ethnic groups. The purpose of this review was to suggest a model for explaining how skeletal muscle fiber type may contribute to reduced aerobic capacity and obesity among non-Hispanic Black subjects. Our review indicates that metabolic properties of type II skeletal muscle (e.g. reduced oxidative capacity, capillary density) are related to various cardiometabolic diseases. Based on the review, non-Hispanic Black subjects appear to have a lower maximal aerobic capacity and a greater percentage of type II skeletal muscle fibers. Combined with reduced energy expenditure and reduced hemoglobin concentration, non-Hispanic Black subjects may be inherently predisposed to a reduced maximal aerobic capacity compared with non-Hispanic White subjects, thereby increasing the risk for obesity and related metabolic diseases.

Differential gene expression in high- and low-active inbred mice.

Numerous candidate genes have been suggested in the recent literature with proposed roles in regulation of voluntary physical activity, with little evidence of these genes' functional roles. This study compared the haplotype structure and expression profile in skeletal muscle and brain of inherently high- (C57L/J) and low- (C3H/HeJ) active mice. Expression of nine candidate genes [Actn2, Actn3, Casq1, Drd2, Lepr, Mc4r, Mstn, Papss2, and Glut4 (a.k.a. Slc2a4)] was evaluated via RT-qPCR. SNPs were observed in regions of Actn2, Casq1, Drd2, Lepr, and Papss2; however, no SNPs were located in coding sequences or associated with any known regulatory sequences. In mice exposed to a running wheel, Casq1 (P = 0.0003) and Mstn (P = 0.002) transcript levels in the soleus were higher in the low-active mice. However, when these genes were evaluated in naïve animals, differential expression was not observed, demonstrating a training effect. Among naïve mice, no genes in either tissue exhibited differential expression between strains. Considering that no obvious SNP mechanisms were determined or differential expression was observed, our results indicate that genomic structural variation or gene expression data alone is not adequate to establish any of these genes' candidacy or causality in relation to regulation of physical activity.

Cross-validation of waist-worn GENEA accelerometer cut-points.

PURPOSE: The purpose of this study was to determine the classification accuracy of the waist gravity estimator of normal everyday activity (GENEA) cut-points developed by Esliger et al. for predicting intensity categories across a range of lifestyle activities. METHODS: Each participant performed one of two routines, consisting of seven lifestyle activities (home/office, ambulatory, and sport). The GENEA was worn on the right waist, and oxygen uptake was continuously measured using the Oxycon mobile. A one-way chi-squared test was used to determine the classification accuracy of the GENEA cut-points. Cross-tabulation tables provided information on under- and overestimations, and sensitivity and specificity analyses of the waist cut-points were also performed. RESULTS: Spearman rank order correlation for the GENEA gravity-subtracted signal vector magnitude and Oxycon mobile MET values was 0.73. For all activities combined, the GENEA accurately predicted intensity classification 55.3% of the time, and it increased to 58.3% when stationary cycling was removed from the analysis. The sensitivity of the cut-points for the four intensity categories ranged from 0.244 to 0.958, and the specificity ranged from 0.576 to 0.943. CONCLUSION: In this cross-validation study, the proposed GENEA cut-points had a low overall accuracy rate for classifying intensity (55.3%) when engaging in 14 different lifestyle activities.

Classification accuracy of the wrist-worn gravity estimator of normal everyday activity accelerometer.

PURPOSE: The purpose of this study was to determine whether the published left-wrist cut points for the triaxial Gravity Estimator of Normal Everyday Activity (GENEA) accelerometer are accurate for predicting intensity categories during structured activity bouts. METHODS: A convenience sample of 130 adults wore a GENEA accelerometer on their left wrist while performing 14 different lifestyle activities. During each activity, oxygen consumption was continuously measured using the Oxycon mobile. Statistical analysis used Spearman's rank correlations to determine the relationship between measured and estimated intensity classifications. Cross tabulations were constructed to show the under- or overestimation of misclassified intensities. One-way χ2 tests were used to determine whether the intensity classification accuracy for each activity differed from 80%. RESULTS: For all activities, the GENEA accelerometer-based physical activity monitor explained 41.1% of the variance in energy expenditure. The intensity classification accuracy was 69.8% for sedentary activities, 44.9% for light activities, 46.2% for moderate activities, and 77.7% for vigorous activities. The GENEA correctly classified intensity for 52.9% of observations when all activities were examined; this increased to 61.5% with stationary cycling removed. CONCLUSIONS: A wrist-worn triaxial accelerometer has modest-intensity classification accuracy across a broad range of activities when using the cut points of Esliger et al. Although the sensitivity and the specificity are less than those reported by Esliger et al., they are generally in the same range as those reported for waist-worn, uniaxial accelerometer cut points.

Association of physical activity, fitness, and race: NHANES 1999­2004.

PURPOSE: Regular physical activity (PA) can be used to improve cardiorespiratory fitness (CRF). Previous research has shown differences in CRF (VO2max) among racial groups, but it is unclear how much of these differences can be explained by PA. Thus, we sought to examine the association between PA and CRF in different racial groups. METHODS: As a part of the National Health and Nutrition Examination Survey (1999­2004), 3115 adults (18­49 yr) completed a submaximal graded treadmill exercise test to estimate VO2max. Independent variables were demographics (race, education, sex, partner status, and waist circumference), behavioral measures (smoking and alcohol consumption), self-reported PA from three domains (leisure-time, domestic, and transportational PA (MET x min x wk(-1)), and the proportion of PA at a vigorous intensity (VMET). CRF was the dependent variable. Multiple linear regression was performed using SUDAAN statistical software. RESULTS: Results indicated that VO2max was significantly higher for Mexican Americans (40.9 T 0.5 mL x kg(-1) x min(-1)) and non-Hispanic whites (40.2 +/- 0.3 mL x kg(-1) x min(-1)) compared with non-Hispanic blacks (37.9 +/- 0.6 mL x kg(-1) x min(-1)) (P = 0.01). Demographics including race explained 18.5% of the variance in VO2max, with race being significant (P < 0.01) in the model. When PA was added to the model, the explained variance in VO2max increased to 19.3% (P = 0.001). VMET was more predictive of VO2max than total PA, and the model including VMET explained 20.4% of the variance in VO2max. Race remained a significant, independent predictor of VO2max after VMET and overall PA were added to the model. CONCLUSION: Race, PA, and exercise intensity are important factors in explaining differences in CRF. After accounting for demographics, PA, and VMET, a large proportion of the variance in CRF remains unexplained. Thus, other factors should also be considered when examining racial/ethnic differences in CRF.