Calculations of mechanisms for balance control during narrow and single-leg standing in fit older adults: A reliability study A.C. A ˚ berg a,b, *, A. Thorstensson a , O. Tarassova a , K. Halvorsen c,d a The Swedish School of Sport and Health Sciences, Stockholm, Sweden b Department of Public Health and Caring Sciences/Geriatrics, Uppsala, Sweden c School of Technology and Health, KTH-Royal Institute of Technology, Stockholm, Sweden d Department of Information Technology, Uppsala University, Uppsala, Sweden 1. Introduction Balance control in upright positions is a prerequisite for independence in activities of daily living (ADL) [1]. Additionally, the definition of human balance, being the dynamics of postural control to prevent falling [2], indicates that balance performance is necessary to avoid such incidents [3]. Correspondingly, balance impairment has been identified as one of the most reliable predictors of falls among the elderly [4]. Though, due to its complex nature, balance is difficult to assess [5]. In clinical practice, different categories of tests are commonly used to screen for balance problems [6]; for example (I) Timed tests such as standing balance [7] or timed up-and-go (TUG) [8], (II) Reaching tests such as functional reach [9], (III) Stepping tests [10], or (IV) Ordinal scales, combining different balance tasks, for example Berg’s balance scale [11]. It has, however, been indicated that the theoretical constructs behind these measures are insufficiently described [6] and that the understanding of balance in clinical environments is largely intuitive. This, however, may lead to a less systematic approach in balance assessment, documentation and intervention [2]. ‘Knowledge translation’ in this area, including theoretical models that are useful for clinical reasoning and practice is, therefore needed. Based on the equations of motion, it is possible to discriminate two balance mechanisms for standing situations when no extra external force (apart from gravity) is applied [12]. The first of these mechanisms (M 1 ) implies moving the Centre of Pressure (CoP) with respect to the Centre of Mass (CoM), to drive it within safe boundaries. This mechanism is commonly described by the inverted pendulum model (IP) for human balance [13], where the body is modelled as a single segment or a ‘stick’ standing on the ground. The second mechanism (M 2 ) identified as ‘counter rotation’ [12] or ‘segment acceleration’ [14] implies that parts of the body are rotated around the CoM, thus causing horizontal reaction forces to accelerate the CoM in Gait & Posture 34 (2011) 352–357 A R T I C L E I N F O Article history: Received 7 December 2010 Received in revised form 9 May 2011 Accepted 31 May 2011 Keywords: Balance Elderly Assessment Reliability A B S T R A C T For older people balance control in standing is critical for performance of activities of daily living without falling. The aims were to investigate reliability of quantification of the usage of the two balance mechanisms M 1 ‘moving the centre of pressure’ and M 2 ‘segment acceleration’ and also to compare calculation methods based on a combination of kinetic (K) and kinematic (Km) data, (K–Km), or Km data only concerning M 2 . For this purpose nine physically fit persons aged 70–78 years were tested in narrow and single-leg standing. Data were collected by a 7-camera motion capture system and two force plates. Repeated measure ANOVA and Tukey’s post hoc tests were used to detect differences between the standing tasks. Reliability was estimated by ICCs, standard error of measurement including its 95% CI, and minimal detectable change, whereas Pearson’s correlation coefficient was used to investigate agreement between the two calculation methods. The results indicated that for the tasks investigated, M 1 and M 2 can be measured with acceptable inter- and intrasession reliability, and that both Km and K–Km based calculations may be useful for M 2 , although Km data may give slightly lower values. The proportional M 1 :M 2 usage was approximately 9:1, in both anterio-posterior (AP) and medio-lateral (ML) directions for narrow standing, and about 2:1 in the AP and of 1:2 in the ML direction in single-leg standing, respectively. In conclusion, the tested measurements and calculations appear to constitute a reliable way of quantifying one important aspect of balance capacity in fit older people. ß 2011 Elsevier B.V. All rights reserved. * Corresponding author at: Uppsala University, Department of Public Health and Caring Sciences/Geriatrics, Uppsala Science Park, SE-751 85 Uppsala, Sweden. Tel.: +46 18 6117960. E-mail addresses: anna.cristina.aberg@gih.se, anna.cristina.aberg@pubcare.uu.se (A.C. A ˚ berg). Contents lists available at ScienceDirect Gait & Posture jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/g aitp os t 0966-6362/$ – see front matter ß 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2011.05.025