Longitudinal changes in sprint performance in relation to fitness development in U-14 soccer players
The 20 m sprint times of the players were measured by infrared photocell sensors (Optical Laptime Watcher, Hamamatsu Photonics K.K.) placed at distances of 5, 10, 15, and 20 m. As described in Kubo, et al. (2011), the sensors detected the release of the subject’s rear foot at the start and the passage of the subject’s trunk through the respective sensors. Each subject performed 3 sprinting trials with a 5min interval rest and the best time taken to cover the 20 m distance was used for data analysis. All measurements were conducted on the same grass soccer field using the same equipment. Body composition was measured using the BODPOD system (LMI, Inc.). Leg length and circumference were measured by a threedimensional photonic scanner (Hamamatsu Photonics K.K.). Muscle crosssectional area (CSA) for trunk and thigh muscles were determined using magnetic resonance imaging (0.2-T Signa Profile, GE) in the same way as earlier studies (Hoshikawa, et al., 2006, 2009). The CSA of the psoas majors, gluteus maximus and thigh muscles at upper(70%), middle(50%) and lower(30%) femur levels were determined on the images. Vertical height of squat jump (SJ) and countermovement jump with the arm swing (CMJ) were measured by filming with height calibration. The top of the head was digitized to measure the vertical jump height for each test performance. The onset of blood lactate accumulation (OBLA) and O2max were measured by an incremental test on a treadmill. The test protocol was a 3-minute submaximal run followed by a 1-minute rest repeated for 4 to 6 times with the running speed increased from 180 to 280 m/min. Blood lactate was measured immediately after each submaximal run to determine OBLA, which was defined as the speed at 4mmol/L lactate concentration. After that, a final run to exhaustion was performed to determine O2max by using a calibrated Meta Max metabolic system (Cortex Biophysic GmbH). Although all subjects were examined four times at intervals of 6 months for a period of 2 years, only the data from the first (M1) and last (M2) measurements were included in statistical analyses. Pearson-product correlations between sprint times for 20 m (20-m ST) and variables of anthropometry and fitness at M1 and between changes during the 2 years were calculated to examine their association. The 10 respective players showing the largest (L; -0.22±0.03 sec) and the smallest (S; -0.01±0.03 sec) improvements in 20-m ST from M1 to M2 were compared by a non-pared t-test. The statistical software SPSS ver.11.5 was used for analyses. Statistical significance was set at p<0.05.