The aim of this study was to determine if gender differences in muscle mass explain the gender differences in running and cycling sprint performance. Body composition (dual-energy X-ray absorptiometry), and running (30 and 300 m test) and cycling (Wingate test) sprint performance were assessed in 123 men and 32 women. Peak power (PP) output in the Wingate test expressed per kg of lower extremities lean mass (LM) was similar in males and females (50.4 +/- 5.6 and 50.5 +/- 6.2 W kg(-1), P = 0.88). No gender differences were observed in the slope of the linear relation between LM and PP or mean power output (MP). However, when MP was expressed per kg of LM, the males attained a 22% higher value (26.6 +/- 3.4 and 21.9 +/- 3.2 W kg(-1), P < 0.001). The 30 and 300-m running time divided by the relative lean mass of the lower extremities (RLM = LM x 100/body mass) was significantly lower in males than in females. Although, the slope of the linear relationship between RLM and 300-m running time was not significantly different between genders, the males achieved better performance in the 300-m test than the females. The main factor accounting for gender differences in peak and mean power output during cycling is the muscle mass of the lower extremities. Although, the peak power generating capability of the muscle is similar in males and females, muscle mass only partially explains the gender difference in running sprints, even when expressed as a percentage of the whole body mass.

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BackgroundExcessive time in front of a single or several screens could explain a displacement of physical activity. The present study aimed at determining whether screen-time is associated with a reduced level of moderate to vigorous physical activity (MVPA) in Spanish adolescents living in favorable environmental conditions.Methodology/Principal FindingsA multi-stage stratified random sampling method was used to select 3503 adolescents (12–18 years old) from the school population of Gran Canaria, Spain. MVPA, screen-time in front of television, computer, video game console and portable console was assessed in the classroom by fulfilling a standardized questionnaire. Bivariate and multivariate logistic regression analyses adjusted by a set of social-environmental variables were carried out. Forty-six percent of girls (95% CI±2.3%) and 26% of boys (95% CI±2.1%) did not meet the MVPA recommendations for adolescents. Major gender differences were observed in the time devoted to vigorous PA, video games and the total time spent on screen-based activities. Boys who reported 4 hours•week−1 or more to total screen-time showed a 64% (OR = 0.61, 95% CI, 0.44–0.86) increased risk of failing to achieve the recommended adolescent MVPA level. Participation in organized physical activities and sports competitions were more strongly associated with MVPA than screen-related behaviors.Conclusions/SignificanceNo single screen-related behavior explained the reduction of MVPA in adolescents. However, the total time accumulated through several screen-related behaviors was negatively associated with MVPA level in boys. This association could be due to lower availability of time for exercise as the time devoted to sedentary screen-time activities increases. Participation in organized physical activities seems to counteract the negative impact of excessive time in front of screens on physical activity.

The main aims of this study were to determine the accuracy of the portable metabolic cart K5 by comparison with a stationary metabolic cart (Vyntus CPX), to check on the validity of Vyntus CPX using a butane combustion test, and to assess the reliability of K5 during prolonged walks in the field. For validation, measurements were consecutively performed tests with both devices at rest and during submaximal exercise (bicycling) at low (60 W) and moderate intensities (130–160 W) in 16 volunteers. For the reliability study, 14 subjects were measured two times during prolonged walks (13 km, at 5 km/h), with the K5 set in mixing chamber (Mix) mode. Vyntus measured the stoichiometric RQ of butane combustion with high accuracy (error <1.6%) and precision (CV <0.5%), at VO2 values between 0.788 and 6.395 L/min. At rest and 60 W, there was good agreement between Vyntus and K5 (breath-by-breath, B×B) in VO2, VCO2, RER, and energy expenditure, while in Mix mode the K5 overestimated VO2 by 13.4 and 5.8%, respectively. Compared to Vyntus, at moderate intensity the K5 in B×B mode underestimated VO2, VCO2, and energy expenditure by 6.6, 6.9, and 6.6%, respectively. However, at this intensity there was an excellent agreement between methods in RER and fat oxidation. In Mix mode, K5 overestimated VO2 by 5.8 and 4.8%, at 60 W and the higher intensity, respectively. The K5 had excellent reliability during the field tests. Total energy expenditure per Km was determined with a CV for repeated measurements of 4.5% (CI: 3.2–6.9%) and a concordance correlation coefficient of 0.91, similar to the variability in VO2. This high reproducibility was explained by the low variation of FEO2 measurements, which had a CV of 0.9% (CI: 0.7–1.5%) combined with a slightly greater variability of FECO2, VE, VCO2, and RER. In conclusion, the K5 is an excellent portable metabolic cart which is almost as accurate as a state-of-art stationary metabolic cart, capable of measuring precisely energy expenditure in the field, showing a reliable performance during more than 2 h of continuous work. At high intensities, the mixing-chamber mode is more accurate than the B×B mode.

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