Effects of Transverse and Frontal Plane Knee Laxity on Hip and Knee Neuromechanics During Drop Landings Shultz SJ, Schmitz RJ. Effects of Transverse and Frontal Plane Knee Laxity on Hip and Knee Neuromechanics

Background Varus-valgus (LAXVV) and internal-external (LAXIER) rotational knee laxity have received attention as potential contributing factors in anterior cruciate ligament injury. This study compared persons with above-and below-average LAXVV and LAXIER values on hip and knee neuromechanics during drop jump landings. Hypothesis People with greater LAXVV and LAXIER values will have greater challenges controlling frontal and transverse plane knee motions, as evidenced by greater joint excursions, joint moments, and muscle activation levels during the landing phase. Study Design Descriptive laboratory study. Methods Recreationally active participants (52 women and 44 men) between 18 and 30 years old were measured for LAXVV and LAXIER and for their muscle activation and transverse and frontal plane hip and knee kinetics and kinematics during the initial landing phase of a drop jump. The mean value was obtained for each sex, and those with above-average values on LAXVV and LAXIER (LAXHIGH = 17 women, 16 men) were compared with those with below-average values (LAXLOW = 18 women, 17 men). Results Women with LAXHIGH verus LAXLOW were initially positioned in greater hip adduction and knee valgus and also produced more prolonged internal hip adduction and knee varus moments as they moved toward greater hip adduction and internal rotation as the landing progressed. These patterns in LAXHIGH women were accompanied by greater prelanding and postlanding muscle activation amplitudes. Men with LAXHIGH versus LAXLOW also demonstrated greater hip adduction motion and produced greater internal hip internal rotation and knee varus and internal rotation moments. Conclusion Participants with greater LAXVV and LAXIER landed with greater hip and knee transverse and frontal plane hip and knee motions. Clinical Relevance: People (especially, women) with increased frontal and transverse plane knee laxity demonstrate motions associated with noncontact anterior cruciate ligament injury mechanisms. Article: Noncontact anterior cruciate ligament (ACL) injury mechanisms are consistently reported as rapid deceleration, plant and cut maneuvers, and 1-foot stopping/landing. 2,11 The position of no return has often been implicated in the ACL injury mechanism and has been described as having components of tibial external rotation and knee valgus. 11 Systematic video observation of team handball players sustaining an ACL injury supports this position of no return, revealing a mechanism of forceful valgus collapse (5° to 20°) with the knee in 5° to 25° of flexion, combined with 5° to 15° of external or internal rotation of the tibia. 23 Moreover, women tend to demonstrate these at-risk frontal and transverse plane knee motions 3,7,17,18,20 and moments 20,21,34,36 more often than men do during landing and cutting maneuvers, which are thought to contribute to their greater risk of suffering ACL trauma. However, factors underlying excessive transverse and frontal plane knee motions and moments are still unknown. Given that the knee is frequently exposed to considerable frontal and transverse plane torques during sport activity, understanding the factors that contribute to excessive transverse and frontal plane motion may be critical to maximizing our prevention strategies. The potential consequences of varus-valgus (LAXVV) and internal-external (LAXIER) rotational knee laxity on functional knee joint neuromechanics and ACL injury risk have been recently noted. Compared with men, women are reported to have approximately 25% to 30% greater LAXVV and LAXIER, whether 19,32 or not 10,28 sex differences in anterior knee laxity are also observed. This greater LAXVV and LAXIER is also associated with decreased torsional stiffness in women, compared with men. 10,24,27 On the basis of these findings and reports of ACL injury being associated with a combination of valgus and internal-external motions about the knee, 23 greater LAXVV and LAXIER may affect the orientation of the tibiofemoral joint and so place greater challenges on the neuromuscular system to stabilize the joint during weightbearing. 10,24,27,32 To date, we are not aware of any study that has examined the consequence of greater LAXVV and LAXIER on weight-bearing knee joint neuromechanics. We therefore compared participants who were above and below average on both LAXVV and LAXIER on their muscle activation and transverse and frontal plane hip and knee biomechanics during the initial phase of a drop jump landing. Our expectation was that individuals with greater LAXVV and LAXIER would have greater challenges controlling frontal and transverse plane knee motions, as evidenced by greater joint excursions, joint moments, and muscle activation levels during the landing phase. Given that men and women are already known to differ on joint laxity and knee joint neuromechanics, we examined the effects of laxity in men and women separately to control for other sex-dependent confounding variables. MATERIALS AND METHODS Ninety-six people participated: 52 women aged 22 ± 3 years (163 ± 6 cm tall, 60 ± 8 kg) and 44 men aged 22 ± 3 years (178 ± 10 cm tall, 81 ± 14 kg). Participants were recreationally active (2.5–10.0 hours per week) and did not smoke. They had no history of knee injury involving the osteochondral surface, ligament, tendon, capsule, or menisci; no history of vestibular or balance disorders; and no medical conditions affecting the connective tissue. Participants were recruited from the university and surrounding community as part of a larger ongoing project examining the effect of sex hormone–mediated knee laxity changes on weightbearing knee joint neuromechanics. Other inclusion criteria for the larger project were a body mass index ≤ 30, an ability to abstain from alcohol for 24 hours before any testing, and no history of vestibular or balance disorders. Additional criteria for female participants included self-reported normal menstrual cycles lasting 26 to 32 days for the past 6 months, consistent cycle lengths that varied no more than 1 day from month to month for the past 6 months, no use of oral contraceptives or other hormone-stimulating medications for the past 6 months, and no history of pregnancy or planning to become pregnant during the course of the study. All participants who were enrolled in the larger study were initially included in the current study. Data were obtained during a single test session where participants were measured on their dominant stance limb (ie, stance leg when kicking a ball) for anterior knee laxity, LAXVV and LAXIER, and hip and knee neuromechanics during the initial landing phase of a drop jump. Participants were familiarized to all testing procedures approximately 2 weeks before the actual testing. Female participants were tested during the first 6 days of their menstrual cycle (based on self-report of the first day of menstrual bleeding) to control for cyclic hormone effects on baseline laxity values. Before participation, participants were informed of all study procedures and then signed a consent form approved by the institutional review board. Specific procedures for obtaining each measure follow. Anterior knee laxity was measured with a KT-2000 knee arthrometer (Medmetric Corp, San Diego, California) with the participant supine and the knee flexed 25° ± 5° over a thigh bolster. 30 Anterior knee laxity was measured (in millimeters) as the anterior displacement of the tibia relative to the femur when a 133-N anteriordirected load was applied to the posterior tibia. The same researcher who had established excellent test-retest reliability, intraclass correlation coefficient2,k (standard error of the mean) = 0.96 (0.3 mm), measured all participants. The average of 3 measures represented the participant’s anterior knee laxity value. Participants were measured for LAXVV and LAXIER using the Vermont Knee Laxity Device (University of Vermont, Burlington, Vermont). 33 Detailed methods for these procedures have been reported and have been shown to yield consistent measures over repeated tests with acceptable measurement error: intraclass correlation coefficient2,k (standard error of the mean) = 0.91 (0.87°) for LAXVV, 0.89 (2.80°) for LAXIER. 33 In brief, the participant was positioned supine in the Vermont Knee Laxity Device, with the knee flexed to 20°, the thigh securely fixed, the foot and ankle tightly restrained in the foot cradle, and with counterweights applied to the thigh and shank to create an initial zero shear and compressive load across the tibiofemoral joint. LAXVV was assessed when valgus and varus torques of 10 N·m were created about the knee through the application of known forces applied to the medial and lateral aspect of the distal tibia at a known distance from the knee with a handheld force transducer (model SM-50, Interface, Scottsdale, Arizona). LAXIER was assessed when internal and external rotation torques of 5 N·m were applied about the long axis of the tibia using a T-handle connected to a 6-degree-of-freedom force transducer (model MC3A, Advanced Medical Technology Inc, Watertown, Massachusetts) affixed to the foot cradle. Joint displacements were collected (100 Hz) using Minibird Electromagnetic hardware (Ascension Technology Corporation, Burlington, Vermont) and MotionMonitor software (Innovative Sports Training, Chicago, Illinois). The average of 3 trials represented the participant’s laxity value. In preparation for surface electromyography (sEMG) measurements during the drop jump landing, the skin was shaved and cleaned with isopropyl alcohol, and 10-mm bipolar Ag-AgCl surface electrodes (Blue Sensor N-00S, Ambu Products, Ølstykke, Denmark) were positioned midway between the motor point and the distal tendon of the lateral and medial quadriceps, hamstrings, and gastrocnemius muscles, oriented perpendicular to the length of the muscle fibers with a center-to-center distance of 20 mm. Absence of cross talk was visually confirmed during manual muscle testing. To normalize the sEMG signal, participants completed 3 maximal voluntary isometric contractions (MVICs) on a Biodex System 3 Dynamometer (Biodex Medical Systems Inc, Shirley, New York) at 25° of knee flexion and 90° of ankle flexion while sEMG signals were obtained using a 16-channel Myopac telemetric system (Run Technologies, Mission Viejo, California) with an amplification of 1 mV/V, a frequency bandwidth of 10 to 1000 Hz, a common mode rejection ratio of 90 dB minimum at 60 Hz, an input resistance of 1 MΩ, and an internal sampling rate of 8 kHz. All sEMG data were acquired, stored, and analyzed using DataPac 2K2 lab application software (version 3.13, Run Technologies, Mission Viejo, California). Torque data obtained during the MVIC knee extension and knee flexion contractions were also recorded and normalized to the participant’s body weight to confirm that LAX groups within each sex were similar in their strength values; that is, previous work has shown a negative association between thigh strength and activation. 29 All biomechanical data were collected and processed using MotionMonitor software. With sEMG electrodes still attached, position sensors (Motion Star, Ascension Technologies, Burlington, Vermont) were attached to the sacrum, the C7 spinous process, the anterior midshaft of the third metatarsal, the midshaft of the medial tibia, and the lateral aspect of the midshaft of the femur of the dominant limb using previously described methods. 29 Joint centers were determined as the midpoint between the medial and lateral malleoli for the ankle, as the midpoint between the medial and lateral joint line for the knee, and by the Leardini method for the hip. 16 Once instrumented, the participants performed 5 barefoot drop jump landings from a 0.45-m wooden platform placed 0.1 m behind the rear edge of the force plate (type 4060, Bertec Corp, Columbus, Ohio). Participants began with toes aligned along the leading edge of the wooden platform and with hands placed at the level of the ears. They were instructed to drop off the platform and perform a maximal vertical jump upon landing, keeping their hands at ear level to eliminate variability attributed to arm swing. To prevent experimenter bias, no other specific instructions were provided with regard to landing mechanics. Along with the familiarization session, practice repetitions (typically, 3) were allowed before test trials to ensure that the participant remained comfortable with the task. Kinematic (100 Hz) and sEMG and kinetic (1000 Hz) data were simultaneously collected during 5 successful drop jump trials and synchronized by the software using a foot contact threshold of 10 N to trigger data collection. A trial was discarded and repeated if the participants lost their balance, did not land bilaterally, let their hands drop below ear level, or failed to land back onto the force plate after the maximal vertical jump. Data Reduction and Analyses Kinematic signals were linearly interpolated to force plate data and low-pass filtered at 12 Hz using a fourthorder, zero-lag Butterworth filter. The segmental reference system for all body segments was defined as the positive Z-axis for the medial to lateral axis, the positive Y-axis for the distal to proximal longitudinal axis, and the positive X-axis for the posterior to anterior axis. Knee motions were calculated using Euler angle definitions with a rotational sequence of Z Y′ X′′. 13 Kinetic data were low-pass filtered at 60 Hz using a fourth-order, zerolag Butterworth filter. Intersegmental data were calculated via inverse dynamics 9 and normalized to each participant’s height and weight: N·m × body weight −1 × height −1 . Kinematic and kinetic data for the initial landing phase (initial contact to peak knee flexion angle) were then normalized to 101 points and averaged across the 5 drop jump trials. Signals (sEMG) of the lateral and medial quadriceps, hamstrings, and gastrocnemius were band-pass filtered from 10 Hz to 350 Hz, using a fourth-order, zero-lag Butterworth filter, 15 then processed using a centered root mean square algorithm for the MVIC trials (100-millisecond time constant) and drop jump trials (25-millisecond time constant). After the 5 landing trials were ensemble averaged, the peak root mean square amplitude obtained from each muscle during the 150 milliseconds immediate before (preactivation) and after (postactivation) initial ground contact were then normalized using the average of the peak sEMG amplitudes obtained over 3 MVIC trials (% MVIC). Within each sex, mean LAXVV and LAXIER were calculated, based on the total range of motion observed during the varus-valgus and internal-external torque loadings, respectively. Participants were then classified as having aboveor below-average values on each measure and so were included in the analyses if they were classified with above-average values on both LAXVV and LAXIER (LAXHIGH) or below-average values on both LAXVV and LAXIER (LAXLOW). Participants were excluded if they were above average on 1 laxity value but below average on the other; that is, our goal was to have a clear separation of those with a high envelope of knee laxity and those with a low envelop of knee laxity. Our rationale for combining the 2 measures to determine laxity status is that knee joint biomechanics represent coupled knee motions and rarely do these motions occur in isolation during sport activity. On the basis of these criteria, 17 women were identified as LAXHIGH (LAXVV > 13.0°, LAXIER > 25.5°) and 18 as LAXLOW (LAXVV ≤ 13.0°, LAXIER ≤ 25.5°), and 16 men were identified as LAXHIGH (LAXVV > 8.3°, LAXIER > 19.3°) and 17 as LAXLOW (LAXVV ≤ 8.3°, LAX IER ≤ 19.3°) using the mean values for each sex as the cut point. With a conservative sample size of 32 (16 per group), an alpha level of P = .05, and a correlation among repeated measures of r = .5, we had 80% power to detect a moderate effect size (f >.36) for overall group main effects and a small effect size (f >.09) for group × time interactions. 4,6 Given that our main interest was to identify meaningful differences in joint motion and moment patterns over the landing phase (group × time), this sample size and associated statistical power were considered acceptable. To analyze the data, simple t tests compared groups on LAXVV, LAXIER, anterior knee laxity, body mass index, and maximal voluntary isometric strength of the thigh muscles. Separate group × time (LAXLOW, LAXHIGH × percentage landing phase increments) repeated measures analysis of variance compared LAX groups on hip abduction/adduction (HIPAA), hip internal/external rotation (HIPIER), knee varus/valgus (KNEEVV), and knee internal/external rotation (KNEEIER) motions and internal moments across the entire landing phase for each sex. Alpha level was set for each analysis at P < .05. Trend analyses (polynomial contrasts), along with graphical presentation of the data, were used to explore significant group × percentage-increment interactions. Trend analysis determines the most reasonable description of how groups differ in the pattern in the data across time (whether linear or nonlinear). 25 This process was followed by selected pairwise comparisons with Bonferroni corrections to compare the magnitudes of the greatest observed mean differences where trends in the data diverged. To examine group differences in muscle activation, separate group × muscle × side (LAXLOW, LAXHIGH × quadriceps, hamstring, gastrocnemius × lateral, medial) repeated measures analysis of variance compared LAX groups on prelanding and postlanding activation amplitude within each sex. For significant interactions, post hoc comparisons consisted of repeated contrasts within groups using Bonferroni corrections. All analyses were performed with SPSS 15.0.01 for Windows (SPSS Inc, Chicago, Illinois).