Landing on an Unstable Surface Decreases ACL Biomechanical Risk Factors

OBJECTIVES: Quadriceps dominant athletes are at a higher risk for anterior cruciate ligament (ACL) injuries because they lack sufficient hamstring activation resulting in a higher Quadricips: hamstring ratio. Muscular co-contraction (low Q:H) is needed to protect the intra-articular structures of the knee. Exercises that promote co-contraction and proprioception have been shown to reduce quadriceps dominance, enhance knee stability and alter neuromuscular firing patterns. The purpose of this investigation was to examine whether landing on an unstable surface (Bosu Ball) induced a greater amount of co-contraction at the knee compared to a stable surface. METHODS: Thirty-one Division I NCAA female athletes performed 3 single leg drop jumps per leg on 2 surfaces. Subjects dropped from a 30 cm step first onto the floor (stable surface), and onto a Bosu ball (unstable surface). Each landing was held for a minimum of 2 seconds. Subjects were familiar with Bosu ball training. Muscle activity of the lateral hamstring and vastus lateralis were used to estimate peak hamstring activity and the Quadriceps:Hamstring (Q:H) co-contraction ratio at the time of peak quadriceps activity. Kinematic data were also collected (Vicon) and used to evaluate the following peak measurements: knee flexion angle, hip flexion angle, and trunk flexion and sway angles (Visual3D). All variables were assessed between the time of landing and the end of deceleration. A 1-level ANOVA was used to test for significant differences across the sports in for each variable of interest. Significance was set at p<0.05. RESULTS: Max co-contraction (Q:H) was significantly reduced when athletes landed on an unstable surface (45% lower, p<0.01 Table 1, Figure 1A) compared to the stable surface. Peak hamstring activity was higher when landing on an unstable surface (15% higher, p=0.05, Table 1) compared to a stable surface. Peak knee flexion angles were 21% greater when athletes landed on a stable surface compared to landing on an unstable surface (Table 1, Figure 1B). Conversely, peak hip flexion angles were 25% less when athletes landed on a stable surface compared to an unstable surface. No significant difference between the surface landings was observed for either trunk flexion or trunk sway. CONCLUSION: The findings of this study highlight the potential utility of unstable surfaces as a landing training tool for ACL injury prevention. In order to reduce the risk of ACL injuries, athletes should land with a low quadriceps:hamstring ratio and increased knee and hip flexion. Training efforts to alter quadriceps:hamstring co-contraction have proved difficult. Unlike increasing knee or hip flexion, it is suspected to be more challenging to alter neuromuscular patterns with a simple command or demonstration compared to knee flexion. Here, we demonstrate that athletes significantly improve and lower their quadriceps:hamstring co-contraction when landing on an unstable surface. This finding should encourage the integration of unstable surface training as a possible method for ACL injury prevention.