Kinematic predictors of single-leg squat performance: a comparison of experienced physiotherapists and student physiotherapists

The single-leg squat (SLS) is a functional clinical test used by physiotherapists in the musculoskeletal assessment of the lower limb. Despite ubiquity of use, little is known of the validity and reliability of the test. Specifically, the relationship that physiotherapy assessment bears with kinematic parameters is unclear. Furthermore, the influence of clinician experience on the validity and reliability of SLS assessment is yet to be established. Thus, a comprehensive investigation of SLS kinematics in light of experienced and inexperienced physiotherapist assessment is warranted to justify the utility of the test.

There is some evidence that kinematic patterns, particularly at the knee, are characteristic of musculoskeletal conditions or related to lower limb injury risk [1–3]. An increase in knee valgus angle during functional tasks, for example, is recognised as an important risk factor for non-contact anterior cruciate ligament injury [1, 4]. Similarly, increased knee valgus angle during weight-bearing tasks is capable of separating individuals with and without patellofemoral pain syndrome [2]. Indeed, dynamic knee valgus observed in the frontal plane during common athletic screening tasks is a reliable method of identifying those at high risk of knee injury [5]. Others have identified relationships between excessive knee frontal plane motion and hip muscle weakness [6]. For clinicians, the application of these findings to practice relies on an ability to recognise abnormal movement patterns through visual observation during the clinical exam.

The SLS test is touted to be a simple method of identifying clinically abnormal movement patterns to assist clinicians with screening and diagnosis. As such, a modicum of descriptive reports on the kinematics of the SLS have emerged [6–17], however many of these reports are focused on laboratory measures and their results may not directly translate to the clinical assessment of SLS performance [6, 8, 11, 12, 15, 16]. Several research groups, however, have considered the nexus of laboratory findings and clinical implications by assessing the relationship between lower limb SLS kinematics and the clinical assessment of SLS performance. Ageberg and colleagues [7] found that the degree of knee medio-lateral motion could be reliably determined by musculoskeletal physiotherapists. Further, Crossley and colleagues [10] found that physiotherapists were able to use the SLS test as a reliable tool for identifying hip muscle dysfunction in asymptomatic participants. DiMattia and colleagues [17] however, were ambivalent of the associations between SLS assessment and its relationship with hip joint kinematics and abductor strength [17]. Rating method was investigated by Chmielewski and co-workers [9], who found that the assessment of SLS movement quality using an ‘overall approach’ rather than a ‘specific/segmental approach’ resulted in greater percent agreement between physiotherapists. The impact of experience on clinician reliability and validity of SLS assessment, however, remains unknown.

Though a small body of work is available lending support to the SLS test and its sensitivity to injury identification, most reports are limited to two-dimensional descriptions of kinematics or confined to a single joint. Furthermore, investigations to date have not considered the association between joint kinematics and clinician assessment or the influence of clinical experience. Therefore, the purpose of the present study was to determine kinematic predictors of perceived SLS quality for experienced and student physiotherapists and assess inter- and intra- rater reliability of SLS performance rating for experienced and student physiotherapists. We hypothesised that (i) physiotherapist and student ratings of SLS performance would be predicted by hip and knee joint kinematic variables and (ii) physiotherapist ratings of SLS performance would be more reliable than physiotherapy student ratings.

In establishing the utility of the SLS test in clinical practice, our goals were to (i) determine the lower limb joint kinematic variables that best predicted therapist ratings of SLS performance, (ii) evaluate whether kinematic predictors of SLS performance were influenced by rater experience, and (iii) assess the reliability of the SLS assessment for novice and experienced physiotherapists. We found that peak knee flexion angle was the strongest predictor of SLS performance and that knee joint medio-lateral displacement enhanced the multivariate model prediction of SLS performance for both experienced and student raters. The model prediction was further enhanced by the inclusion of peak hip adduction angle for experienced physiotherapists, indicating that they took into account both hip and knee movement when determining SLS performance. As a result, experienced physiotherapists demonstrated higher inter- and intra- rater reliability than their less experienced counterparts.

Around 70% of all ACL injuries are incurred through a non-contact mechanism [26]. Accumulating evidence suggests that undesirable movement patterns, such as excessive knee valgus, underpin the mechanism and heighten the risk of ACL injury [1, 27]. The SLS test provides an attractive and clinically-efficient means of identifying undesirable movement patterns during screening and rehabilitation. However, before clinicians could apply and interpret the SLS test results with confidence, there was a need to establish the validity of clinical judgement related to the test. Our findings are important as they have established the convergent validity and reliability of the SLS test for both novice and experienced physiotherapists.

Peak knee flexion was the strongest predictor of SLS performance. Unlike others [7, 9, 13, 14], we purposely chose not to standardize the depth of each SLS in an endeavour to reflect a typical clinical scenario. Despite all observations being made from directly in front of each subject, our results highlight the importance placed on adequate knee flexion by clinicians in making their assessment. In fact, knee flexion during the SLS is found to differ between men and women [11], which is supported by our sex-specific kinematic analysis, albeit in a relatively small sample. It appears necessary then that ratings are sensitive to this kinematic parameter. For studies investigating biomechanical differences (e.g. between sexes), however, we recommend standardizing knee flexion during the SLS.

In agreement with our hypothesis both physiotherapists’ and students’ scores were predicted by knee medio-lateral displacement. Not surprisingly, differences in knee motion in the frontal plane have been identified in different patient populations [28, 29]. Indeed, frontal plane angular deviation of the thigh during the SLS is greater for patients with patellofemoral pain syndrome than for normal, healthy individuals [28]. Furthermore, ‘valgus collapse’ appears to be a common injury mechanism for non-contact ACL injury [30, 31] and such observations are corroborated by laboratory-based studies of knee joint kinematics in ACL-injured patients [29]. Our findings demonstrate that physiotherapist and student assessment is capable of detecting medio-lateral position changes at the knee.

In partial agreement with our hypothesis, peak hip adduction was a significant predictor of SLS performance, however only for physiotherapists. In fact, predictors for students were only apparent at the knee. This distinction may be clinically important as significant differences in hip adduction have been reported for patient groups [2]. For example, Willson and Davis [2] report greater peak hip adduction for women with patellofemoral pain syndrome than for normal, healthy women. Interestingly, individuals who adopt a greater ‘dynamic knee valgus’ position during the SLS test, are more likely to exhibit greater hip adduction angles during dynamic activities such as running and jumping [2]. Such patterns of movement have been suggested to place those individuals at greater risk of musculoskeletal injury. Consequently, the ability to detect excessive hip adduction angle during the SLS test may be an important clinical screening skill.

We found that both physiotherapists and physiotherapy students were capable of reliable SLS assessment, although clinical experience enhanced both inter- and intra-rater reliability. The fact that physiotherapy students also demonstrated good within- and between-rater reliability in their assessments supports the robustness of the test itself. This finding is in support of Crossley and colleagues [10] who demonstrated similarity in ratings between experts and novices for the SLS test. Our results suggest that clinicians of all levels of experience can use the SLS test with confidence to identify undesirable movement patterns, at least in generally healthy individuals.

Although our design and testing protocol were intended to closely reflect routine clinical practice, several limitations warrant acknowledgement. Firstly, our sample included only healthy, young individuals comprising a mix of sexes. Exclusion of participants with lower limb pathology may be considered a limitation, though that significant findings were forthcoming highlights the sensitivity of the assessment. As we were primarily interested in the assessment of the SLS rather than a characterisation of the kinematics, reporting sex-specific data was not considered necessary. Secondly, the requirement for multiple raters and repeat measures necessitated the use of videos for rating, which might reduce the authenticity of the clinician-patient interaction. Finally, a relatively short period of time (i.e. two weeks) was used between the first and second ratings; however, our intention was to emulate a reasonable patient follow-up timeframe for maximum clinical applicability.

In conclusion, we found that both physiotherapists and physiotherapy students were capable of reliably rating SLS performance in a cohort of healthy young adults. Further, we found that physiotherapist ratings were more strongly related to three-dimensional kinematics at both the hip and knee joint, as opposed to physiotherapy student ratings which were solely related to knee joint kinematics. In order to better understand the utility of the SLS test in clinical practice (and athletic screening), future work will investigate gender differences and the contribution of lower limb muscle activity in patient populations.