The goal of this study was to use spectral analysis of EMG data to test the hypothesis that the O2 uptake VO2) slow component is due to a recruitment of fast fibers. Thirteen runners carried out a treadmill test with a constant speed, corresponding to 95% of the velocity associated with maximal VO2. The VO2 response was fit with the classical model including three exponential functions. Electrical activity of six lower limb muscles (vastus lateralis, soleus, and gastrocnemius of both sides) was measured using electromyogram surface electrodes. Mean power frequency (MPF) was used to study the kinetics of the electromyogram discharge frequency. Three main results were observed: 1) a common pattern of the MPF kinetics in the six muscles studied was noted; 2) MPF decreased in the first part of the exercise, followed by an increase for all the muscles studied, but only the vastus lateralis, and gastrocnemius muscles of both sides increased significantly (P < 0.05); and 3) the beginning of the MPF increase of the four muscles mentioned above corresponded with the beginning of the slow component. Our results suggest a progression in the average frequency of the motor unit discharge toward the high frequencies, which coheres with the hypothesis of the progressive recruitment of fast-twitch fibers during the VO2 slow component. However, this interpretation must be taken with caution because MPF is the result of a balance between several phenomena.

Previous research has shown that young male soccer players who are born early in a cohort are overrepresented on elite soccer teams. Selection advantages such as this have been termed 'relative age effects' (RAEs). Few studies have examined RAEs in elite women's youth soccer. Therefore, the aim of this study is to investigate the occurrence of RAEs in the Fédération Internationale de Football Association (FIFA) U-17 Women's World Cup competition and their link to playing positions. In the entire cohort of 672 players, we found significant RAEs in the geographical zones of Europe and North and Central America, no RAEs in the zones of Asia, Oceania, and South America, and significant inverse RAEs in the zone of Africa. Additionally, significant RAEs were found for goalkeepers and defenders from Europe and North and Central America. Inverse RAEs occurred for African goalkeepers, defenders, and strikers. Goalkeepers of all zones were significantly taller than players of all other playing positions. The results of this study show that remarkable RAEs do exist at elite women's youth soccer. Similar to men's soccer, there is a bias toward the inclusion of relatively older players, and a link between RAEs and playing positions.

Relative age effects (RAEs) refer to age differences between children in the same selection year. The present study investigated the prevalence of RAEs and their link to playing positions in Swiss junior soccer. Swiss male junior soccer players (n=50,581) representing 11% of the age-matched population - members of extra-curricular soccer teams - were evaluated to determine the influence of RAEs on Swiss junior soccer. Subgroups were the national talent development programme (n=2880), and U-15 to U-21 national teams (n=630). While no RAEs were found for the self-selected extra-curricular soccer teams or for the U-20 teams (P>0.05), significant RAEs were found for talent development and the national U-15 to U-19 and U-21 teams (P<0.01). Additionally, defenders born early in the year were significantly overrepresented compared with goalkeepers, midfielders and strikers (P<0.05). In Switzerland, RAEs apparently have substantial influence on the talent identification process for U-15 to U-18 teams, significantly influencing the selection of players in talent development teams already at an early age, but do not influence self-selected participation in extra-curricular soccer. Additionally, the RAE bias may be a predictor of playing positions in national teams. To minimise RAEs in Swiss soccer, systematic education for all coaches regarding RAEs should be established, in addition to a slotting system with rotating calendar cut-off dates.

Relative age effects (RAEs) refer to age differences among athletes in the same selection year. This study analyzed birth date distributions of 301,428 female athletes (aged 10-20 yr.) in Swiss Youth sports and the subgroup (n = 1,177) of the National Talent Development Program (TDP) in individual sports. Comparisons showed significant RAEs in the distribution of athletes' birth dates in alpine skiing, tennis, athletics, fencing, and snowboarding. Significant "reverse" RAEs with an overrepresentation of athletes at the end of the year were found in table tennis. In the TDP, significant RAEs were found in alpine skiing and tennis. No RAEs were detected in athletics. In table tennis, fencing, and snowboarding, "reverse" RAEs were found. Clearly, RAEs are complex and vary across individual sports for females.

Relative age refers to age differences between children in the same selection year. The present study investigated the prevalence of relative age effects (RAEs) at the Grand Prix Migros (GPM), which is the most popular alpine skiing race for children aged 7 to 14 years in Europe. In total, 17,992 Swiss junior alpine skiers, separated into female skiers (n = 7,227) and male skiers (n = 10,765), were evaluated in the 2010, 2011, and 2012 races. Chi-square analyses revealed no RAEs (p > 0.05) for the entire group of finishers in the qualification race for females in the Under U-8 to U-13 categories (n = 7,010) and all males (n = 10,410). Significant inverse RAEs were detected in the qualification race among female skiers in the U-14 and U-15 age categories (p < 0.01; odds ratio OR = 0.79; 95% confidence interval (CI) [0.64–0.98], and among disqualified male skiers (p < 0.01; OR = 0.54; [CI, 0.40–0.74]. However, significant RAEs were found for the entire group of both female and male skiers who qualified for the final race (p < 0.01; OR = 1.49; [CI, 1.28–1.73] of females, respectively OR = 2.18; [CI, 1.87–2.53] of males). RAEs were additionally apparent in all age categories of female and male finalists. The GPM is apparently influenced by RAEs, which may be an initial step towards RAEs in youth sports and may lead to an unequal participation in Swiss skiing.