Different Exercise Training Interventions and Drop-Landing Biomechanics in High School Female Athletes

Different Exercise Training Interventions and Drop-Landing Biomechanics in High School Female Athletes
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   Journal of Athletic Training   2013;48(4):450–462doi: 10.4085/1062-6050-48.4.06   by the National Athletic Trainers’ Association, srcinal research Different Exercise Training Interventions and Drop-Landing Biomechanics in High School Female Athletes Kate R. Pfile, PhD, ATC*; Joseph M. Hart, PhD, ATC † ; Daniel C. Herman, MD,PhD ‡ ; Jay Hertel, PhD, ATC, FNATA, FACSM § ; D. Casey Kerrigan, MD, MS||;Christopher D. Ingersoll, PhD, ATC, FNATA, FACSM ¶ *College of Charleston, SC; †Department of Orthopaedics, University of Virginia, Charlottesville; ‡University of Florida,Gainesville; §Department of Kinesiology, University of Virginia, Charlottesville; ||JKM Technologies, Charlottesville,VA; ¶The Herbert H. and Grace A. Dow College of Health Professions, Central Michigan University, Mt Pleasant Context:   Anterior cruciate ligament (ACL) injuries arecommon in female athletes and are related to poor neuromus-cular control. Comprehensive neuromuscular training has beenshown to improve biomechanics; however, we do not knowwhich component of neuromuscular training is most responsiblefor the changes. Objective:   To assess the efficacy of either a 4-week corestability program or plyometric program in altering lower extremity and trunk biomechanics during a drop vertical jump(DVJ). Design:   Cohort study. Setting:   High school athletic fields and motion analysislaboratory. Patients or Other Participants:   Twenty-three high schoolfemale athletes (age ¼ 14.8 6 0.8 years, height ¼ 1.7 6 0.07 m,mass ¼ 57.7 6  8.5 kg). Intervention(s):   Independent variables were group (corestability, plyometric, control) and time (pretest, posttest).Participants performed 5 DVJs at pretest and posttest.Intervention participants engaged in a 4-week core stability or plyometric program. Main Outcome Measure(s):   Dependent variables were 3-dimensional hip, knee, and trunk kinetics and kinematics duringthe landing phase of a DVJ. We calculated the group means andassociated 95% confidence intervals for the first 25% of landing.Cohen d effect sizes with 95% confidence intervals werecalculated for all differences. Results:   We found within-group differences for lower extremity biomechanics for both intervention groups ( P     .05).The plyometric group decreased the knee-flexion and kneeinternal-rotation angles and the knee-flexion and knee-abduc-tion moments. The core stability group decreased the knee-flexion and knee internal-rotation angles and the hip-flexion andhip internal-rotation moments. The control group decreased theknee external-rotation moment. All kinetic changes had a strongeffect size (Cohen d  . 0.80). Conclusions:   Both programs resulted in biomechanicalchanges, suggesting that both types of exercises are warrantedfor ACL injury prevention and should be implemented by trainedprofessionals. Key Words:   anterior cruciate ligament, plyometrics, corestability Key Points  Kinematic and kinetic changes occurred in high school female athletes after an in-season, 4-week training programof core stability and plyometric exercises.  The plyometric group demonstrated changes only at the knee joint, but the core stability group demonstratedchanges in kinetics at the hip joint and kinematics at the knee joint.  Core stability and plyometric exercises are warranted in programs designed to prevent anterior cruciate ligamentinjury because they contribute different biomechanical adaptations. T he rate of noncontact anterior cruciate ligament(ACL) injury is more than 3 times higher in adultand adolescent females than in their male counter- parts. 1  Noncontact ACL injuries commonly occur duringdynamic activities when the individual is decelerating, suchas landing from a jump or changing direction. 2 Kinematic patterns thought to be associated with greater risk for injuryinclude landing in an extended posture through the knee,hip, and trunk, resulting in increased shear force on theACL. 3,4 Frontal- and transverse-plane movements, includ-ing increased knee abduction and internal rotation and decreased hip abduction, also are thought to place rotationalforce on the static stabilizer. 5–9 A link between ACL injury and proximal lower extremityand trunk neuromuscular control has been established.Hewett et al 6 found that individuals who sustained an ACLinjury had larger external knee-abduction moments thatwere correlated with the hip-adduction moment. Inaddition, females who had greater lateral trunk displace-ment in response to a sudden force were more likely toincur an ACL injury. 9 These results suggest that the risk for noncontact ACL injury may be related to forces at the kneeaffected by decreased neuromuscular control at the hip and trunk.Biomechanical and neuromuscular control patterns have been shown to be modifiable in response to training. 5,10,11 450  Volume 48    Number 4    August 2013  Training programs that have resulted in favorable changesto biomechanical patterns have involved a broad approach,incorporating balance, lower extremity strength, plyomet-ric, and agility components to address all aspects of neuromuscular control. 7,8,10,11 These comprehensive pro-grams often involve lengthy training sessions and mayrequire equipment that is not always easily accessible for group training purposes. Furthermore, we do not knowwhether all components of comprehensive training pro-grams are effective or necessary in altering biomechanical patterns. The variety and volume of the componentsincluded in an intervention program possibly can bereduced to make it more manageable to incorporate invarious athletic settings. Researchers 12,13 have investigated the contributions of specific muscles during commonly prescribed lower extremity and trunk exercises; however,little information exists about how a group of exercisesaffects lower extremity and trunk biomechanics during adynamic landing task. In 1 study, 14 9 weeks of lower extremity strength training did not result in any lower extremity biomechanical changes despite an increase instrength. In contrast, researchers 10 who compared tradition-al strength training and plyometric training found similar changes in kinematic and kinetic variables for both groups.By gaining a better understanding of how individualcomponents effectively alter neuromuscular patterns, clini-cians may be able to develop more effective and efficientinjury-prevention programs.Therefore, the purpose of our study was to assess theefficacy of either a 4-week core stability program or  plyometric program in altering lower extremity and trunk  biomechanics during a drop vertical jump (DVJ). Wehypothesized that (1) the plyometric group would decreaselateral trunk-flexion, hip-adduction, hip internal-rotation,knee-abduction, and knee internal-rotation angles; (2) the plyometric group would increase hip- and knee-flexionangles; (3) the plyometric group would decrease their external flexion, abduction, and external-rotation momentsat the hip; (4) the plyometric group would decrease their external flexion, abduction, and internal-rotation momentsat the knee; (5) the core stability group would decreaselateral trunk-flexion, hip internal-rotation and adductionangles and external joint moments; and (6) the controlgroup would not show changes in kinematic or kineticvariables. METHODS We used a cohort design in which the independentvariables tested were group (core stability, plyometric,control) and time (pretest, posttest). The dependentvariables were lower extremity kinematic and kinetic groupmean values during the first 25% of stance phase at pretestand posttest. The kinematic variables assessed were lateraltrunk-flexion angle; hip-flexion, adduction, and internal-rotation angles; and knee-flexion, abduction, and internal-rotation angles. External joint moments for hip flexion,adduction, and internal rotation and for knee flexion,abduction, and internal rotation also were collected. Participants Twenty-three girls from 3 area high schools participated in this study (Figure 1). Participants (age  ¼  14.8  6  0.8years, height ¼ 1.7 6 0.07 m, mass ¼ 57.7 6 8.5 kg) wereactive on a junior varsity lacrosse or soccer team, had nohistory of trunk or lower extremity surgery, and had noinjury within the 6 weeks before the study that limited their athletic or physical activity. The control and core stabilitygroups comprised athletes from both sports, whereas the plyometric group consisted solely of lacrosse players.Furthermore, participants had no neurologic disorders thataffected balance and had not been involved in a formal corestability or plyometric training program. The study wasapproved by the Institutional Review Board for HealthSciences Research of the University of Virginia. Parents or guardians of the participants provided written informed consent, and the participants provided written informed assent. Instrumentation A force platform (OR6-7; AMTI, Watertown, MA) wasused to collect raw ground reaction forces at 1000 Hz and interfaced with a 10-camera motion analysis system (model624; Vicon Peak, Lake Forest, CA) to capture the 3-dimensional position of markers at 250 Hz. Testing Procedures Participants reported to the Motion Analysis Laboratoryfor pretesting within the first third of the high school springathletic season. Anthropometric measurements, includingheight, mass, leg length, knee width, and ankle width, weretaken and recorded. Participants were fitted with runningshoes (model Radius 06; Brooks Sports, Inc, Bothell, WA).Retroreflective markers were placed bilaterally on thefollowing anatomic landmarks to represent the lower extremity segments in accordance with the Vicon ClinicalManager (Vicon, Centennial, CO) protocol: second meta-tarsal head, calcaneus, lateral malleolus, lateral midshank,lateral femoral condyle, and lateral midthigh. A 4-marker cluster was secured around the hips over the sacrum withelastic tape. To capture trunk motion, markers also were placed on the sternum, xiphoid process, C7 and T10spinous processes, and bilateral acromion processes. Astatic marker trial was collected before the dynamic testing.Participants were instructed in how to perform the DVJtask, and a demonstration was given to ensure comprehen-sion. No instructions or feedback on landing performancewas provided. For the DVJ, participants were directed tostand on a 25-cm box and lead with their right lower extremities to step off the box, landing on both feet. Theright and left foot contacted separate embedded force plates, and the participant performed a maximal vertical jump immediately upon contacting the ground. Each participant practiced the task until she felt comfortable,then 5 test trials were collected for analysis. The height of the box is the average maximal vertical jump heightachieved by adolescent girls when performing a DVJ. 15 Kinetic and kinematic data were collected for all partici- pants, and the mean values were used for analyses.Each participant repeated the testing after the 4-week intervention. All participants completed testing within 10days after the final session of intervention exercises. Theywere retaught how to perform the DVJ and allowed toreacquaint themselves with the task. After the posttesting Journal of Athletic Training  451  session, participants were dismissed from the study (Figure1). Intervention Programs Teams were allocated to 1 of 3 groups, and the athletes participated as an entire team in either the plyometric or core stability program 3 times each week for 4 weeks. Thetester (K.R.P.) was not blinded to which school wasallocated to the control group but was unaware of thespecific intervention group assignment for the remaining 2groups. The control group continued its normal teamactivities for 4 weeks: 1 to 2 games per week and 3 to 4 practices per week, depending on the game schedule. Thecoaches were given an attendance log to monitor compliance of the athletes enrolled in the study. They alsowere provided with a standardized exercise protocol thatincluded directions for the athletes and pictures of how tocorrectly perform the exercises, common mistakes madeduring each exercise, and potential corrections to make based on common errors (see Supplemental Appendixes S1and S2, available online at The coaches were not given atutorial on the exercises other than the material presented tothem via the standardized manual, and no script was provided to read for each intervention session. As anoutside assessment, a certified athletic trainer (AT) fromeach school observed 1 session each week and completed aform for 6 criteria (Table 1). The plyometric and corestability programs were designed to be conducted within 20minutes and require no additional exercise or rehabilitationequipment.The plyometric program (Table 2) consisted of a series of double-limb and single-limb jumps and of skippingexercises focused on quality takeoff and landing form.The included exercises were adapted from various ACLinjury-prevention and neuromuscular training programs inwhich the emphasis was placed on soft, balanced, and controlled landings. 5,10,16–18 The plyometric program wasdivided into 2 phases, with a progression occurring after thesixth session that increased the level of difficulty byincorporating more single-legged landings and multiplanar movements. The participants performed the exercises with partners to help reinforce the use of correct form; however,no specific partner instructions were given. Exercises for the core stability group were targeted at improving Figure 1. Flowchart outlining the progression of testing order and participant dropout. 452  Volume 48    Number 4    August 2013  coordination of the abdominal and lumbar stabilizers and hip extensors, external rotators, and abductors (Table3). 14,19,20 After completing 6 sessions, participants pro-gressed to a second phase of exercises that incorporated more challenging positions and combined maneuvers from phase 1 that focused on increasing trunk stability with moretraditional strength-gain exercises. All exercises in the plyometric and core stability programs were performed  bilaterally. Data were included in the analysis for all who participated in at least 9 of the 12 sessions. Data Analysis A Woltring filtering technique was applied to the marker data with a predicted mean square error value of 20according to recommended Vicon processing protocols.Ground reaction force data were synchronized with theVicon system for simultaneous collection. The ground reaction forces were filtered using a low-pass, antialiasingfilter with a cutoff frequency of 30 Hz. Initial contact wasidentified by marking the point at which the ground reactionforce vector first appeared, and toe-off was identified in asimilar manner by indicating the point at which the vector was no longer present; the ground reaction force vector wasassociated with a 20-N threshold. The data between initialcontact and toe-off were normalized to 101 data points for the stance phase. The ground reaction force data were timesynchronized with the kinematic data and processed usingPlug-in Gait (Vicon) to determine hip- and knee-jointmoments. Joint moment calculations were based on thefollowing variables: mass and inertial characteristics of each lower extremity segment, the derived linear and angular velocities and accelerations of each lower extremitysegment, and estimates of ground reaction force and joint-center position. Moments were normalized to a product of mass and height and reported in newton meters per kilogram (Nm/kg  m). Statistical Analysis We made within-group comparisons for all dependentvariables. We implemented an intention-to-treat analysis,carrying the last data point forward (baseline) for the 1 participant in the plyometric group who did not report for  posttest. Group means and associated 95% confidenceintervals (CIs) were calculated for each percentage of thelanding phase. Data during the first 25% of landing werecompared to assess intervals for which the CI bands did notoverlap. We chose this period because most noncontactACL injuries are reported in the early phase of landing. 21 We set the  a  level at .05 to determine differencesthroughout the landing phase by identifying periods inwhich the 95% CI bands for the 2 data sets did not cross. 22 Confidence interval bootstrapping allows for comparisonsduring a period of the landing phase rather than peak  points; the latter tend to represent only a discrete minimumor maximum value of the landing phase (Figure 2). Effectsize (Cohen d) was calculated for each joint moment at the point where the mean difference between pretest and  posttest scores was the largest and where the CI bands did not cross. Cohen d was calculated by taking the meandifference between pretests and posttests and dividing bythe pooled standard deviation. An effect size of 0.8 or larger with a CI that did not cross zero was considered a Table 1. Assessment Tool Used by the High School Certified Athletic Trainer, Criteria and Means 6 SDs Grading Criteria Result a 1. The coach is instructing the athletes how to correctly perform the exercises. 3.00 6 0.762. The coach is emphasizing key mistakes to look for. 2.75 6 0.713. The coach is giving constructive feedback during the exercises. 3.13 6 1.254. The athletes are listening and following the directions given. 3.5 6 0.535. The athletes are working hard to perform the exercises correctly. 2.88 6 0.645. The athletes seem challenged by the exercises they are asked to perform. 2.75 6 0.46 a The scores were based on a 5-point Likert scale: 0 ¼ never  , 1 ¼ rarely  , 2 ¼ some of the time  , 3 ¼ most of the time  , 4 ¼ always  . Table 2. Plyometric Group Exercise Progression PhaseSets 3 Repetitions1 (weeks 1 and 2)Forward/backward single-legged line jumps 1 3  30Side-to-side single-legged line jumps 1 3  30High skips 1 3  field lengthDistance skips 1 3  field lengthBroad jumps a 2 3  10Tuck jumps a 2 3  10Alternating single-legged lateral jumps 2 3  102 (weeks 3 and 4)Forward single-legged hop, hop, hop, and stick a 1 3  10Squat jumps a 2 3  10Single-legged maximal vertical jump a 1 3  10Single-legged jump for distance a 1 3  10Broad jump, jump, jump, vertical jump a 1 3  5180 8  jumps a 1 3  10Single-legged lateral jumps a 1 3  10 a Indicates the exercise was performed with a partner watching. Table 3. Core Stability Group Exercise Progression PhaseSets 3 Repetitions1 (weeks 1 and 2)Abdominal draw in 10 3  5 sSide plank knee bent 2 3  20 sSide-lying hip abduction 3 3  10Side-lying hip external rotation (clam shells) 3 3  10Crunches 3 3  15Lumbar extension, hands on head 3 3  10Walking lunges, hands on hips Field length2 (weeks 3 and 4)Hamstrings bridge with abdominal draw-in 3 3  20 sSide plank legs extended with abdominal draw-in 2 3  10 sQuadruped hip extension/external rotation/abduction 2 3  10Crunches, opposite elbow to knee 3 3  15Lumbar extension, upper extremities straight 3 3  10Squats with upper extremities overhead a 3 3  10Lunges with ball toss a 3 3  10 a Indicates the exercise was performed with a partner watching. Journal of Athletic Training  453  Figure 2. Confidence interval analysis graphs. Each graph depicts a group change from pretest to posttest for a specific dependentvariable that was different. The lines surrounding the mean scores represent the upper and lower 95 %  confidenceintervals. The dependentvariable was graphed for the entire stance phase but only differences that occurred within the first 25 %  of stance were analyzed. A, Controlgroup knee internal-rotation (IR) moment. B, Plyometric group knee-flexion angle. C, Plyometric group knee IR angle. Abbreviation: ER,external rotation. 454  Volume 48    Number 4    August 2013
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