INCREASED LEG STIFFNESS IN ACL RECONSTRUCTED KNEES.

Kremenic IJ, Gartman EJ, Schneider K, Ben-Avi SS, Gleim GW, McHugh MP, Nicholas SJ

Nicholas Institute of Sports Medicine and Athletic Trauma and The Cooper Union School of Engineering, New York, NY

The goal of ACL reconstruction is to restore normal knee function, allowing return to sport within 6 months. A spring is a simple but useful model of lower extremity stiffness in many activities. We used this spring model to examine lower-extremity stiffness during one-legged hopping at a frequency of 2 Hz in 29 subjects. Of these, 15 (9 male, 6 female) had no history of knee pathology (NORM), and 14 (7 male, 7 female) were post-ACL reconstruction (mean f/u 7.8 +/- 2.5 months, ACL). Subjects hopped on a force plate (Kistler, sampled at 1200 Hz) and were filmed using infra-red cameras (Qualisys, 60 Hz). Spring stiffness was computed using the motion of the anterior-superior iliac spine (ASIS) and the peak force (at the lowest point of the hop) and corrected for body weight. In ACL, stiffness was higher (p = 0.03) in the involved leg (178 +/- 9 N/m/kg) than the uninvolved leg (154 +/- 6 N/m/kg). In NORM, stiffness was not different between dominant (159 +/- 6 N/m/kg) and non-dominant (160 +/- 9 N/m/kg) legs. Gender did not affect stiffness in either group. Knee flexion angle at peak force was lower for ACL (involved, uninvolved: 39.8 +/- 2.0, 43.8 +/- 1.4 deg.) than NORM (dominant, non-dominant: 47.4 +/- 1.3, 45.2 +/- 1.9 deg.) (group effect, p = 0.04). Ankle and hip angles did not exhibit a difference between group or side. Hop height was not different between groups and sides. In conclusion, leg stiffness was 16% higher in the involved vs. the uninvolved side during a simple hopping test despite no differences in kinematics, hop height, or hop frequency. This change in stiffness may be a strategy to increase functional stability of the knee.

Medicine and Science in Sports and Exercise. 32: S267, 2000.