Flexibility is, along with strength, speed and endurance, one of the basic physical attributes for sports performance and adolescence is one of the most important and sensitive periods in the development of these attributes in an athlete. Flexibility, defined as the intrinsic property of the body tissues that determines the range of motion achievable without damage,1 is known to progress negatively during an athlete's life,2–4 although its evolution during adolescence, when exposed to high intensity training, remains unknown. Growth appears to positively influence physical performance in terms of strength, speed and endurance,5–8 but the lack of evidence on whether flexibility is negatively influenced by growth through adolescence remains unclear. Important musculoskeletal changes occur during this period, when different body segments rapidly grow longitudinally,9,10 challenging the tissue capacity to adapt to new movement patterns. Some studies report that flexibility decreases during adolescence,4,11 others report that this decrease can start as early as 5–8 years of age,12,13 but others suggest that adolescent growth is not associated with any changes in flexibility.14 However, most of these studies were not carried out on a sporting population and specifically not on athletics population.

Flexibility can also be influenced by the type of physical activity and sports specialization undertaken. It is unclear if it is the sport itself which influences an athlete's flexibility or is the individual athlete's flexibility characteristics that work as a “natural selection” to allow a better sports performance, particularly if an athlete has commenced the sport at an early age. Swimmers have shown to have more flexibility in ankles and shoulders,15 pitchers and tennis players greater gleno-humeral external rotation,16,17 karate players increased hip flexibility,18 or wrestlers greater neck and shoulder abduction.19 A specific event or position may also be influential; football goalkeepers are more flexible than field players20 and linemen less flexible than any other player in American football.21 Performance level is also key; increased flexibility is demonstrated in high level swimmers,15 gymnasts22 and wrestlers19 compared to amateurs.

To be able to define an optimal flexibility for each sport or discipline, reference values are needed for specific athletic populations. There have been many studies describing physiological profiles for different sports such as football, gymnastics, wrestling, or taekwondo,21–24 but a specific flexibility profile has not, as yet, been defined in athletics or any of its disciplines. Considering athletics is a combination of multiple disciplines, throwers, sprinters, jumpers and long-distance runners, different lower limb flexibility demands may be required, with adolescence being a key period for adaptation to the event-specific demands. Establishing profiles for adolescent athletes according to their athletics event specialization may help adjusting realistic expectations according to their expected lower limb flexibility. This study aims to explore any specific lower limb flexibility patterns which may emerge, for the main athletics disciplines and age groups from 12 to 18 years old in highly-trained adolescent athletes.

A retrospective analysis of all lower limb flexibility data collected for the athletics squad in an elite sports academy in the Middle East was undertaken. Flexibility measures were taken for 9 consecutive seasons (2006/07 through 2014/15) as part of the physical screening tests routinely carried out in the academy. Ethical approval for the use of the flexibility measures, which are part of the routinely collected screening data for scientific purposes, was given by the Anti-Doping Lab Qatar Institutional Review Board (Ethics number E20140000012). Informed consent was granted by the parents at the beginning of each season prior to any data collection.

A video-based measurement of flexibility designed for adolescent athletes was used, according to the protocol described by Fourchet et al. (2013).25 Eight lower limb muscle groups were tested bilaterally: quadriceps (QUAD), hamstrings (HAMS), adductors (ADD), gastrocnemius (GAST), soleus (SOL), hip flexors (HIPFLEX), hip internal rotators (INTROT) and hip external rotators (EXTROT). The measurement endpoint for each muscle was reflected by the angle of the joint of interest with the application of a standardized force on the distal part of the segment. The force was applied by means of a hand-held dynamometer (Compact force gauge, Mecmesin, Slinfold, UK) just proximal to the malleolus level for each tested muscle groups, except for the hip flexors (above the patella) and calf muscles (on the heads of metatarsals at the plantar side of the foot). A specific force was defined for each muscle group (QUAD=78N, HAMS=68N, ADD=39, GAST=147N, SOL=147N, HIPFLEX=98N, INTROT=49, EXTROT=49N). Once the endpoint angle was reached, a video capture was made (Digital video camera recorder, DCR-SR220E, Sony Corporation, Tokyo, Japan) by a second operator, which is then analyzed using digital video analysis software (Dartfish Software, Fribourg, Switzerland). The results were then entered into a password protected database which was managed by the medical staff for screening and injury prevention purposes. The standard positions for data collection for each muscle group and the protocol for angle correction are showed in Fig. 1A–H.

Figure 1.

Video-capture of the endpoint angle positions for eight different muscles based on standardized dynamometer values, and calculation of final value for each muscle (being ‘x’ the primary measurement). (A) Quadriceps=180°−x. (B) Hamstrings=x−90°. (C) Adductors=x. (D) Gastrocnemius=180°−x. (E) Soleus=180°−x. (F) Hipflexors=180=180°−x. (G) Hip medial rotators=x. (H) Hip lateral rotators=x.

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Flexibility measurements were captured by the academy physiotherapy team who were trained on the technical procedures for the tests on each occasion. The same room, time (7–9 am) and procedures (no previous warm up, same sequence of muscle groups and consistent pairs of testers and recorders) were used annually for the first 4 seasons (2006/2007 to 2009/2010) and then 3 times per season (beginning, mid and end of season) for the next 5 seasons (2010/2011 to 2014/2015). Testing calendar and number of athletes tested per session is presented in Table 1.

A total lower limb flexibility value (TF) was calculated for each athlete-test by summating all muscle group results, higher values indicating greater overall lower limb flexibility.

All athletes in the athletics squad, tested for flexibility for the 9-season period, were included for analysis. Full time athletes (all males) followed comprehensive training sessions 8 times per week around the school calendar and competed at national to international level events. All athlete-tests were reviewed, analyzed and categorized by age and event following specific criteria; seven age groups were established according to the age on the day of testing with one-year increments from under 13 years (U13; age <13.0) to above 18 years (>18).

The athletics program was structured towards eventual athletics specialization, with younger athletes (12–15 years) following a general athletics development program (non-specialized) and older athletes (above 15) specializing in one of the four event-groups: sprints, jumps, long distance running and throws. Sprints and jumps were training as a single group for most of the seasons, therefore they were categorized together as sprints & jumps, while the rest of the groups were categorized according to the event the athlete was part of on the day of testing.

Mean values were calculated for each muscle group by age group and event and expressed in degrees±standard deviation (STD). A paired T-test was used to ascertain potential differences between left and right. Statistical analysis was carried out to explore comparisons between events and age groups for TF and all muscle groups individually. Analysis of Variances (ANOVA) was used when comparisons were made among events and age groups. Logistic regression was done to analyze the relationship between age and total flexibility. The level of significance was established at p<0.05. Statistical analyses were carried out using Statistical Package for Social Sciences (version 17, SPSS Inc., Chicago) software.

One-hundred and twenty-seven male youth athletes underwent, in total, 604 flexibility tests corresponding to 18 test batteries from October 2006 to May 2015. Mean age at test date was 15.4±1.8 years, and athletes’ origin were essentially Middle Eastern countries (72%), North & East Africa (23%), and others (5%). Athlete distribution by age group was: U13=52, U14=103, U15=116, U16=110, U17=106, U18=68 and >18=49.

Mean value for total flexibility (TF) was 1175±94.3. Mean values for each muscle group are described in Table 2. There were no significant differences between left and right for any muscle group, so a single reference value per muscle group, resulting from the mean left and right results was calculated.

When analyzing muscle flexibility by age groups, statistically significant differences were found for QUAD, GAST, SOL, hip INTROT and hip EXTROT (p<0.01). In all cases, the evolution of flexibility was negative, showing lower values in older age groups. Age did not appear to influence HAMS (p=0.19), ADD (p=0.58) and HIPFLEX (p=0.09). Mean values for each muscle group and TF by age group are showed in Table 3.

A tendency of a decrease in TF in the older age groups was observed and a negative correlation index was found (r=−0.21; r2=0.04; p<0.01) when a regression analysis was conducted between age and TF (Fig. 2), indicating that overall lower limb flexibility tends to decrease towards late adolescence.

Figure 2.

Regression analysis age vs total lower limb flexibility (TF). Significant decrease p<0.01.

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When categorizing all athlete-tests by athletics event, 205 belonged to non-specialized athletes, 114 to long distance runners, 182 to sprints & jumps, and 103 to throwers. Mean values for each muscle group per athletics event are presented in Table 4. Throwers were less flexible for all muscle groups (p<0.001), while long distance runners were the most flexible overall and for QUAD, HAMS and EXTROT. Sprints & jumps had more HIPFLEX flexibility. In terms of TF, there were also statistically significant differences (p<0.001) between groups, throwers being significantly less flexible than any other group.

This study has resulted in lower limb flexibility being described for the first time in youth athletics. A consistent methodology for lower limb flexibility assessment from 2006/07 through 2014/15 allowed comparisons between highly-trained youth athletes in a full-time sports academy. Different patterns were observed among athletics events, throwers being the least flexible group and long distance the most flexible one, especially for hamstrings and hip rotators. Flexibility showed a trend to decrease in late adolescence for all groups, although hamstrings, adductors and hip flexors remained unchanged. Important considerations should be taken when interpreting these results.

No funding was received from any institution for this study.

The authors declare that they don’t have any conflict of interest.