서강대학교

초록 (요약문)

Anterior cruciate ligament (ACL) injuries are common during sports activities. More than 70% of ACL injuries occur in a noncontact situation. Since the muscle strength and coordination have a direct effect on the mechanical loading of the ACL during sport movements, muscle strengthening has been included in the ACL injury prevention programs. A better understanding of the connection between muscle strengthening and lower extremity joint biomechanics associated with the ACL injury risk helps to improve the current intervention program, thereby decreasing the risk of ACL injury. Thus, the objectives of this dissertation was to examine the biomechanical and neuromuscular changes in lower extremity joints after muscle strengthening and to contribute an in-depth understanding to improve the design and effectiveness of strength training in ACL injury prevention programs. A 3-dimensional motion capture system equipped with a synchronized force plate was used to obtain the trunk, hip, knee, and ankle kinematic/kinetic data, and the muscle activations of the selected trunk and lower extremity muscles were obtained using an EMG system while the participants performed side-step cutting, single-leg landing, or level walking. Recreationally active male and female participants were randomly assigned to the whole-body strength training group, core strengthening group, strength training targeting medial quadriceps and hamstrings group, training non-targeting medial quadriceps and hamstrings group, or control group. Concentric isokinetic muscle strength of quadriceps and hamstrings were measured using an isokinetic dynamometer before and after completing each strength training program. To test the core stability, a trunk endurance test was used. In addition, the muscle thickness of individual quadriceps and hamstrings derived from ultrasound imaging was obtained, since measuring the individual medial and lateral thigh muscle strength using the isokinetic dynamometer is unavailable. A 2 × 2 (group × time) analysis of variance was conducted for each dependent variable to determine the effects of each strength training. Pearson correlation coefficient was used to determine the relationship between the muscle strength differences and the changes in significant kinematic/kinetic variables found from post hoc tests. Stepwise multiple regression analysis, a follow-up Fisher’s r to z test to examine the sex as a moderator, and independent t tests to evaluate the sex difference were also performed. Both quadriceps (P = 0.005) and hamstrings (P = 0.030) muscle strength were increased after whole-body circuit strength training. An increased biceps femoris activation (P = 0.003) and hamstring-to-quadriceps (H:Q) coactivation ratio (P = 0.016), as well as decreased gastrocnemius muscle activation (P = 0.012) during the pre-contact phase, were found, but no significant differences were found in knee kinematics and kinetics. Thus, whole-body strength training alone would not be a sufficient intervention program to reduce the ACL injury risk factors, although it altered some muscle activations of lower extremity muscles in a decreasing direction of ACL injury risk (Specific aim 1). On the other hand, core strength training decreased knee valgus (P = 0.020) and hip adduction angles (P = 0.012), but increased trunk flexion angle (P = 0.005), rectus abdominis to erector spinae coactivation ratio (P = 0.027), H:Q coactivation ratio (P = 0.007), and vastus medialis to vastus lateralis coactivation ratio (P = 0.003). In addition, the knee valgus angle at initial contact was negatively correlated with the VM:VL activation ratio in the pre-contact phase (R2 = 0.188, P < 0.001) but was positively correlated with the hip adduction angle (R2 = 0.120, P < 0.005). As a result, training core muscles can modify the biomechanics associated with ACL injuries in the side-step cutting task; thus core strength training might be considered in ACL injury prevention programs to alter the lower extremity alignment in the frontal plane and muscle activations during sports-related tasks (Specific aim 2). When investigating the relationship between the muscular parameters of individual quadriceps/hamstrings derived from ultrasound imaging and the knee joint biomechanics, there was a significant relation between the muscular parameters and the knee joint moments. Specifically, both knee valgus moment (R2 = 0.466, P < 0.001) and tibial external rotation moment (R2 = 0.330, P < 0.001) at peak anterior tibial shear force during single-leg landing were negatively correlated with medial-to-lateral (M:L) thigh muscle thickness ratio regardless of sex. In addition, the muscular parameters of the individual quadriceps and hamstring determine the correlative properties of knee joint moments during walking and the gait spatiotemporal parameters. Thus, the medial/lateral thigh muscle thickness could be a potential contributor to knee joint loading during dynamic movements (Specific aim 3, 4). Based on the above-reported findings, the effect of novel strength training targeting the medial quadriceps and hamstrings on the knee joint biomechanics was investigated. The strengthened medial quadriceps and hamstrings decreased the coronal plane knee excursion (P = 0.036), peak knee valgus moment (P = 0.042), and peak hip internal rotation moment (P = 0.037) during single-leg landing. In addition, VM:VL and M:L coactivation ratios in the post-contact phase were increased (P = 0.043 and P = 0.036, respectively), and the onset timing differences of VL – VM and biceps femoris – semitendinosus were altered after training. These results provide evidence that the novel strength training targeting medial quadriceps and hamstrings can alter the biomechanical risk factors of ACL injury as a single intervention, thus it might be taken into account when planning the ACL injury prevention programs aimed at reducing dynamic knee valgus occurring in single-leg landing (Specific aim 5). With the information gathered from this study results and previous research, incorporating the strength training designed to target the medial quadriceps and hamstrings in the ACL injury prevention program with core strengthening can effectively modify the “at-risk” lower extremity joint kinematics/kinetics and muscle activity associated with noncontact ACL injuries, thereby reducing the incidence of ACL injuries.