This study design was reviewed and approved by Kanazawa University Medical Ethics Review Committee (reference number (2016 − 115) and conducted in accordance with the Declaration of Helsinki; informed consent was obtained from participants and their parents after thorough explanation, in both written and oral formats, regarding the study before enrollment.
We enrolled 47 male preadolescent football players who play for an amateur team under the umbrella of a professional soccer club with an average age of 12.4 ± 0.6 years, height of 153.3 ± 8.9 cm, body weight of 43.9 ± 8.2 kg, and body mass index (BMI) of 18.5 ± 2.0 kg/m2. All participants took baseline surveys as described below. The survey items were height, body weight, BMI, anteflexion in sitting, and quadriceps and hamstring tightness. Height and body weight were measured using a portable stadiometer and weight meter with graduations of 0.1 cm and 0.1 kg, respectively. To assess anteflexion in sitting, we used a digital machine for anteflexion measurement (Long-seat anteflexion meter; Takei Scientific Instruments Co., Ltd, Niigata, Japan). Participants were asked to sit with their back against the wall, extend the elbow joint, and grasp the instrument between the thumb and index finger. By moving the instrument forward as much as possible with the knees remaining extended, we could measure the forward movement distance in 0.5 cm as an anteflexion. To assess the quadriceps and hamstring tightness, we measured the heel-hip distance (HHD; cm) in the prone position and the straight-leg-raise angle (SLR; degrees) in the supine position. Quadriceps and hamstring tightness was measured as described below. An examiner applied increasing force on the participant’s lower limb until just before the examiner felt resistance. Concomitantly, the second examiner obtained all measurements using a ruler and goniometer for lower limb muscle tightness. All items were measured thrice, and the average values were used for statistics. All examiners were orthopedic surgeons or physiotherapists with more than 5 years of experience.
Figure 2 presents the technique for performing the JKS. In the starting position, the participant squats while holding their ankle joints with their hands. Subsequently, there is gradual extension of the knee joints while maintaining contact between the chest and thighs. The maximum extension position is achieved when the quadriceps femoris is at maximal contraction (Fig. 1) with the position being held for 10 seconds. One set consists of five JKS repetitions with each repetition being held for 10 seconds. We instructed participants to do one set of JKS daily for half a year. The participants were asked to perform daily stretching and indicated it on the day. After 6 months, we checked all survey items and compliance for JKS frequency.
The participants were divided into two groups according to the JKS frequency as follows: group A included participant who performed their JKS stretching more than one time every 3 days and group B included participants who performed their JKS stretching less than one time every 3 days. Among the 47 participants, we excluded 3 and 4 patients with Osgood-Schlatter disease and low back pain, respectively, as well as 2 and 4 patients in Group A and B, respectively, who were lost to follow-up. Finally, groups A and B each consisted of 17 healthy participants.
All data were analyzed using SPSS 23.0 (IBM SPSS Statistics 23). The paired t-test was used to determine significant differences in the pre- and post-exercise height, weight, BMI, anteflexion in sitting, HHD, and SLR. Between-group comparisons were performed using Student’s t-test. Statistical significance was set at 0.05 (two-sided) with 95% confidence intervals. The sample size was calculated using G-Power 3.1 (effect size 0.8, α-error 0.05, and target power 0.95), and a minimum of 15 participants per group was recommended.