Evidence that Matters for Function: Proximal Control of Dynamic Knee Valgus
Cannon J, Cambridge EDJ, McGill SM. Anterior Cruciate Ligament Injury Mechanisms and the Kinetic Linkage: The Effect of Proximal Control During Bilateral Landings. J Orthop Sports Phys Ther, 2019 49: 601-610.
The above article will be the subject of two blogs and two accompanying videos in the Gray Institute® series called “Evidence that Matters for Function.” In spite of some limitations, which the authors acknowledge, this study has a lot to offer practitioners of any movement discipline / profession. First, their rationale for the study design covers a lot of valuable information in a logical manner (63 references). The scientific rationale for the study validates the Chain Reaction® approach advocated by Dr. Gary Gray for more than 40 years. Second, the results of the study support the Applied Functional Science® of addressing links in the kinematic chain other than where the injury / symptoms are located.
The initial study group consisted of 18 university-aged women. The subjects performed Drop Vertical Jumps (DVJ) from a height of 31 centimeters, onto two adjacent force plates. The landing was transformed into a maximum vertical jump without any pause. In addition to force plate data, motion analysis of the joints and electromyography (EMG) of 24 muscles was recorded and analyzed. The mean of three jumps was averaged, and subjects were categorized as having bilateral, unilateral, or no valgus. Four subjects demonstrated bilateral valgus and four subjects did not demonstrate valgus in either knee. These two groups were then compared with regard to the other data collected to see what characteristics of the DVJ differentiated the two groups.
This blog will focus on the findings related to hip muscles activation and the calculated rotational stiffness at the hip joint. The mean medial knee displacement (MKD) in the low valgus group of four subjects was 1.4 cm, while the mean for the high valgus group was 2.1 cm. Joint Rotational Stiffness (JRS) was modeled from the EMG data and the motion data. They calculated JRS for the frontal plane (gluteus medius) and transverse plane (gluteus maximus) from surface EMG. From an Applied Functional Science® perspective, it is impossible to assign any of the three gluteal muscles to a single plane.
Nevertheless, the calculated JRS differences begin to explain part of the differences in the knee valgus motion for the two subgroups. The frontal plane JRS for the low valgus group was 170% greater than the group that demonstrated high valgus. The transverse plane JRS showed a very similar difference. In spite of the low number of subjects, the differences between groups were statistically significant. The effect size calculations indicate that the differences are of large practical importance.
JRS should not be perceived as a lack of motion at a joint. Instead the JRS numbers, based on the model, represent a place on a stiffness–compliance continuum. Two much stiffness can be as dangerous as too little. Greater stiffness indicates greater control of the hip motion. Greater control of hip motion will translate into better control of knee motion. This is consistent with a recent appreciation (both scientifically and clinically) of the influence that the hip has on the knee.
The Chain Reaction® seminars, taught by Dr. Gary Gray, have been “beating this drum” for more than 30 years. Gary would say that it is not surprising that research is providing evidence that the hip influences the knee. He would say to look at the femur. The femur is the knee and the femur is the hip. The femur can’t move at the hip without influencing the knee joint. The logical progression of this anatomical logic, increasingly supported by science, is that the best way to train and rehabilitate the knee is to focus on the hip joints and the powerful muscles that control the motions.