Since the publication of the seminal review on youth resistance training by Kraemer and colleagues in 1989, a compelling body of evidence has found that resistance training can be a safe, effective, and worthwhile method of conditioning for children and adolescents. New perspectives for promoting resistance exercise as part of a long-term approach to youth physical development highlight the importance of integrating resistance training into youth fitness programs. Youth who do not enhance their muscular strength and motor skill proficiency early in life may not develop the prerequisite skills and abilities that would allow them to participate in a variety of activities and sports with confidence and vigor later in life. The identification of asymptomatic children with muscular weaknesses or imbalances may facilitate the development of a management plan which should rectify movement limitations and educate children and their families about the importance of daily physical activity.
Avery D. Faigenbaum, Rhodri S. Lloyd, and Gregory D. Myer
Erich J. Petushek, Edward T. Cokely, Paul Ward, and Gregory D. Myer
Instrument-based biomechanical movement analysis is an effective injury screening method but relies on expensive equipment and time-consuming analysis. Screening methods that rely on visual inspection and perceptual skill for prognosticating injury risk provide an alternative approach that can significantly reduce cost and time. However, substantial individual differences exist in skill when estimating injury risk performance via observation. The underlying perceptual-cognitive mechanisms of injury risk identification were explored to better understand the nature of this skill and provide a foundation for improving performance. Quantitative structural and process modeling of risk estimation indicated that superior performance was largely mediated by specific strategies and skills (e.g., irrelevant information reduction), and independent of domain-general cognitive abilities (e.g., mental rotation, general decision skill). These cognitive models suggest that injury prediction expertise (i.e., ACL-IQ) is a trainable skill, and provide a foundation for future research and applications in training, decision support, and ultimately clinical screening investigations.
Christopher A. DiCesare, Scott Bonnette, Gregory D. Myer, and Adam W. Kiefer
Biomechanical analysis can effectively identify factors associated with task performance and injury risk, but often does not account for the interaction among the components that underlie task execution. Uncontrolled manifold (UCM) analyses were applied to data from 38 female, adolescent athletes performing single-leg drop landings and were used to differentiate successful and unsuccessful task performance by examining the frontal plane joint variance within the UCM (V UCM) that stabilized the horizontal center of mass position (V UCM) and within the orthogonal subspace (V ORT). The UCM revealed stronger coordination, indicated by the V UCM/V ORT ratio, in the successful condition. This may inform future research examining reduced motor coordination in failed movement tasks and its relation to injury risk and allow for targeted interventions that consider coordination processes rather than joint-specific outcomes.
Jensen L. Brent, Gregory D. Myer, Kevin R. Ford, Mark V. Paterno, and Timothy E. Hewett
As high school female athletes demonstrate a rate of noncontact anterior cruciate ligament (ACL) injury 3–6 times higher than their male counterparts, research suggests that sagittal-plane hip strength plays a role in factors associated with ACL injuries.
To determine if gender or age affect hip-abductor strength in a functional standing position in young female and male athletes.
Prospective cohort design.
Over a 3-y time period, 852 isokinetic hip-abduction evaluations were conducted on 351 (272 female, 79 male) adolescent soccer and basketball players.
Before testing, athletes were secured in a standing position, facing the dynamometer head, with a strap secured from the uninvolved side and extending around the waist just above the iliac crest. The dynamometer head was positioned in line with the body in the coronal plane by aligning the axis of rotation of the dynamometer with the center of hip rotation. Subjects performed 5 maximum-effort repetitions at a speed of 120°/s. The peak torque was recorded and normalized to body mass. All test trials were conducted by a single tester to limit potential interrater test error.
Main Outcome Measure:
Standing isokinetic hip-abduction torque.
Hip-abduction torque increased in both males and females with age (P < .001) on both the dominant and nondominant sides. A significant interaction of gender and age was observed (P < .001), which indicated that males experienced greater increases in peak torque relative to body weight than did females as they matured.
Males exhibit a significant increase in normative hip-abduction strength, while females do not. Future study may determine if the absence of similar increased relative hip-abduction strength in adolescent females, as they age, may be related to their increased risk of ACL injury compared with males.
Azahara Fort-Vanmeerhaeghe, Ariadna Benet, Sergi Mirada, Alicia M. Montalvo, and Gregory D. Myer
Context: Understanding how neuromuscular and biomechanical deficits that are associated with knee injuries change as youth mature may improve injury prevention strategies in this population. Objective: To investigate sex and maturation differences in jump-landing technique performance in youths using a practical clinical tool. Design: Cross-sectional study. Setting: High Performance Center Laboratory. Participants: A total of 165 youth athletes were included in this study. Main Outcome Measures: The main outcome measures were each of the 10 items of the modified tuck jump assessment and the total score. These measures include (1) knee valgus at landing, (2) thighs do not reach parallel, (3) thighs not equal side to side, (4) foot placement not shoulder width apart, (5) foot placement not parallel, (6) foot contact timing not equal, (7) excessive landing contact noise, (8) pause between jumps, (9) technique declines prior to 10 seconds, and (10) does not land in same footprint. Results: Only knee valgus at landing had a significant sex × maturation interaction. The main effect of maturation was significant for items 2, 3, 6, 7, 9, and total score. Plyometric technique performance improved with increasing maturation. The main effect of sex was significant for items 1 and 9, with males performing better than females. Conclusions: Female athletes demonstrate increased knee valgus at landing and fatigue relative to males during jump-landing performance. Overall, there was a trend of improved jump-landing performance with maturation.
Kevin R. Ford, Gregory D. Myer, Laura C. Schmitt, Timothy L. Uhl, and Timothy E. Hewett
The purpose of this study was to identify alterations in preparatory muscle activation patterns across different drop heights in female athletes. Sixteen female high school volleyball players performed the drop vertical jump from three different drop heights. Surface electromyography of the quadriceps and hamstrings were collected during the movement trials. As the drop height increased, muscle activation of the quadriceps during preparatory phase also increased (p < .05). However, the hamstrings activation showed no similar increases relative to drop height. Female athletes appear to preferentially rely on increased quadriceps activation, without an increase in hamstrings activation, with increased plyometric intensity. The resultant decreased activation ratio of the hamstrings relative to quadriceps before landing may represent altered dynamic knee stability and may contribute to the increased risk of ACL injury in female athletes.
Christopher A. DiCesare, Adam W. Kiefer, Scott Bonnette, and Gregory D. Myer
Context: Laboratory-based biomechanical analyses of sport-relevant movements such as landing and cutting have classically been used to quantify kinematic and kinetic factors in the context of injury risk, which are then used to inform targeted interventions designed to improve risky movement patterns during sport. However, the noncontextual nature of standard assessments presents challenges for assessing sport-relevant skill transfer. Objective: To examine injury-risk biomechanical differences exhibited by athletes during a jump-landing task performed as part of both a standard biomechanical assessment and a simulated, sport-specific virtual reality (VR)-based assessment. Design: Observational study. Setting: Medical center laboratory. Participants: Twenty-two female adolescent soccer athletes (age = 16.0 [1.4] y, height = 165.6 [4.9] cm, and weight = 60.2 [11.4] kg). Interventions: The landing performance was analyzed for a drop vertical jump task and a VR-based, soccer-specific corner-kick scenario in which the athletes were required to jump to head a virtual soccer ball and land. Main Outcome Measures: Hip, knee, and ankle joint kinematic differences in the frontal and sagittal planes. Results: Athletes exhibited reduced hip and ankle flexion, hip abduction, and frontal plane ankle excursion during landing in realistic sport scenario compared with the standard drop vertical jump task. Conclusion: VR-based assessments can provide a sport-specific context in which to assess biomechanical deficits that predispose athletes for lower-extremity injury and offer a promising approach to better evaluate skill transfer to sport that can guide future injury prevention efforts.
Sylvia Moeskops, Jon L. Oliver, Paul J. Read, Gregory D. Myer, and Rhodri S. Lloyd
Purpose: To quantify speed, peak momentum, standing long jump (SLJ), and the ratio of vertical to horizontal take-off velocity (Ratiovert–hori TOV) in young female gymnasts of different maturity status and their influence on vaulting vertical TOV. Methods: One hundred twenty gymnasts age 5–14 years were subdivided into maturity groupings using percentage of predicted adult height. Participants performed three 20-m sprints, SLJ, and straight jump vaults that were recorded using 2-dimensional video and analyzed using digitizing software. Results: All speed intervals, peak speed, peak momentum, SLJ distance, vault height, and vertical TOV increased between the early prepubertal and late prepubertal (P < .001; d = 0.65–1.10) and early prepubertal and pubertal (P < .001; d = 0.75–1.00) groups. No differences between these metrics were observed between the 2 most mature groups (d = 0.01–0.55). Multiple regression analyses revealed peak speed had the strongest association with vertical TOV (R2 = 59%) and also identified the Ratiovert–hori as a secondary determinant (R2 = 12%). A separate regression model indicated that maturity status (percentage of predicted adult height) moderately influences vertical TOV during vaulting (R2 = 41%). Conclusion: Speed and SLJ performance increase between the early prepubertal and late prepubertal years in young female gymnasts. However, given that peak speed and Ratiovert–hori combined to explain 71% of the total variance in vaulting vertical TOV, in order to increase aerial time for more advanced vaulting, practitioners should attempt to enhance peak speed alongside takeoff technique to develop gymnasts’ ability to transfer linear speed to vertical TOV.
Kevin R. Ford, Christopher A. DiCesare, Gregory D. Myer, and Timothy E. Hewett
Context: Biofeedback training enables an athlete to alter biomechanical and physiological function by receiving biomechanical and physiological data concurrent with or immediately after a task. Objective: To compare the effects of 2 different modes of real-time biofeedback focused on reducing risk factors related to anterior cruciate ligament injury. Design: Randomized crossover study design. Setting: Biomechanics laboratory and sports medicine center. Participants: Female high school soccer players (age 14.8 ± 1.0 y, height 162.6 ± 6.8 cm, mass 55.9 ± 7.0 kg; n = 4). Intervention: A battery of kinetic- or kinematic-based real-time biofeedback during repetitive double-leg squats. Main Outcome Measures: Baseline and posttraining drop vertical jumps were collected to determine if either feedback method improved high injury risk landing mechanics. Results: Maximum knee abduction moment and angle during the landing was significantly decreased after kinetic-focused biofeedback (P = .04). The reduced knee abduction moment during the drop vertical jumps after kinematic-focused biofeedback was not different (P = .2). Maximum knee abduction angle was significantly decreased after kinetic biofeedback (P < .01) but only showed a trend toward reduction after kinematic biofeedback (P = .08). Conclusions: The innovative biofeedback employed in the current study reduced knee abduction load and posture from baseline to posttraining during a drop vertical jump.
John M. Radnor, Jon L. Oliver, Charlotte M. Waugh, Gregory D. Myer, and Rhodri S. Lloyd
Purpose: To determine the differences in muscle architecture of the lower limb in pre-peak height velocity (PHV), circa-PHV, and post-PHV boys. Method: Muscle architecture variables from both the gastrocnemius medialis (GM) and vastus lateralis (VL) were derived from ultrasonographic images in 126 school-aged boys. One-way analysis of variance using Bonferroni post hoc comparisons was employed to determine between-group differences, and effect sizes were calculated to establish the magnitude of these differences. Results: All muscle architecture variables showed significant small to large increases from pre-PHV to post-PHV, excluding GM fascicle length (d = 0.59–1.39; P < .05). More discrete between-group differences were evident as GM and VL muscle thickness, and physiological thickness significantly increased between pre-PHV and circa-PHV (d > 0.57; P < .05); however, only the VL muscle thickness and physiological thickness increased from circa-PHV to post-PHV (d = 0.68; P < .05). The post-PHV group also showed larger GM pennation angles than the circa-PHV group (d = 0.59; P < .05). Conclusion: The combined results showed that maturation is associated with changes in muscle morphology. These data quantify that the maturity-related changes in muscle architecture variables provide a reference to differentiate between training-induced adaptations versus changes associated with normal growth and maturation.