Background
Muscle function may influence the risk of knee injury and outcomes following injury [
1‐
6]. Clinical tests of muscle function are meant to resemble conditions of daily life and more strenuous activities [
7] and are easy to administer in the clinical setting and in research. High-demand tasks such as hop tests, may not be appropriate, nor replicate daily activities, for less physically active individuals. The single-limb mini squat may be more appropriate as it resembles conditions of daily life, such as stair descent.
A lower number of single-limb mini squats in 30 seconds indicate poorer function [
8]. However, the quality of movement during functional tasks may also be important, and may encompass an aspect not reflected by tasks measured in distance, height or frequency [
9].
One component of movement quality is postural orientation. This involves the ability to maintain an appropriate relationship between the body segments when performing a dynamic task [
10]. At the knee, the medio-lateral position relative to the ankle joint during functional activity involving hip and knee flexion is thought to indicate movement quality. A knee-medial-to-foot position, i.e., when the knee is not aligned over the ankle in the frontal plane, is related to an increased risk of anterior cruciate ligament (ACL) injury [
11‐
15], is more common in individuals with ACL injury or patellofemoral pain syndrome (PFPS) than in non-injured controls [
12,
16‐
18], and is related to worse patient-reported function after knee injury [
9]. Moreover, preventing a medial position of the knee is suggested to reduce the risk of ACL injuries [
18‐
21] and forms an integral component of ACL rehabilitation through neuromuscular training interventions. Therefore, a knee-medial-to-foot position is deemed inappropriate (less optimal), indicating poor postural orientation. A knee-over-foot position, i.e., when the joints in the lower extremity are well aligned, is considered appropriate (optimal), indicating good postural orientation.
The medio-lateral knee motion can be measured quantitatively with modern motion analysis technology. However, valid and reliable observational clinical tests that can be used in large groups of people are needed. The reliability of visual inspection of the medio-lateral knee motion has been tested in clinical tests such as drop-jump landings [
21,
22], single-limb squats [
23], and lateral step downs [
23]. In observational tests, the knee-medial-to-foot position is thought to reflect "knee valgus" or "valgus collapse" [
22‐
24]. The validity of such tests, in terms of the lower limb motion that determines the appearance of a knee with and without a medial position in relation to the foot, has not been established.
The aim of this study was to validate an observational clinical test; the single leg mini-squat, for assessing the position of the knee in relation to the ankle joint. This was done by comparing the two- and three-dimensional biomechanics of the lower limb between people who perform the test with a knee-medial-to foot position and those with a knee-over-foot position. In addition, the inter-rater reliability of the clinical test was assessed.
Discussion
The frontal plane 2-D data indicate that in subjects scored as having a knee-medial-to-foot position during the single-limb mini squat their knee was more medially positioned relative to their hip and ankle, resulting in more 2-D knee valgus than those with a knee-over-foot position. In 3-D, the hip was more internally rotated in subjects with a knee-medial-to-foot position than in those with a knee-over-foot position, but there was no difference between the groups in knee valgus angle. High inter-rater reliability was found for the observational test. These results suggest that the test provides a valid and reliable clinical method to delineate between those with knee-over-foot and knee-medial-to-foot positioning during a single limb mini-squat.
The subjects with a knee-medial-to-foot position displayed a knee valgus angle in 2-D nearly 7 degrees greater than those with a knee-over-foot position. A knee valgus position in 2-D, also called frontal plane knee valgus, has been observed in video analysis studies, assessed by visual inspection [
14] or using a digital measuring tool [
15]. It is likely that other movements of the lower limb contribute to a frontal plane knee valgus position during movement [
24]. This was confirmed in the present study, where the knee valgus position in 2-D was accompanied by a more medially placed tibia and thigh in 2-D, but a greater internal hip rotation in 3-D in those with a knee-medial-to-foot position. It was suggested that the 2-D approach could be used to screen for and evaluate excessive knee valgus [
12,
30,
31]. Because the medio-lateral knee motion assessed by visual inspection during the single-limb mini squat was valid in 2-D, the clinical test may be used as proxy.
The actual movement (in 3-D) for the knee-medial-to-foot position was a greater internal rotation of the hip (about 11 degrees) compared with the knee-over-foot position (about 5 degrees). However, there was no difference between the groups in knee valgus angle in 3-D (mean difference 1.1 degrees). In other words, the appearance of a knee-medial-to-foot position is mainly exhibited as increased internal hip rotation. Thus, a frontal plane knee valgus may not be representative of knee valgus in 3-D.
Greater internal hip rotation has been seen in subjects with patellofemoral pain syndrome compared with controls [
32,
33]. Our results showed increased internal hip rotation along with greater frontal plane knee valgus. A greater knee valgus movement in 3-D has been reported during functional tests [
12,
31,
34]. In these studies, more strenuous tasks were used [
12,
31,
34], possibly creating a greater demand on the hip stabilizing musculature and, thus, stressing knee valgus movement more than the single-limb mini squat.
A ROC curve was used to assess whether the observational test could discriminate between those with and without a medial knee position. An area under the curve close to 0.5 indicates a poor test, and a value close to 1.0 indicates a good test. The area under the curve for knee valgus in 2-D was reasonably close to 1.0, denoting that the test can discriminate between those with and without a medial knee position.
It has been suggested that the knee-medial-to-foot position is due to poor sensorimotor control. This has been reported, e.g., as a relation between greater internal hip rotation and hip abductor weakness [
32,
33], and differences in muscle activation patterns of the lower limb and trunk in those with greater compared with smaller knee valgus in 2-D [
34]. 2-D valgus anatomical alignment of the knee, measured in standing, was not related to dynamic 2-D knee valgus during a single-limb squat [
35], indicating that knee valgus measured statically cannot be used to predict knee valgus during movement. The relative contribution of valgus anatomical alignment, and sensorimotor control that determine a knee-medial-to-foot position during the single-limb mini squat, are subject for further study.
The utility of any assessment tool depends on its validity and reliability. Agreement was good [
29], and there was no systematic bias, indicating that visual analysis of the medio-lateral knee motion during single-limb mini squat is reliable between raters. Other studies have failed to report high agreement between observers [
23,
36]. Possible reasons for this are vague guidelines, and that more than two scoring categories were used [
23,
36]. The importance of clear and simple standardizations, and adequate rater training, has been highlighted [
22,
37]. The examiners in the present study received explicit guidelines and thorough training prior to study start, likely contributing to the achieved high reliability. The high reliability also indicates that previous experience of the clinical test is not a necessity for obtaining consistency in measurements.
We have validated a clinical test of assessing the quality of movement by visual analysis. The test resembles conditions of daily life, is easy to administer in the clinical setting and in research, requires no expensive or advanced equipment, and seems to have adequate standardization contributing to high reliability. It also enables the examiner to give immediate feedback to the person being assessed. Further studies may reveal whether the single-limb mini squat can be used as a simple clinical test for screening and evaluation of medio-lateral knee motion in those with or at high risk of knee injury and knee osteoarthritis.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
EA and ER contributed to the conception of the study. All authors contributed to the design of the study, participated in data interpretation, and contributed to manuscript revision. MC, MH, and MS collected the data. MC performed data management. EA performed the data analysis, was in charge of data interpretation, and drafted the manuscript. All authors read and approved the final version.