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00005768-201206000-0000200005768_2012_44_993_thijs_participation_6article< 93_0_13_5 >Medicine & Science in Sports & Exercise©2012The American College of Sports MedicineVolume 44(6)June 2012p 993–998Is High-Impact Sports Participation Associated with Bowlegs in Adolescent Boys?[CLINICAL SCIENCES]THIJS, YOURI1; BELLEMANS, JOHAN2; ROMBAUT, LIES3; WITVROUW, ERIK11Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, Ghent University, Ghent, BELGIUM; 2Department of Orthopaedic Surgery, University Hospital Pellenberg, Catholic University of Leuven, Leuven, BELGIUM; and 3Department of Rehabilitation Sciences and Physiotherapy, Ghent University – Artevelde University College, Ghent, BELGIUMAddress for correspondence: Youri Thijs, P.T., Ph.D., Faculty of Medicine and Health Sciences, Ghent University, Ghent University Hospital (3B3) (REVAKI), De Pintelaan 185, 9000 Ghent, Belgium; E-mail: youri.thijs@ugent.be.Submitted for publication July 2011.Accepted for publication November 2011.ABSTRACTPurpose: The purpose of this study was to investigate whether the participation in load bearing sports is associated with significant knee alignment adaptations in adolescent boys, which might cause a higher risk for the development of knee osteoarthritis in later life.Methods: Five hundred twenty-one healthy boys (from 7 to 18 yr), selected from local primary and secondary schools, participated in the study. Two hundred sixty-five of them practiced competitive sports (track and field, field hockey, basketball, volleyball, tennis, badminton, and squash) for at least 3 h·wk−1. The other remaining 256 boys did not practice any kind of sports. Genu varum/valgum was determined by measuring the intercondylar (IC) and intermalleolar (IM) distance with the subjects in a relaxed erect standing position. The IC and/or IM distance was measured using a caliper. Both measurements were combined to one parameter: the IC–IM distance. A one-way ANOVA was performed to analyze differences between the different age groups within the sporting and nonsporting boys separately. For each age group, the IC–IM distances of the sporting and nonsporting boys were compared by ANOVA with post hoc Bonferroni corrections.Results: A comparison between the sporting and nonsporting boys showed that the sporting boys had a significantly higher degree of genu varum from 13 to 15 yr or older (P = 0.01).Conclusions: From the results of this study, it can be concluded that practicing load bearing sports in general is associated with the same knee varus alignment in adolescent boys as previously has been indicated in intense soccer-playing adolescents.It is an accepted fact that sports participation improves general health (13). Consequently, over the world, millions of children are encouraged to practice sports at various levels and intensities. Although it must be stressed that exercising sports promotes overall well-being, the question of whether load bearing sports participation is associated with possible adaptations of the growing skeleton of children and adolescents also merits particular attention. More specifically, the possible relationship between high-impact load bearing sports participation and axial development adaptations of the lower extremities of growing adolescents deserves particular notice.At present, little is known about the relationship between load bearing sports participation and knee alignment adaptations in adolescents during growth. In healthy children, it has been demonstrated that the knee angle progresses from bowlegs in the infant to knock knees in early childhood (6,11,20). The development of bowlegs during late childhood is considered atypical (11,28). However, it has already been demonstrated that soccer participation is associated with an increased degree of genu varum in adolescent boys (27,28). This has important clinical implications because axial deviations in the knee such as genu varum are associated with an increased risk of developing knee osteoarthritis in later life (9,14,16). This statement is supported by a study by Chantraine (7), who documented both a higher incidence of osteoarthritis in the knee and a higher prevalence of knee varus in veteran soccer players in comparison with a random population of the same age. In addition, besides the development of osteoarthritis, genu varum has been shown to predispose athletes to other overuse and traumatic lesions like patellofemoral pain syndrome (15,18) and meniscal lesions (25).A possible explanation for the high prevalence of genu varum in soccer players might be provided by the Hueter–Volkmann law (12,26). Almost 150 yr ago, Hueter (12) and Volkmann (26) reported that increased pressure parallel to the axis of the epiphysis inhibits growth, whereas decreased pressure promotes it, and that changes in compressive forces cause asymmetrical growth of a joint. Moreover, the “chondral modeling” theory of Frost (10) suggests that physiological loading stimulates growth, whereas loads outside this range, either higher or lower, will inhibit it.The hypothesis that the demands of soccer participation on the growing skeleton may contribute to a growth deformity in the lower extremities may however also be applicable for other load bearing sports. However, it is yet unknown whether there is a relationship between practicing high-impact load bearing sports in general and the alignment of the knee in adolescent boys similar to the one seen in soccer players.It has been propounded that people who participate in load bearing sports have an increased risk of developing osteoarthritis compared with sedentary people, especially in sports that subject joints to more intense impact and torsion loading (4,5,21). The possibility that practicing load bearing sports in general may be associated with knee alignment changes during growth is pertinent. Although the Hueter–Volkmann law describes the effect of high-impact loading on axial deformations of growing joints, strikingly, besides for soccer, no studies have yet investigated this hypothesis in sporting growing adolescent boys. Because bowlegs predispose to osteoarthritis (9,14,16) and other injuries (15,18,25) and millions of children over the world participate in load bearing sports during youth (19), there is a great need to scientifically investigate this relationship. Therefore, the purpose of this study was to investigate whether participating in load bearing sports besides in soccer is associated with knee alignment adaptations in adolescent boys during growth. It was hypothesized that load bearing sports participation leads to the development of bowlegs in adolescent boys.MATERIALS AND METHODSSubjectsFive hundred twenty-one healthy boys (from 7 to 18 yr) who did not practice soccer were selected from local primary and secondary schools. All of the included subjects were Caucasian boys. None of them had any history of musculoskeletal disorders. Subjects were excluded from the study if they underwent a surgical procedure involving the hip, knee, lower leg, ankle, or foot in the past. Two hundred sixty-five of the 521 boys practiced competitive sports (track and field, field hockey, basketball, volleyball, tennis, badminton, and squash). Within this group, from the boys age 9 yr and older, these boys practiced sports at least 3 h·wk−1 for at least three consecutive years. The number of years of sports participation in the 7- and 8-yr-old sporting boys ranged from 1 to 2. The intensity of sports participation for all of the sporting boys ranged from training two times 1 h·wk−1 to two times 2 h·wk−1 and playing a weekly match with a duration ranging from 1 to 2 h. The remaining 256 boys had not ever practiced any kind of sports in their lives, except for the sports they practiced at school for 1 h·wk−1.Both groups were divided into four age groups: group 1 (7–9 yr) (n = 32 for the nonsporting boys and n = 52 for the sporting boys), group 2 (10–12 yr) (n = 35 for the nonsporting boys and n = 64 for the sporting boys), group 3 (13–15 yr) (n = 111 for the nonsporting boys and n = 92 for the sporting boys), and group 4 (16–18 yr) (n = 78 for the nonsporting boys and n = 57 for the sporting boys). Within each age group, the group of sporting and nonsporting boys had similar weight, length, and body mass index distributions (Table 1). The distribution of the number of individuals in the different age groups over the different sport disciplines is presented in Table 2. The study was approved by the ethical committee of Ghent University, and all subjects and their parents gave written informed consent to participate.TABLE 1 Mean values, SEM, and P values of the length, weight, and body mass index (BMI) of the sporting (sport) and nonsporting (nonsport) boys within the different age groups (yr) (α = 0.05).TABLE 2 Number of individuals in the different age groups participating in the different sport disciplines.MeasurementsThe genu varum/valgum was determined by measuring the intercondylar (IC) and intermalleolar (IM) distance. The measurement of the IC–IM distance was done by asking the subject to take a relaxed erect standing position on a specially developed instrument with the feet at shoulder width, each foot on a different platform (Fig. 1). The subjects were instructed to position their feet in such a manner that the medial border of the feet was above the inner border of the platform. This positioning made sure that all subjects pointed their feet straight forward. Then, using a motor, the two platforms were moved to each other at a constant low speed until the medial condyles or malleoli touched. Subsequently, in subjects with genu varum, in whom only the medial malleoli touched, the bony IC distance (mm) was measured with a caliper using the method described by Arazi et al. (2). In subjects with genu valgum, in whom only the medial femoral condyles touched, the bony IM distance (mm) was measured. Both measurements were combined to one parameter: the IC–IM distance. In the subjects with genu varum, the IC–IM distance was expressed as a positive value. In the subjects with genu valgum, the IC–IM distance was expressed as a negative value. If, in an individual, both the medial malleoli and femoral condyles touched, the IC–IM distance was defined as zero. In some subjects, during the IC–IM measurement, the medial sides of both feet touched with neither the malleoli nor the condyles touching. In these subjects, the bony IC and IM distance was measured, and the distance between the malleoli was subtracted from the distance between the condyles. This resulted, in all subjects in whom this occurred, in a positive IC–IM distance value. All IC–IM distances from all subjects were measured by the same examiner. The intraclass correlation coefficient (ICC) for the used technique for measuring the IC–IM distance proves to be high for the intertester reliability (ICC = 0.95) and the intratester reliability (ICC = 0.96) (27).FIGURE 1. Measurement apparatus for positioning the subject during the IC–IM distance measurement.STATISTICAL ANALYSISStatistical analysis was performed using SPSS 15.0 for Windows (SPSS, Inc., Chicago, IL). Within the group of sporting and nonsporting boys, a one-way ANOVA was performed to analyze differences between the five age groups separately. Post hoc tests were performed with Bonferroni correction. For each age group, the sporting and nonsporting boys were compared on the basis of IC–IM distance by ANOVA with post hoc Bonferroni corrections. Statistical significance was accepted at the level of P < 0.05.RESULTSThe nonsporting boys showed a negative IC–IM measurement (genu valgum) until the age group of 13–15 yr. In this group of nonsporting boys, the IC–IM distance turned slightly positive in the age group of 16–18 yr (Fig. 2).FIGURE 2. Plot of the mean values of the IC–IM distances from the nonsporting and sporting boys of the different age groups (group 1, 7–9 yr; group 2, 10–12 yr; group 3, 13–15 yr; group 4, 16–18 yr). *P < 0.05.The sporting boys showed a negative IC–IM distance until the age of 10–12 yr. Within this group, the IC–IM distance turned positive (genu varum) from 13 to 15 yr or older and showed an increasing evolution in the age group of 16–18 yr (Fig. 2).In the sporting boys, statistical analyses revealed a significant difference in IC–IM distance between age group 4 and age groups 1, 2, and 3 (P < 0.001) and between age group 3 and age groups 1 and 2 (P = 0.001 and P < 0.001, respectively). The difference in IC–IM distance between age groups 1 and 2 was not significant (P = 1.00).In the nonsporting boys, the IC–IM distance was significantly different between age group 4 and age groups 1 and 2 (P = 0.001 and P < 0.001, respectively) and between age groups 3 and 2 (P < 0.001). The differences in IC–IM distance between age group 1 and age groups 2 and 3 (P = 0.95 and P = 0.12, respectively) and between age group 3 and age group 4 (P = 0.11) were nonsignificant within the nonsporting boys.A comparison between the sporting and nonsporting boys showed a significantly different IC–IM distance in groups 3 and 4 (P = 0.01 and P < 0.001, respectively). In these age groups, the sporting boys showed a significantly greater IC–IM distance, indicating a higher degree of knee varus angulation, than the nonsporting boys. The mean IC–IM values and SD of the sporting and nonsporting boys for the different age groups are shown in Table 3.TABLE 3 Mean values, SEM, and P values of the ANOVA of the IC–IM distances (mm) of the sporting and nonsporting boys within the different age groups (yr).DISCUSSIONThe results of this study show that from the age period of 13 to 15 yr, a varus evolution was present in the measured sporting and nonsporting boys (Fig. 2). This finding is in agreement with a study by Cahuzac et al. (6), who demonstrated that a certain varus pattern is noticeable in boys at the end of the growth spurt. Striking in this present study, however, is that from 13 to 15 yr or older, the IC–IM distance was significantly larger in the sporting boys compared with the boys who did not practice any kind of sports. In this age group, the knees of the nonsporting boys still showed a slight valgus angulation, whereas the knees of the sporting boys showed a varus angulation. This indicates that from this age on, the boys who practiced load bearing impact sports markedly developed a greater degree of genu varum than the boys who did not practice any kind of sports.Interestingly, the age period at which this significant difference in knee alignment was observed between both groups corresponds with the period of the growth spurt in boys (17,24). Therefore, the results of this study support the hypothesis that stress and strain imposed on the knee joint during growth through practicing load bearing impact sports might be related to a varus axed growth deformity in the knee. According to the Hueter–Volkmann law, compression forces will halt physeal growth, whereas distraction will lead to overgrowth. Hence, as stated earlier by Yaniv et al. (28), proximal tibial growth changes due to repeated altered stress over the growing growth plate may be a possible mechanism of axis change. Cook et al. (8) provided evidence that mechanical overload of the proximal medial tibial physis plays a major role in the development of genu varum and proved that restricted physeal growth due to excessive compressive loading leads to progressive varus deformity in the knee.However, the question of which specific activities during the participation of load bearing sports may lead to this asymmetric excessive compressive loading of the knee and might be associated with a varus angulation in the knee in growing adolescents remains. Reports of previous studies have suggested that in soccer players, kicking the ball may play a prominent role in the development of bowlegs in male soccer players (27,28). This kicking action requires an important adduction moment, which may develop strong adductor muscles leading to an alteration in the players’ normal adductor/abductor strength ratio and the possible development of a varus knee axis deviation (27).Yaniv et al. (28), however, postulated that a combination of internal forces with muscular tension into flexion and varus during side kicking and external forces such as those exerted in crossover cutting maneuvers during play may underlie the deformity evolution in soccer players.In this present study, however, the investigated sporting boys who did not practice soccer but were enrolled in other load bearing sports (track and field, basketball, volleyball, field hockey, tennis, badminton, and squash) showed a significantly increased degree of genu varum from 13 to 15 yr or older compared with the sedentary boys. In this investigated group of athletes, the side kicking action, which is a typical action for soccer, was not performed during sports participation. However, the external forces during running, sidestepping, and crossover cutting tasks are equally frequently exerted in the knee in these sports.A major determinant of medial-to-lateral load distribution in the knee joint is the moment that tends to abduct the knee during almost the entire stance phase of normal gait (22,23). This abduction moment is strongly related to the magnitude of the total intrinsic compressive load on the medial compartment of the joint (22). Load bearing sports that involve intensive running cause a strong increase of these abduction moments in the knee. Moreover, Besier et al. (3) measured the external loading in the knee joint during running and cutting maneuvers. They concluded that the external varus/valgus and internal/external rotation loads placed on the joint increased dramatically during cutting tasks compared with normal running. According to the Hueter–Volkmann law and Frost’s chondral modeling theory, these strongly increased abduction moments in the knee during load bearing sports activities may halt physeal growth in the proximal medial tibial physis as a result of repeated supraphysiological compressive loads that they cause over the growing growth plate at that location. This might form a possible explanation for the observed relationship between practicing load bearing sports and the knee varus evolution in the investigated adolescent boys of this study.Furthermore, during cutting tasks, compression forces may be applied to the proximal medial tibia by repeated contraction of the pes anserinus (sartorius, gracilis, and semitendinosus muscles) and semimembranosus muscle (28). Frequently repeated contractions of these muscles during sports may also contribute to the mechanical overload of the proximal medial tibial physis.The results of this study suggest that participation in load bearing sports such as track and field, basketball, volleyball, field hockey, tennis, badminton, and squash, in which intense running and cutting maneuvers are highly frequently exerted, is associated with an increased knee varus evolution in adolescent boys. This has a very important implication because a higher than normal abduction moment in the knee joint is related to the development and progression of medial tibiofemoral osteoarthritis (1,23). Several studies in the literature have shown that axial deviations in the knee such as genu varum are associated with the development of knee osteoarthritis in later life (9,14,16). In 1985, Chantraine (7) documented both a higher prevalence of knee varus and a higher incidence of osteoarthritis in the knee in veteran soccer players. Because, today, millions of children over the world are enrolled in and practice load bearing sports, the results of this study warn of the possible deleterious consequences in the knee in later life.A second important question one should ask is what efforts can be done to reduce this evolution in adolescent athletes. In a previous study in male soccer players, it has been proposed that if a muscular imbalance around the knee joint would prove to be associated with the development of genu varum, prevention should emphasize on preservation of this muscular balance (27). Future studies are, however, needed to investigate this relationship. Further future research is also warranted to investigate whether other preventative measures can be taken to reduce the possible risk of developing bowlegs in millions of sporting growing adolescents and to reduce their risk of developing knee osteoarthritis later on.There are some limitations of this study that have to be considered. First, the different sport disciplines in which the sporting boys of this study were enrolled were pooled into one group of load bearing sports. However, more accurate information concerning the relationship between sports participation and a varus evolution in the knee may be obtained if the different sport disciplines would be investigated separately because different forces may act on the knee in athletes practicing different types of sports. This should be addressed in future studies examining larger cohorts of subjects. Second, the cross-sectional design of our study does not allow us to establish cause-and-effect relationships. As previously has been hypothesized in soccer players (28), a natural selection might be present in athletes practicing load bearing sports by which varus axed knees may confer an advantage in sports participation. Yaniv et al. (28) suggested that in soccer players, bowlegs may play a role in the player’s balance and stability and may serve the player to adapt more readily to the increased difficulty to maintain balance during the game. This, however, might also apply for athletes practicing other load bearing sports and may result in the fact that more children with genu varum end up being athletes. Furthermore, the observed varus angulation in the sporting boys may also be genetically present and rather might be a product of normal development than a result of training, as previously has been suggested by Chantraine (7). Therefore, future prospective studies would be required to draw any definitive conclusions in this regard.The present study focused on the relationship between sports participation and the frontal plane knee axis evolution in adolescent boys. 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