Physical activity plays an important role in the bone health (Mackelvie et al., 2002) and overall growth and development of children (Roemmich et al., 2001). The effectiveness of exercise-training on performance and physical adaptation depends on the training load and on the individual's ability to adapt to training. Too little or too much training will have relative impact on the desired physical, mental and performance outcomes. Therefore, finding objective measures to quantify the balance between training load and athlete's tolerance is essential. Intensive sports training and competition are often associated with endocrine deficiencies in adult men (Tanskanen et al., 2011). In young athletes, the stress of intense physical training, combined with calorie restriction, can alter cellular homeostasis and the normal pattern of pubertal development (Adiyaman et al., 2004; Roemmich et al., 2001). The long term benefit from intense training in adolescents and the mechanisms for how intense exercise training may affect tissue anabolism without taking into account the normal growth are not well known (Roemmich et al., 2001). The impact of strenuous exercise on the pubertal development of child and adolescent athletes in several sports is still not well understood.
Normal growth in children and adolescents is regulated to a great extent through the actions of the growth hormone/insulin-like growth factor-I (GH/IGF-I) axis (Adiyaman et al., 2004). Physical exercise plays an important role in the regulation of the GH/IGF-I axis by increasing GH secretion (Kanaley et al., 1997; Kraemer and Ratamess, 2005). Literature data concerning the effect of training on GH response to exercise training are mixed. In over-trained athletes, endogenous GH production is suppressed, exercise tolerance is reduced and performance is decreased (Schmikli et al., 2012). Therefore, the GH response to exercise is complex and appears to be affected by many variables (eg, exercise-type, intensity, duration, age ...).
The effects of physical training on IGF-1 responses remain incompletely understood. Some investigations have found that endurance-training increased IGF-1 circulating levels (Maimoun et al., 2004), whereas other authors have reported a decrease (Eliakim et al., 1998). Moreover, acute exercise and exercise-training have been reported to increase IGFBP-3 (Di Luigi et al., 2001). Divergent results concerning GH, IGF-1 and IGFBP-3 may be explained by differences in exercise-type, intensity and duration as well as the subject's training level, dietary status, body composition, and age.
Soccer is the most popular sport in the world, especially among children and adolescents. Optimizing the physical potential of young soccer players is one of the main objectives of youth soccer academies. Indeed, elite soccer players must be prepared to perform and sustain high-load training. The most important variables for measuring performance in soccer are physical fitness and technical and tactical performance (Rosch et al., 2000). The physical fitness of soccer players is usually measured in terms of endurance, speed, power, and strength (Hoff, 2005). It is relatively easy to test the physical fitness of young players, but it is a more challenging task to differentiate between the adaptations of soccer training and growth-mediated development (Vanttinen et al., 2011).
Some cross-sectional studies have demonstrated that soccer practice induces positive hormonal adaptations (Mejri et al., 2005; Vanttinen et al., 2011). However, effects of long-term intense soccer training on the adaptations of GH/IGF-I axis are very scarce. To the best of our knowledge, only one study examined the effects of intense soccer training over a competitive season on hormones related to growth. Mejri et al. (2005) observed, in young adult soccer players (19 years of age), that soccer training decreased exercise-stimulated GH levels throughout the competitive season, but did not have any effect on basal IGF-1 and IGFBP-3 levels. However, this study had subjects at the end of adolescent growth and they did not include a control group to differentiate exercise training from normal growth (Bouix et al., 1997).
To the best of our knowledge no longitudinal investigation (more than 1-yr.) has studied the impact of intense exercise training in elite adolescent soccer players on markers of growth and development. Therefore, the purpose of this study was to compare changes in growth related hormones (GH, IGF-1 and IGFBP-3) between elite youth soccer players belonging to youth national team and non-athletic controls over a 2-year period. We hypothesized that intense soccer-training would elicit greater physical, hormonal and physiological changes than just growth alone.
Thirty-six adolescent boys were recruited to participate in our study. Eighteen were residents at the youth academy of elite soccer players (age: 14.5 [+ or -] 0.4 years at the start of the study) and were members of the U-17 national team. These athletes were preparing for the U-17 Championship of African Nations which is a qualifier for the world cup. This group of elite players were selected from among 800 young male soccer players from six regional centres of football throughout Tunisia. The criteria used for selection was based on technical tests and physical fitness parameters. They had been playing soccer, in addition to their school physical education, for 11 months of the year, for at least 5 years, at a rate of 5 practice sessions and one competitive game per week. In general, soccer training sessions lasted ~1.5 hours, with about 15-20 min of warm up consisting of low-intensity games and stretching exercises, 15-25 min of technical soccer exercises (kicking, dribbling, jumping, and running with fast accelerations and decelerations), 20-30 min of match practice, and 10-15 min of active recovery.
Eighteen subjects (age: 14.3 [+ or -] 0.3 years at the beginning of the study) were assigned to the control group (non-athletic boys). They participated only in the compulsory physical education curriculum at school (two weekly sessions of 50 min). Individuals in the control group were randomly chosen from nearby schools, were healthy and were representative of the general population. Written informed consent was obtained from the parents of each subject before the study, and the study was approved by the Ethical Committee on Human Research of the University of Manouba, Tunisia.
The experiment was conducted over five time-points for both soccer players and the control group (Figure 1). Testing was conducted over 2 days. On both testing days, all subjects performed the identical standardized 15 min warm-up consisting of low-intensity running, a series of dynamic stretching exercises (high knee lifts, butt kicks, straight line skipping, etc.) and short accelerations. On the first test day, subjects performed a squat jump, counter movement jump using arms, a 5 consecutive jump test and a 30m sprint. On the second testing day, the subjects performed a Yo-Yo Intermittent Recovery Test--Level 1 (2).
Puberty stage assessment
Puberty (Tanner) stage was determined and recorded by a pediatrician experienced in the assessment of secondary sex characteristics according to the method of Tanner (1975). It was determined that all subjects were at Tanner stage 2-3 at the beginning of the study and 2 years after they attained Tanner stage 4 and 5.
Each participant came to the laboratory for a medical examination and anthropometric measurements performed by a pediatrician at each time period (T0, T1, T2, T3 and T4). Body height and body mass were measured with standard techniques to the nearest 0.1 cm and 0.1 kg, respectively for each subject. To estimate the adiposity, skin-fold thickness was measured at four sites (triceps, biceps, subscapular and suprailiac) (During and Webster., 1985) using a Harpenden skin-fold calliper (British Indicators Ltd., Luton). All measurements were taken in the morning between 07:30 and 08:30 am by the same investigator for all time periods.
Physical fitness characteristics
Vertical Jump: Each subject performed three maximal jumps of the following...