The single leg squat in clinical testing : aspects of reliability, validity, and associated factors

Background: The Single Leg Squat (SLS) test is a functional test widely used in clinical settings to examine and evaluate rehabilitation goals. Research indicates that the SLS is reliable when the knee relative to the foot is dichotomously assessed. However, the assessment of functional movements often comprises more complex analyses of the whole kinetic chain with several body segments which highlights the need to develop and test a standardised multi-segmental SLS. Movement quality is an important aspect when using prevention programs in the clinical context, and the SLS can be used for this purpose. As knee injuries are common among athletes and especially among female soccer players, further investigation of the SLS in this population is warranted. Overall, the development of both quantitative and qualitative measurements needs to be studied to improve clinical testing. Clinically, portable marker-less motion capture (MMC) systems are suggested to be an adequate substitute for a three-dimensional analysis system, and one such novel MMC system is the QinematicTM. Before a test such as the SLS, or any other new measurement instrument, can be used in clinical settings, it is important to explore its measurement properties. Aim: The overall aim of this thesis was to develop and assess aspects of reliability and validity of the SLS among physically active people, and from a biopsychosocial perspective investigate factors associated with the SLS in a sample of female soccer players. Methods: Study I was a systematic review and meta-analysis that investigated the current literature regarding the intra- and inter-rater reliability of visually assessed SLS, including the Forward Step-Down (FSD) and Lateral Step-Down (LSD) tests. Study II was a laboratory-based test-retest reliability and validity study of a three-dimensional MMC system, the QinematicTM. Study III was an intra- and inter-rater reliability study of a standardised multi-segmental SLS developed from the findings in Study I. Study IV was a cross-sectional observational study using linear regression models to explore if demographic and biopsychosocial factors associated with the outcome of the SLS, assessed as a total score for all segments and as a separate knee segment in a sample of elite and sub-elite female soccer players. An additional analysis investigated the possibility of the SLS to discriminate injured soccer players from non-injured players. Results: In Study I, the pooled results of ICC/