Background: Women’s susceptibility
to injuries involving the anterior cruciate ligament remains unexplained.
Volitional contraction of the knee musculature is known to increase
the resistance of the knee to shear deformation, raising the possibility
that muscles play a part in protecting the anterior cruciate ligament
during hazardous activities. We therefore tested the hypothesis
that a volitional co-contraction of the knee muscles increases the
sagittal-plane shear stiffness (or resistance to anterior tibial
translation) of the knee more in men than in women.
Methods: Twenty-three volunteers (ten men and thirteen
women; mean age, 24.7 ± 5.4 years), all with anterior
tibial translation of 6 mm, agreed to participate in the study.
Each subject underwent a subjective evaluation of knee function
and activity level, an arthrometric measurement of passive anterior
tibial translation, and an isokinetic dynamometer strength test
at 60Â°/sec. A dynamic stress test was then performed to
measure anterior tibial translation while simultaneously monitoring lower-extremity
Results: Maximum co-contraction of the knee musculature
significantly decreased mean anterior tibial translation in both
men and women (from 7.8 mm to 2.2 mm in men and from 6.5 mm to 3.1
mm in women). The corresponding percentage increase in shear stiffness
of the knee was significantly greater (p = 0.003) in men
(379%) than in women (212%).
Conclusions: The results suggested that women have
a diminished potential for muscular protection of passive structures
of the knee in anterior tibial translation.
Clinical Relevance: Maximal muscular protection
of the anterior cruciate ligament in women may be less than that
in men. This may be one factor explaining why more women than men
are apt to sustain injuries to the anterior cruciate ligament.