Description
The knee abduction moment (KAM) is one component of the anterior cruciate ligament (ACL) injury mechanism(1). As the injury happens immediately upon ground contact(2), the early stance phase KAM is therefore a potential risk factor for ACL injury. While risk factor studies have generally used a regular cutting or bilateral drop jump movement tasks, the occurrence and magnitude of the KAM has been shown to be task dependent. Identifying the best movement tasks to maximize the early peak KAM is therefore an important intermediate step for prospective studies that evaluate KAM as a risk factor for ACL injury. The aim of this study was to compare the early stance phase KAM between four athletic movement tasks in female athletes.
Methods:
16 female athletes aged 18-22 warmed up for 5 minutes on a treadmill before performing 10 repetitions per leg of four tasks: cutting, crossover-cutting, single-leg drop jumps, and side shuffles. Athletes were instructed to perform the movements with maximum speed. Marker data was collected at 1400 Hz with 12 infrared cameras while ground reaction forces were collected at 2000Hz. The raw signals were used to calculate the normalized KAM (Nm/kg) before applying a low-pass filter with a cut off of 6Hz. The highest KAM during the first 70ms was calculated and a mixed linear regression model was performed to compare this value between the movement tasks adjusted for leg (right vs left).
Results
The statistical model was a good fit for the data with an marginal r squared of 0.31. There was a difference in early peaks between all the movement tasks compared to the drop-jump reference value. Compared to the drop jump, the the early peak KAM was 0.18, 0.39, and 0.57 Nm/kg higher for the crossover cut, cut, and side shuffle respectively with all p<0.000001.
Discussion
Compared to the drop-jump, all the movements with a sideways motion produced higher early stance phase KAM, confirming the movement task specificity of the variable. The most studied movement, a regular cut, showed a definite potential to identify early peak KAM but the side shuffle clearly produces the greatest early stance KAM by a wide margin. The side shuffle shows potential to identify athletes that display higher KAM during the early stance phase. Thinking more broadly, results such as these can indicate that researchers should perhaps embrace more variety in the selection of movement tasks for motion analysis.
