# Kinematic Determinants of the Swimming Push Start in Competitive Swimmers
## 2 Materials and Methods  
Seventy-four swimmers (33 males with 16.5 \(\pm\) 1.29 years of age, 62.34 \(\pm\) 7.22 kg, 1.73 \(\pm\) 0.05 m and 41 females with 15.5 \(\pm\) 1.29 years old, 50.50 \(\pm\) 6.53 kg, 1.60 \(\pm\) 0.05 m) voluntarily participated in the present study. All swimmers competed at the national level and accredited a personal best in the 100 m event of butterfly, backstroke or front-crawl within the 85% of world record in long course. According to the recent standardization of swimming results proposed by [PERSON] et al. [22], participants in the present study would be identified as level two and three swimmers. Their training programs consisted of between 12 and 20 h of water training and between 3 and 6 h of dry-land training per week. Swimming experience was 7.12 \(\pm\) 2.35 years for males and 5.90 \(\pm\) 1.68 years for females. A written informed consent was signed by all participants or their tutors, and all experimental procedures were conducted according to Declaration of Helsinki for human research [23]. The study was approved by the Local University Ethics Committee. Tests were conducted in an indoor 50 m pool, with water temperature at 27 \({}^{\circ}\)C and at the usual training time. Prior to the experiment, the swimmers' hip joint center was marked with a waterproof adhesive tape of a round shape (diameter 25 mm) to assist in the video analysis. After a standardized warm up consisting of 5-10 min of dynamic dry-land exercises and 20-25 min of swimming (including some easy swim, technical drills, descend 50 m repeats and some speed exercises), the swimmers performed one repetition of 25 m at maximal intensity from an in-water push start. Swimmers were instructed to begin with feet situated approximately one meter below the water surface to avoid the wave drag resistance during the wall push-off [24]. The 25 m effort was performed at the preferred stroke (butterfly, backstroke or front-crawl) and, in the case of swimmers that accredited a personal best in 100 m events faster than 85% of the world record, two or three 25 m repetitions (at the different strokes in random order) were performed with at least three minutes of passive rest between each of them. In total, 46 trials were performed at front-crawl, 53 at butterfly and 24 at backstroke. No breaststroke trials were included for the aims of the present study as FINA rules do not allow to perform a free UUS subsection and do not limit the underwater segment to 15 m.  
Two sequential video cameras (JVC GY-DV500E), situated behind the underwater windows of the pool at a distance of 3.5 and 8 m from the starting wall, recorded all trials at a frame rate of 50 Hz and with image resolution of 758 \(\times\) 576 pixels. Cameras were located at 10 m away from the swimmers' lane, with their optical axis perpendicular to the sagittal plane of swimmers and with both fields of view filming the swimmers' trials in sequence. Before the commencement of the trials, a rectangular calibration frame (5 m length and 2 m wide) containing eight calibration points was located in the middle of the swimming lane and perpendicular to the cameras field of view. An experienced observer manually digitized all trials footage by mean of Photo 23D software [25]. The hip center was identified at the beginning and/or end of the of the sections of the push start to represent the swimmers' position [26] and screen coordinates were transformed to real spatial coordinates in two dimensions by means of Direct Linear Transformation algorithms [27]. Eight control points in the calibration frame (different from those employed for calibration purposes) were used to estimate accuracy in the coordinates' reconstruction. RMSE (root-mean-squared error) in the horizontal and vertical axes was lower than 0.035 m, which represented less than 0.6% of the field of view of each camera.