Impact of Effective Frequency Jack-knife Stretching on Preadolescent Male Football Players -Prospective Cohort Study-Junsuke

Objective: We aimed to investigate the effective frequency of jack-knife stretching in preadolescent male football players. Methods: We enrolled 47 male preadolescent football players (average age: 12.4 ± 0.6 years old). All participants took surveys at baseline and after intervention for 6 months. The survey items were height, weight, body mass index, anteexion in sitting, and quadriceps and hamstring tightness. The participants were divided into two groups according to the stretching exercise frequency, where group A performed stretching at least once every 3 days and group B performed stretching less than once every 3 days. After exclusion, 17 participants were enrolled in each group. Results: There was a signicant increase in anteexion in the sitting position in group A but not group B; moreover, straight leg raising was signicantly larger in Group A Conclusions: Performing jack-knife stretching at least once every 3 days’ intervals could improve hamstring exibility in preadolescent male football players.


Introduction
Stretching of muscle-tendon complexes occurs with bone growth in length with adaptation involving an increase in the sarcomere number in muscle bers. In case of an unbalanced increase in the sarcomeres and bone length, there is an increase in muscle tightness [1,2]. Further, there is a signi cant increase in thigh muscle tightness with skeletal maturation in preadolescent males [3]. Furthermore, tight hamstring and/or quadriceps muscles have been reported as risk factors for disorders, including Osgood-Schlatter disease and lumbar spondylolysis in preadolescents and adolescents [4,5]. Increased muscle exibility allows for increased freedom of movement, which could be of bene t to preadolescents and adolescents. Therefore, maintaining exible quadriceps and hamstrings muscle could be important for preventing Osgood-Schlatter disease and lumbar spondylolysis.
Previous studies have described three muscle stretching techniques, namely, static, dynamic, and precontraction stretches. The traditional and most common type is static stretching, which involves a speci c position being held with the muscle being tensed until a stretching sensation is achieved and the position is repeated. There are two types of dynamic stretching: active and ballistic. Active stretching is a means of increasing muscle exibility while concomitantly improving the function of antagonist muscles. Active stretching involves several limb movements through the full motion range to end ranges. Ballistic stretching involves fast and sudden movements to increase exibility. Ballistic stretching is no longer recommended given its increased risk for injury. Pre-contraction stretching involves contraction of the primary muscle or its antagonist before stretching. The most common type of pre-contraction stretching is proprioceptive neuromuscular facilitation stretching. Resistance is provided by a partner or an elastic band or strap [6,7].
Static, dynamic, and pre-contraction stretching can effectively increase muscle extensibility and exibility.
Among these stretching types, static and dynamic stretching exercises are easy and safe to perform by Page 3/10 anyone, including children; moreover, they do not require special equipment or specialized assistants, e.g., a physical therapist. A randomized controlled trial by Meroni et al. compared active dynamic stretching and static stretching and showed that dynamic stretching was more time-e cient and required less compliance to produce effects on exibility [8]. In a study by Sairyo et al., participants performed active-static stretching for the hamstring muscles in the standing position and this technique was termed as "jack-knife stretching (JKS)" [9]. In our study, we assessed JKS as it is a very simple stretching exercise.
However, it is di cult for preadolescent children to continue a certain number of daily stretching exercises.
Furthermore, the JKS frequency and its effectiveness in preadolescents remain unclear. We aimed to investigate the effective frequency of JKS in preadolescent male football players. We hypothesized that lower limb tightness changes with JKS at least once every 3 days' interval.

Methods
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/m 2 . All participants took baseline surveys as described below. The survey items were height, body weight, BMI, ante exion 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 ante exion in sitting, we used a digital machine for ante exion measurement (Long-seat ante exion meter; Takei Scienti c 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 nger. 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 ante exion. 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. 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 ve 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 signi cant differences in the pre-and post-exercise height, weight, BMI, ante exion in sitting, HHD, and SLR.
Between-group comparisons were performed using Student's t-test. Statistical signi cance was set at 0.05 (two-sided) with 95% con dence 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.

Results
There were no signi cant between-group differences in baseline height, weight, BMI, HHD, and SLR; however, there was a signi cant between-group difference in the ante exion in sitting position, which was signi cantly smaller in group A. There was a signi cant increase in height and weight after 6 months. In group A, there was a signi cant increase in ante exion in the sitting position and SLR as well as a decrease in HHD. Contrastingly, there was no signi cant change in group B in all items. At 6 months, only SLR showed a signi cant between-group difference, with it being signi cantly larger in Group A (Table).

Discussion
Our ndings indicate that it is necessary to perform at least once every 3 days to improve hamstring exibility in preadolescent male football players. Contrastingly, the HHD results indicate the need for additional stretching to improve quadriceps exibility.
The hamstrings are major muscles that control hip and knee joint movement and control pelvis and spine alignment [10,11]. The hamstrings play an important role in postural alignment, where hamstring shortening could cause posterior pelvic tilt [12]. Changes in the posture caused by hamstring shortening could result in lower back and limb pain [13,14]. Hamstring stretching is important for preadolescent male football players with a high incidence of lumbar spondylolysis and Osgood-Schlatter disease.
Winter et al. reported that static stretching is characterized by external stretch stimulation on muscle contraction, whereas active stretching is characterized by a reciprocal innervation mechanism for relaxing antagonist muscle contraction [15]. Static stretching adversely affects muscle performance. A meta-analysis by Simic et al. showed that static stretching just before exercise reduces maximum muscle strength [16], whereas active stretching improves muscle strength. Active stretching improves muscle strength by an average of 13.3% [17]. JKS is considered as active-static stretching of the hamstring muscles and is very simple without the need for tools nor partners. Sairyo et al. reported that performing JKS for 4 weeks increased the nger-to-oor distance to an average of 22.2 cm and increased the pelvis forward inclination angle to an average of 33° [9]. In our study, 6 months of JKS increased ante exion in the sitting position and SLR to an average of 5 cm and 7°, respectively.
Our ndings indicate that a 12-year-old boy grows by about 4 cm and 3.5 kg in height and weight, respectively, within 6 months. Given this growth, there was decreased hamstring exibility when the stretching frequency was about once a week. Cipriari et al. assessed the effect of stretch frequency on hamstring exibility and found that approximate hamstring stretching thrice per week was as effective as stretching once daily [18].
Guidelines indicate that the joint range of motion is transiently improved after stretching and chronically improved after 3-4 weeks of regular stretching at a frequency of at least twice to thrice a week among adults [19]. Our ndings indicate that JKS performed once every 2 days can improve hamstring exibility in preadolescent boys.
Holding static stretching for 10-30 seconds at the point of tightness or slight discomfort enhances the joint range of motion, but holding for longer durations has an insigni cant bene t [20]. In our study, we instructed the participants to stretch for 10 seconds; however, there is a need for future studies to assess different durations of stretching. Moreover, optimal repetitions for JKS remain unclear. Regarding static stretching, two and four repetitions are recommended [21]. We employed 5 sets to improve upon compliance; however, future studies should assess this problem.
This study has several limitations. First, the JKS has only been studied once and it has not been assessed over a 6-month period. Second, we measured quadriceps tightness using HHD; therefore, the measured values could have changed depending on the muscle tone in the gluteal area, which might have led to inaccurate measurements. Furthermore, the test-retest reliability for ante exion in sitting, HHD, and SLR remains uncon rmed. Therefore, measurement reproducibility should be considered in future studies. Third, the team we evaluated comprised only boys; therefore, only boys were included in this study. It is unclear how the results of this study apply to girls. However, to our knowledge, this is the rst study to report the effect of JKS in preadolescent male football players for 6 months prospectively.

Conclusion
Performing jack-knife stretching at least once every 3 days' intervals could improve hamstring exibility in preadolescent male football players. There are no con icts of interest to declare.
There are no funding.