THE EFFECTS OF HIP AND HAMSTRING MUSCLE STRENGTH ON DYNAMIC KNEE VALGUS AND JOINT TORQUES DURING A DOUBLE LEG DROP JUMP AND DOUBLE LEG SQUAT

Abstract

INTRODUCTION: Amongst the injuries that occur in the lower extremities, anterior cruciate ligament (ACL) injuries have been shown to be the most common lower extremity injury impacting the young adult and athletic populations on a year-to-year basis and these injuries are typically occurring via non-contact mechanisms. Insights into the mechanisms of these non-contact ACL injuries can give researchers guidance as to which direction prevention strategies should utilized rather than the implementation of broad neuromuscular and proprioceptive training protocols in use today. While knee valgus has been shown to be a risk factor for ACL, the role of proximal hip muscle strength and hamstring strength to prevent this knee valgus has not been established. PURPOSE: The purpose of this research was to determine the effect of hip and hamstring muscle strength on knee abduction angles and torques during double leg squats and drop jumps. METHODS: 20 healthy subjects were recruited and enrolled in the study. The Humac NORM dynamometer was used to assess muscle strength from the hip extensors, hip abductors, knee flexors, and knee extensors. Subjects then performed double leg squats and double leg drop jumps while motion analysis and force plate data were simultaneously collected. DATA REDUCTION: Lower extremity angles and torques were calculated from the motion and force plate data. Strength testing data (EMG, Humac Position, and Humac Torque) was collected from the Humac Norm Dynamometer. STATISTICAL APPROACH: We divided the overall group of participants into categories of strong vs. weak overall hip extensor and knee flexor strength. We then compared the knee abduction angles and joints torques between the two groups. We also explored the relationship between hip extensor and knee flexor strength on knee abduction angles and torques through a correlational analysis. RESULTS: Only the hip extensor showed significant correlations and significant differences between high vs low muscle strength with any of the knee angles or torques during the double leg drop jump task. Findings of this current investigation indicate that the strength values for only the hip extensor muscle group are indicative of a relationship between strength and knee valgus angles/torques. During the double leg drop jump task, the hip extensor strength group significantly predicted knee flexion angle at initial contact and knee flexion angle at peak vertical ground reaction force accounting for 26% and 24% of the variance respectively, leaving 74% and 76% of the variance unaccounted for by all other muscle strength variables. CONCLUSION: The current study found no statistically significant relationships between the maximum isokinetic strength between the knee extensor, knee flexor, and hip abductor muscle groups with the peak knee valgus angles and torques displayed during double leg drop jumps and squats. Findings of this current investigation indicate that the strength values for only the hip extensor muscle group are indicative of a relationship between muscle strength and knee valgus angles/torques based off a correlational analysis and stepwise regression.