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 Qinematic™. 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 Qinematic™. 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/kappa showed a “moderate” agreement for inter-rater reliability and a “substantial” agreement for intra-rater reliability of the SLS, including the FSD and LSD. In Study II, the Qinematic™ showed “substantial relative reliability” but “poor absolute reliability”. Regarding validity, a “moderate” agreement between the visual assessment and Qinematic™ data for various knee angles was shown and the best discriminative ability of the SLS was found at a knee angle of 6°. In Study III the proposed multi-segmental SLS showed a “moderate” inter-rater reliability and an “almost perfect” intra-rater reliability. In Study IV, the outcome of the SLS was associated with previous injuries and various demographic-, biomechanical- and psychosocial factors depending on the tested leg. The total score associated with hip strength for both the dominant and the non-dominant leg, and the knee segment associated with division inherency for both the dominant and non-dominant leg. The additional analysis showed that the SLS was not able to discriminate between players with and without previous or present injuries.

Conclusion: The SLS seems to be a reliable and clinically useful multi-segmental test of movement quality in contrast to the Qinematic™ system. The SLS was, in a sample of female elite and sub-elite soccer players, associated with a variety of biopsychosocial factors when assessed as a total score or as a separate knee segment. The results imply that several factors need to be considered when assessing the SLS among female soccer players such as leg dominance, division inherency, hip strength, and psychosocial factors.

List of scientific papers

I. Ressman J, Grooten WJA, Rasmussen Barr E. Visual assessment of movement quality in the single leg squat test: a review and meta analysis of inter-rater and intrarater reliability. BMJ Open Sport Exerc Med. 2019. 5(1):e000541.
https://doi.org/10.1136/bmjsem-2019-000541

II. Ressman J, Rasmussen-Barr E, Grooten WJA. Reliability and validity of a novel Kinect-based software program for measuring a single leg squat. BMC Sports Sci Med Rehabil. 2020. 12:31.
https://doi.org/10.1186/s13102-020-00179-8

III. Ressman J, Grooten WJA, Rasmussen-Barr E. Visual assessment of movement quality: a study on intra- and interrater reliability of a multi-segmental single leg squat test. BMC Sports Sci Med Rehabil. 2021. 13(1):66.
https://doi.org/10.1186/s13102-021-00289-x

IV. Ressman J, Von Rosen P, Grooten WJA, Rasmussen-Barr E. Factors associated with the single leg squat in female soccer players: a cross-sectional study. [Submitted]