This study was approved by the ethical committee of the National Institute of Fitness and Sports in Kanoya and was consistent with its requirements for human experimentation. Prior to the experiment, all subjects were informed of the experimental procedures and possible risks of the measurements. Written informed consent was obtained from each subject. The purpose and hypothesis of the study were not explained to the subjects until termination of measurements, to exclude any potential bias that might affect the results.
Participants
Male sprinters (N = 10; age 20.1 ± 0.6 years; height 174.6 ± 4.4 cm; weight 66.7 ± 3.5 kg; 100-m personal best time, 11.46 ± 0.57 s; means ± SDs) participated in this study. They were free of cardiovascular, metabolic, and immunologic disorders and orthopedic abnormalities, and were not using any medications that affected their muscle functions. All participants had been competing in short distance track and field events (100, 200, and 400 m) for more than 4 years. All participated in specific training programs for sprinters with a coach for at least 3 h/day on 5 days/week.
Experimental procedure
We compared sprint performance at baseline with sprint performance after conducting different conditioning activities, one of which involved actual sprinting. Participants were assessed in three experimental sessions with an interval of 2 days between each session. In each session, they performed a different conditioning activity. Figure 1 presents the study design. One hour prior to the baseline assessment (described below), participants first performed their usual pre-competition warm-up exercises, which were not specified but left to their discretion. After completing warm-up exercises and before the treatment, all participants performed a baseline 60-m maximal sprint on a synthetic surface track wearing spiked shoes (Pre). They took a 10-min rest, and then performed one of the three conditioning activities for 10 min with a coach’s encouragement. In the 60-m free sprint, participants sprinted twice for over 60 meters, at maximal effort, with an interval of 5 min between sprints, starting from a crouched position. In the 60-m bounding jump, which required explosive and horizontal jumping movements, participants jumped three times, each time as far as possible horizontally, using alternating legs and with their arms swinging, with an interval of 3 min between jumps. In the mini-hurdle drill, which involved movements with high step frequency, participants ran three times, each time as fast as possible, over 10 mini-hurdles (height 22 cm and spaced 90 cm apart) with an interval of 3 min between trials. The order of the treatments over days was randomized across participants. After completing each treatment, participants ran 60 m again with maximal effort (Post).
60-m sprint performance test
Running kinematics during the 60-m sprint before and after each treatment was recorded using a high-speed camera (EX-F1, CASIO, Japan) at a sampling frequency of 300 frames/s (shutter speed, 1/1600 s). The camera was set 30 m away from the lateral edge of the running course, such that it panned across the runway. Lines were drawn every 10 m on the lane, and markers were set between the centers of each line and the camera. The running time, running velocity, and the number of steps were averaged over every 10 m using the films. Step length was determined by dividing running velocity by step frequency, and was averaged over every 10 m. The highest running velocity among the 6 phases was labeled the maximal running velocity, and step frequency and step length in the corresponding phase were adopted as the maximal step frequency and step length, respectively. In addition, the mean values of running velocity, step length, and step frequency were calculated by averaging the corresponding values across the phases. In this article, the term “step frequency” is defined as the number of steps per second in half the running cycle, that is, from foot contact to the next contact of the opposite foot. The term “step length” is defined as the distance from foot contact to the next contact of the opposite foot.
Statistical analysis
Independent variables in the analysis were maximal and mean running velocity, step length and step frequency. Descriptive statistics are presented as means and SDs. A one way repeated measures analysis of variance (ANOVA) with a Bonferroni post hoc test was conducted to test the significance of differences among the three conditions for each of the independent variables at Pre. Intra-class correlation coefficients (ICCs) and confidence intervals (CIs) were calculated for the corresponding values. Coefficients of variation (CVs) for sprint time, maximal running velocity, mean running velocity, step length, and step frequency for 60 m at Pre were calculated by dividing SDs by means across all sessions, and multiplying the values by 100 (%). A two-way repeated measures ANOVA (2 times: Pre, Post × 6 phases of running distance) was conducted to test the significance of differences for each of the independent variables. A two-way repeated measures ANOVA (3 conditions × 2 times: Pre, Post) was conducted to test the significance of differences between Pre and Post for 60 m time, maximal running velocity, step length, and step frequency for each of the independent variables. When a significant interaction was found, a simple main effects test was used for post hoc comparison. Effect size (ES) was calculated for each condition as (means value at Post − means value at Pre)/SDs at Pre. ES was classified as trivial (ES < 0.2), small (0.2 < ES < 0.5), moderate (0.5 < ES < 0.8), or large (ES > 0.8). Statistical significance was set at p < 0.05. All data analyses were conducted using the statistical software (SPSS 19.0 for Windows, IBM, Japan).