Pairwise comparison tests indicated a statistically significant increase (mean change; 95% confidence interval) in knee extensor strength from prepubertal to pubertal status (0.073; 0.00–0.146 Nm/kg) and from prepubertal to late/postpubertal (0.119; 0.066–0.172 Nm/kg), but no statistically significant change from pubertal to late/postpubertal (0.046; −0.118 to 0.026 Nm/kg). Pairwise comparison tests for hamstrings-to-quadriceps ratio demonstrated a statistically significant decrease for prepubertal to pubertal (0.071; 0.000–0.143 Nm/kg) and from prepubertal to late/postpubertal (0.109; 0.034–0.184 Nm/kg) but no statistically significant change from pubertal to late/postpubertal (0.038; −0.028 to 0.103 Nm/kg). Likewise, pairwise comparison tests indicated a statistically significant drop in hip strength from prepubertal to pubertal (0.171; 0.069–0.272 Nm/kg) and from prepubertal to late/postpubertal (0.290; 0.119–0.461 Nm/kg) but no significant change in hip strength was observed from pubertal to postpubertal (0.119; −0.21 to 0.260 Nm/kg).
The purpose of this study was to investigate the longitudinal trajectories of lower extremity strength normalized to body mass across maturational stages for a cohort of female adolescent student athletes. It was hypothesized that as subjects progressed from prepubertal to pubertal and late/postpubertal levels, deficits in strength relative to body mass would emerge. The results from this study provide evidence that while knee extension strength relative to body mass steadily increases throughout maturation, knee flexor strength remains about the same. This leads to an increased imbalance in hamstring-to-quadriceps strength. The results from this study also indicate that female adolescents may experience a significant regression in hip abduction strength relative to body mass in the year they transition from prepubertal to pubertal status. Collectively, these results indicate that imbalances in lower extremity activation and strength relative to body mass emerge during puberty for female athletes.
Patellofemoral pain is a painful condition that peaks in incidence during the middle school years3 and eventually affects nearly 1 of 4 school-aged youth.32 Likewise, ACL injuries in females have higher rates following the onset of the pubertal growth spurt.2 Although ACL injuries and the emergence of PFP increase for both males and females around the time of puberty, females sustain ACL injuries and experience PFP at least 4 to 6 times more frequently4–6 and are affected by PFP 2 to 10 times more often than their male counterparts.7,8 Altered or reduced control of the limbs during physical activities may result in excessive knee abduction joint loads in females. This neuromuscular dysfunction appears to increase risk of acute ACL injury and chronic PFP in females.3,25,33,34 Deficits and imbalances in lower extremity strength have also been linked to both ACL injury and PFP.3,25,28,32 The timing of the emergence of these deficits appears to coincide with the emergence of strength imbalances for female adolescents during puberty. Thus, the results of this study may provide a potential explanation for the timing of increased incidences of PFP and ACL injury observed in female adolescents.
The rapid increases in height and body weight adolescents experience during puberty have led to many hypotheses that link pubertal maturation to concomitant decreases in motor control and subsequently increased risk for sports and recreation-related injuries. However, past studies related to these hypotheses have yielded inconsistent results. For example, Davies and Rose23 found no evidence for impaired coordination during puberty, while Loko et al35 found that female adolescents exhibited plateaus and regressions in a number of motor abilities. Findings from a recent systematic review indicate that many of the inconsistencies in our understanding of the relationship between maturation and motor control abilities may relate to a number of methodological limitations and incongruities.36 For example, a number of previous studies have used motor skill performances (eg, throwing distance and accuracy, vertical jump, and running speed) as measures of motor control.23,35,37 However, the use of motor skill performances as indicators of motor control abilities can be problematic because skill performance levels can be strongly influenced by confounding variables such as experience and type of measurement. Other common limitations in studies on maturation and motor control include operational definitions of maturation groups based on chronological age groupings, the combination of males and females within a single-subject pool, and a lack of longitudinal follow-ups on subjects across the pubertal process. These types of operational definitions and subject-pooling strategies can mask progression and regression trends because of the wide variability in maturational status of adolescent children of the same chronological age and potential sex-specific disparities.18,19,38–41
To address limitations in previous studies, this study incorporated several important design features: (1) motor control variables related to neuromuscular strength rather than motor skill variables were used, (2) multiple potential confounding sex-specific disparities were accounted for through the inclusion of only females, (3) pubertal stages were used instead of chronological age groups to minimize error due to the wide variability in age at onset of puberty, and (4) longitudinal follow-ups that covered a single group of subjects across multiple pubertal stages were used for analyses instead of cross-sectional data. The longitudinal nature of the design and use of pubertal stages were particularly useful for identifying specific points in the maturation process where strength deficits might be expected to emerge. For example, hip abductor strength appears to undergo a fairly rapid change relative to body mass during the year when females transition from prepubertal to pubertal status. Imbalances between hamstring and quadriceps strength, on the contrary, appear to emerge more gradually over the 3-year period leading from prepubertal to late/postpubertal status.
The results from this study and other studies indicate that a potential window of opportunity may exist for the optimal initiation of integrative neuromuscular training based on measures of somatic maturity. Specifically, the most beneficial and thus desirable time to initiate integrative training programs may be during preadolescence prior to the period of obvious pubertal maturation when youth are growing most rapidly. Children with earlier somatic maturation (growth) may particularly benefit from earlier participation in integrative neuromuscular strength training.14,42 In a recent longitudinal study, Ford and colleagues19 noted that pubertal females had an increased change in abnormal landing mechanics over time. In addition, important contributing risk factors for knee injury were significantly greater across consecutive years in young postpubertal female athletes compared with males. Integrative neuromuscular training programs have been successful at reducing these abnormal biomechanics43–46 and appear to decrease PFP and ACL injury rates in female athletes.47,48
One limitation of this study is that all of the subjects in this study were females. There is evidence to suggest that males and females may differ in the strength and neuromuscular control abilities that are present during puberty.18–20,49,50 In addition, the longitudinal nature of this study may have been limited by the nature of the design in that the subjects with data that were available for 3 consecutive years were all athletes that made the school team for multiple years. Therefore, future studies should include males and nonathletes to determine the generalizability of these results beyond female adolescent athletes.
The findings of this study indicate that hamstring and quadriceps strength deficits and imbalances between hamstrings and quadriceps strength appear to emerge during pubertal maturation. As these strength variables have been linked to increased risk for ACL injury and PFP, these data provide further support that preadolescence may be an optimal time to institute programs aiming to reduce deficits (eg, increased knee abduction motion and load) that accelerate during maturation and lead to increased musculoskeletal injury risk in female adolescents.
The authors thank Dr Mark Paterno, Dr Carmen Quatman, Dr Laura Schmitt, Chad Cherny, Jensen Brent, Kim Foss, and Staci Thomas for their assistance with data collection and other aspects of the study.
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adolescent; female; puberty/physiology; strength