The Effcts of Isolated Hip Abductor and External Rotator Muscle Strengthening on Dynamic Knee Valgus During Single-Leg Landing

Anterior cruciate ligament (ACL) injury is regarded as one of the most frequent knee joint injuries. ACL injury is typically non-contact and occurs during activities such as cutting, accelerating, and landing after a jump. According to the video analysis, the mechanism of non-contact ACL injuries consist of several common components such as hip adduction and internal rotation, knee valgus, tibia external rotation, and ankle eversion; Combination of these components called "dynamic knee valgus". In this situation, the role of hip abductor and external rotator muscles is to eccentrically control hip adduction and internal rotation. In closed-kinetic-chain, adequate stability in hip joint provides better knee control. Evidence for this is seen in a reported greater hip abduction correlated with less knee valgus during single leg squat. Specifi cally, lower eccentric hip abductor peak torque is associated with higher peak knee valgus during landing Although the literature showed the positive effects of the preventive programs, most of them include exercises that involve combination of muscles groups, so the mechanisms by which muscles group reduce dynamic knee valgus are unclear. To the best of our knowledge, no study has addressed the effectiveness of an isolated hip abductor and external rotator strengthening on dynamic knee valgus during jump-landing task. Therefore, the purpose of the current study was to determine whether an isotonic strengthening which targeting the hip abductor and external rotator would improve the dynamic knee valgus during landing. We hypothesized that dynamic knee valgus would improve post isolated hip abductor and external rotator strengthening program.

Present study is a randomized controlled trial carried out by single-blinded parallel group.  Thirty-two males with knee valgus angle more than 8 degree were randomized into a training (n=16) or control group (n=16). In pre-screen session, in order to determine dynamic knee valgus eligibility for participants, subjects were asked to perform single-leg jump-landing task. Subjects landed forward from the 31-cm height box with their dominant leg onto a marked place at a distance of 40% their height. The subjects landed on their dominant leg and immediately performed a countermovement jump to achieve maximal height. A digital video camera was set up on tripod at the height of the subject’s knee, 2 meter in front of the subjects landing place, and lined up vertical to the frontal plane. The frame that knee has highest medial displacement was exported into Auto CAD 2016 software (version 20.1) for calculation of knee dynamic valgus angle. In pre-test sessions, each subject warmed up for 5 minutes (walking and jogging around the laboratory at a self-selected pace), and then was instrumented with 40 retro-reflective markers. The positions of the markers were recorded at 200 Hz using a motion capture system (Qualisys AB, Gothenburg, Sweden) with eight Ocus cameras and were synchronized with a force plate (Kistler AG, made in Switzerland) collected data at 2000 Hz. Each subject completed an initial static, standing calibration trial to align local coordination system with global coordinate system, and then performed 3 attempts to single-leg jump-landing task (as described in pre-screen) with dominant leg (30 seconds rest between each attempt). Eccentric and concentric peak torques of dominant leg hip abductor (side-lying position) and external rotator (seated position with 90° flexion in the hip and knee) were quantified by a single examiner using KinCom 500H isokinetic dynamometer (Chattecx Corp. Inc. Hixson, TN). The training group carried out 8 weeks of bilateral hip abductor and external rotator strengthening 3 times per week. Each session included 3 phases: 1: warm-up, 5 minutes Walking and jogging, 2: strengthening exercises, 35 minutes hip abductor and external rotator muscles exercises, 3: cool down, 5 minutes walking. In the post-test session, participants followed the same procedure used for the pre-test session. Each participant wore the same shoes in pre and post-test sessions to prevent injury and control footwear differences effect, and all tests were completed at the same time in pre and post-test sessions.

During the course of the study, two subjects withdraw from the study due to personal problems (one subject in CG and one subject in TG) and one subject sustained injury due to an accident (in TG). So, data from 29 subjects were used for statistical analysis.The ANCOVA evaluating changes in hip strength from baseline to the end of the 8-week hip strengthening program revealed a significant difference between TG and CG for concentric (P<0.001) and eccentric hip abduction (P=0.002) and concentric (P=0.007) and eccentric hip external rotation (P=0.001) peak torques. Paired-sample t tests were performed to determine the training effect on the hip muscle strength. At the post-test, the TG demonstrated greater concentric (1.6±0.55 vs 2.01±0.42 Nm/Kg; P<0.001) and eccentric hip abductor (1.41±0.51 vs 1.77±0.43 Nm/Kg; P=0.004) and concentric (0.94±0.21 vs 1.07±0.19 Nm/Kg; P=0.01) and eccentric hip external rotator (0.92±0.18 vs 1.11±0.21 Nm/Kg; P<0.001) peak torques compared with pre-test. No significant differences in muscle strength were observed for the control group between pre-test and post-test.The ANCOVA evaluating changes in knee kinematic from baseline to the end of the 8-week hip strengthening program revealed a significant difference between TG and CG for peak knee valgus (P=0.02) and knee valgus range of motion (ROM) (P=0.04). After the 8-week intervention, the TG demonstrated lower peak (-5.62 ± 7.36 vs -2.74 ± 5.94; P=0.02) and range of motion (ROM) on dynamic knee valgus angle (-9.66 ± 5.56 vs -7.31 ± 3.87; P=0.04) compared with pre-test, whereas significant changes were not noted in the control group (P>0.05).

Hip abductor and external rotator weakness is a well-documented impairment in subjects with increased dynamic knee valgus during landing and has been postulated to contribute to non-contact ACL injury. This study was conducted to investigate the effects of isolated hip abductor and external rotator exercises on lower extremity kinematic, as well as concentric and eccentric abductor and external rotator torques in male subjects who present with excessive dynamic knee valgus during landing. The most important finding of the current study was a reduction in peak and knee valgus range of motion during landing in training group. These changes might be due to an increase in concentric and eccentric hip abductor and external rotator peak torques in training group as a result of the 8-weeks resistance training. Our findings support the isolated hip abductor and external rotator strengthening as a viable preventive and rehabilitative exercises program option for subjects who shows excessive dynamic knee valgus impairment in the hope of preventing non-contact ACL injury.