'Practise makes perfect' is a well-known expression in most sports, including top-level soccer. The expression is often used as an argument for the assumption that the soccer players who have the highest and most effective training load will also achieve the highest skill and performance level (Ericsson et al., 1993). Studies illustrate that training load exposure increases from age 11 to 19 among elite youth soccer players (Baxter-Jones and Helms, 1996; Brito et al., 2012; Malina et al., 2000). Furthermore, McMillan and colleagues (2005) found that 18 year-old Scottish youth professional soccer players had an average training load (calculated as number of training hours per week) of 9.4 to 12.2 hours during a season.
In talent development, the monitoring of training and match load is important. Talented soccer players are often identified at a young age and are given the opportunity to train and compete on different teams, including a higher-level team within their club, a regional team or a national team. Furthermore, the label "soccer talent" often comes with high expectations from both the player and the players' environment (i.e., the club and coach). For the most talented players, the pressure to be successful is felt at an early age (Hill, 2013). Being labelled a soccer talent often implies playing in more matches and participating in fewer training sessions during a week (McMillan et al., 2005). As the intensity during a match is higher than in training sessions (Capranica et al., 2001), it is reasonable to assume that the physical activity (PA) level for talented players could be higher compared to players not identified as talented. A potential downside in the increased number of matches is the increased risk of injury, i.e. overuse (Brito et al., 2012). Le Gall and colleagues (2006) found older players to be injured more often during matches, while younger (U14) players incurred more injuries in training and sustained more growth-related overuse injuries. The injury risk is found to be significantly higher in soccer compared to other sports (Baxter-Jones and Helms, 1996). Furthermore, the pressure to be successful could lead to a potential lack of motivation and burnout. Hill (2013) found that one out of four 13-16 year-old players selected to an English professional club experienced symptoms of burnout at least once in the players career.
From a talent development perspective, it is important to investigate talented players' PA-level. Physical activity is defined as any bodily movement produced by skeletal muscles resulting in energy expenditure (Caspersen et al., 1985). Researchers previously attempted to describe a normal training week for both professional (Bangsbo et al., 2006) and youth soccer players (Akubat et al., 2012; Wrigley et al., 2012). These studies have, however, only focused on club related PA-load (training, match) with different approaches and measurements, including how both external (Rampinini et al., 2007; Hill-Haas et al., 2009; Brink et al., 2010) and internal (Akubat et al., 2012) training and match load can influence players' physiological response. To understand talented players' "total" PA-level, one could argue that it is important to also include activity outside the club. Several approaches have been used in an attempt to understand athletes' training and match load across a range of sports, including soccer (Wrigley et al., 2012). However, when reviewing the literature, studies using objective measurements to assess daily physical activity in talented soccer players were not discovered.
In this study, the focus is on the players daily PAlevel. The use of accelerometers to assess physical activity provides an opportunity to improve understanding about talented soccer players' PA-level. The aims of this study are: (1) to describe talented players' mean daily PA-level during three weeks in different periods of the season (2) to describe talented players' PA level in five training sessions within the same three weeks.
Participants and setting
Participants included 23 male youth soccer players (age = 17.8 [+ or -] 1.3yrs) who represented a premier level club in Norway. Data were collected from March 2011 through October 2011, with physical activity assessments during three weeks of the year. The three weeks ranged from the 18th to 25th of March, the 10th to 17th of June and the 14th to 21st of October. Average daily monitoring time was 12.5 [+ or -] 1.1 hours, ranging from 12.64 [+ or -] 1.02 in March (n = 14), 12.64 [+ or -] 2.14 in June (n = 13) and 12.16 [+ or -] 0.30 in October (n = 13).
Informed written consent was obtained from players, parents of under-age players and coaches. The study (ethics clearance) was in accordance and approved by the Norwegian Social Science Data Services.
Objective measure of daily physical activity
ActiGraph GT3X accelerometers (Manufacturing Technology Incorporated, Fort Walton Beach, FL) were used to obtain an objective assessment of physical activity. ActiGraph GT3X are small (3.8 x 3.7 x 1.8 cm), lightweight (27 g) activity monitors that measure and record time varying accelerations ranging in magnitude from approximately 0.05 to 2.5 G's. The accelerometer output is digitised by a twelve-bit Analog to Digital Convertor (ADC) at a rate of thirty times per second (30 Hz). Once digitised, the signal passes through a digital filter that band-limits the accelerometer to the frequency range of 0.25 to 2.5 Hz. This frequency range has been carefully chosen to detect normal human motion and to reject changing accelerations within the pass band. Each...