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2000 Publications

INCREASED LEG STIFFNESS IN ACL RECONSTRUCTED Kremenic IJ, Gartman EJ, Schneider K, Ben-Avi SS, Gleim GW, McHugh MP, Nicholas SJNicholas Institute of Sports Medicine and Athletic Trauma and The Cooper Union School of Engineering, New York, NY - last modified 2013-02-10 00:00
Medicine and Science in Sports and Exercise. 32: S267, 2000.


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.