Reproducibility of a new signal processing technique to assess joint sway during 1 standing 2

20 Postural control strategies can be investigated by kinematic analysis of joint 21 movements. However, current research is focussing mainly on the analysis of centre of 22 pressure excursion and lacks consensus on how to assess joint movement during postural 23 control tasks. This study introduces a new signal processing technique to comprehensively 24 quantify joint sway during standing and evaluates its reproducibility. Fifteen patients with 25 non-specific low back pain and ten asymptomatic participants performed three repetitions of a 26 60-second standing task on foam surface. This procedure was repeated on a second day. 27 Lumbar spine movement was recorded using an inertial measurement system. The signal was 28 temporally divided into six sections. Two outcome variables (mean absolute sway and sways 29 per second) were calculated for each section. The reproducibility of single and averaged 30 measurements was quantified with linear mixed-effects models and the generalizability 31 theory. A single measurement of ten seconds duration revealed reliability coefficients of . 75 32 for mean absolute sway and . 76 for sways per second. Averaging a measurement of 40 33 seconds duration on two different days revealed reliability coefficients higher than . 90 for 34 both outcome variables. The outcome variables’ reliability compares favourably to previously 35 published results using different signal processing techniques or centre of pressure excursion. 36 The introduced signal processing technique with two outcome variables to quantify joint sway 37 during standing proved to be a highly reliable method. Since different populations, tasks or 38 measurement tools could influence reproducibility, further investigation in other settings is 39 still necessary. Nevertheless, the presented method has been shown to be highly promising. 40 Introduction 41 Postural control is defined as the ability to keep or regain a specific posture, such as 42 standing (Pollock et al., 2000). Commonly, this ability is quantified by centre of pressure 43 excursion (Mazaheri et al., 2013). Postural control strategies are described as a feedback 44 mechanism derived by the interaction of sensory input and adapted motor output (Hodges, 45 2004). Centre of pressure excursion represents whole body movement and does not 46 differentiate between joints. Kinematic measures of joint sway would give more insight into 47 postural control strategies. Joint sway was previously assessed by the standard deviation of 48 angular displacement (Mientjes and Frank, 1999). Standard deviation is one measure of sway 49 but quantifies only its amplitude. This study introduces a new signal processing technique 50 with two outcome variables to comprehensively quantify joint sway, including amplitude and 51 frequency. The technique and its clinical application are demonstrated at the lumbar spine 52 with both, patients suffering from low back pain and asymptomatic participants. Since 53 filtering is a major issue in movement analysis, this study presents a new approach to finding 54 an optimal filter, evaluating the reproducibility of the outcome variables, and recommending a 55 reliable measurement protocol. 56 Methods 57 Participants 58 Fifteen adult patients with non-specific low back pain for longer than four weeks and 59 ten asymptomatic, adult participants were recruited for this study. A detailed description of 60 the recruitment procedures, as well as inclusion and exclusion criteria, is provided elsewhere 61 (Schelldorfer et al., 2015). The study was approved by the local ethics committee. All 62 participants signed informed consent prior to the study. 63