DOES AXIAL ROTATION DIFFER BY GENDER? EFFECTS OF KNEE FLEXION ANGLE AND MUSCLE CO-CONTRACTION

Participants: J.P. Boylan, L.J. Huston, E.M. Wojtys, J.A. Ashton-Miller

Key words: knee, muscle, gender

Introduction

External loads applied to the knee are typically resisted by a combination of ligaments and muscles. Load-sharing between the ligaments and muscles is generally in proportion to their relative stiffnesses. The lower the muscle stiffness, the greater the percentage of the load that must be borne by the ligaments. Hence, for a given external load, insufficient muscle stiffness poses an increased risk for ligamentous damage. Muscle co-contraction is known to reduce anterior-posterior and rotational displacements at the knee under standardized test loads. Males are known to have greater leg muscle strength which is related to cross sectional area. Because muscle stiffness has been documented to increase two to five-fold in a maximal contraction, we theorized that males might be able to protect their ACL more effectively than females due to known absolute strength discrepancies. The aim of this research was to test the (null) hypotheses that gender would affect neither the absolute internal rotational displacement nor the percent decrease in rotational motion affected with a quadriceps and hamstring muscle co-contraction in a group of size and strength-matched men and women. We also tested the hypothesis that an increase in knee angle would significantly decrease knee rotation in response to a standard impulsive force.

Materials and Methods

Fourteen size-matched gender pairs of healthy young adults underwent an isokinetic knee leg strength test (60 degrees/sec), and an isometric knee/ankle strength test in external rotation. Knee rotation was measured in response to a standardized impact load acting in internal rotation applied using a ballistic pendulum. Four tests were conducted both with relaxed and fully co-contracted leg muscles, and knee flexion angles of 30 or 60 degrees with the seated subject.

Results

No significant gender differences were found in BMI, quadriceps/hamstring strength, or skin motion data.

1.Comparison of Gender Differences in Absolute Knee Rotations: In all groups except at 30 degrees tensed, there were statistically significant gender differences. Males had significantly less motion in response to the impulse than their matched females in three out of four states.

2. Effect of Gender on Reduction of Axial Rotation With Co-Contraction: No significant gender differences were found in the ability to reduce rotation with a muscle co-contraction. Percentage increase in male knee stiffness ranged from 114-259% at 30° and 183-342% at 60°. Female knee stiffness increases ranged from 126-277% at 30 ° and 110-300% at 60°. Rotational motion was the value measured with each trial and stiffness is the ratio of change in motion between two trials.

3. Effect of Knee Flexion on Knee Axial Rotation: Knee flexion angle had a highly significant effect on knee rotation, as motion decreased with greater knee flexion. The knee rotated less upon impact with the knee at 60° of flexion than at 30° of flexion for both relaxed and tensed trials (p<.001 for each).

Conclusions

The results showed that males had significantly more passive joint stiffness at both 30 and 60 degrees of knee flexion and significantly more active joint stiffness at 60 degrees. No significant gender differences were found in active joint stiffness at 30 degrees. No significant gender difference was found in the ability to stiffen the knee using a maximum volitional co-contraction. Across most subjects, however, a maximum co-contraction significantly decreased axial rotation under impact loading, particularly at 60 degrees of knee flexion. In conclusion, these results suggest that there is a significant difference in passive joint stiffness between size matched men and women, but no significant gender difference in active joint stiffness at 30 degrees of knee flexion.

Progress

Currently, a manuscript is being written.