ACTIVE KNEE STIFFNESS DIFFERS BETWEEN YOUNG MEN AND WOMEN

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

Keywords: knee, muscle, stiffness

Introduction

Women’s susceptibility to ACL injuries remains unexplained. We hypothesize that neuromuscular factors may be involved. Shear forces applied to the knee must be resisted by load sharing between the passive and active structures, in proportion to their relative stiffnesses. The active generation of stiffness across the knee joint by tensing the knee musculature may be an important protective mechanism, because the greater the muscle stiffness, the lower the force carried by the ACL and other passive structures.

Materials and Methods

Twenty-three volunteers (10 males, 13 females, average age = 24.7+ 5.4 years) agreed to participate in the study. The activity levels ranged from sedentary to NCAA Division I elite level athletes. Each subject underwent a subjective evaluation of knee function and activity level, arthrometer measurement of static anterior tibial translation (ATT), and isokinetic dynometer strength and endurance tests at 60 and 240°/sec. A dynamic ATT stress test evaluated lower extremity muscle response while simultaneously monitoring ATT. The ATT stress test was first conducted with muscles relaxed and then in the fully co-contracted state in order to find the change in knee stiffness.

Results

Maximum contraction of the knee musculature significantly decreased ATT in both men and women (Table 1). However, the percent increase in knee stiffness was significantly greater in males (473% vs. 217%, p=0.003).

Table 1: Mean (SD) ATT (in mm)

The gender difference in physical activity levels did not show statistical difference (p=0.17). Even though males were found to generate significantly higher torques in both the quadriceps and hamstrings, a multiple regression analysis determined that muscle strength was not the predominant contributor to knee stiffness.

Discussion

A significant gender difference was found in the ability to voluntarily stiffen the knee joint, for women could not stiffen their knee complex to the degree that men did. This is consistent with known differences in musculotendonous elasticity properties between the genders.^1,2 This significant gender difference may mean that the passive structures in women’s knees are subjected to higher strains than men during physical activities.

Conclusions

The results suggest that the ability to increase knee stiffness is not determined by the subject’s muscle strength or body height/weight ratio, but primarily by their gender. Men were found to generate significantly higher knee stiffness values compared to women. This finding may partially help to explain the discrepancy in knee injury rates seen between the sexes.

References 1. Winter EM, Brookes FBC: Eur J Applied Physiol 63: 124-128, 1991.

2. Komi PV: Exercise and Sport Science Reviews (12): 81-121, 1984.