Age-related difference on weight transfer during unconstrained standing Janina M. Prado a , Mauro C.M. Dinato b , Marcos Duarte a, * a Neuroscience Program and School of Physical Education and Sport, University of Sa˜o Paulo, Sa˜o Paulo, Brazil b Instituto Vita, Sa˜o Paulo, Brazil 1. Introduction When people stand naturally during daily activities, they are usually not forced to stand as still as possible and they stand in an unconstrained manner. During such unconstrained standing for a prolonged period, besides a continuous low-amplitude and slow sway of the body (which is similar to what is observed during standing still), one can also observe postural changes characterized by voluntary fast and gross body movements [1–3]. Although the exact reasons for producing postural changes are not clear, postural changes are sought, mainly in an ergonomic perspective, as a response to avoid discomfort and complications caused by the continuous pressure of a static posture [1]. During unconstrained standing, Duarte et al. [1,2] detected and classified postural changes measuring the center of pressure of subjects while standing on a force plate and found that young adults tended to produce about two postural changes per minute during uncon- strained standing [1]. Freitas et al. [4] employed this paradigm and discovered that elderly individuals produced fewer large-amplitude postural changes compared to young adults. They observed that elderly individuals tended to maintain the same posture and did not shift from one posture to another during unconstrained standing as often the young adults did. A possible underlying mechanism for this altered behavior is that elderly individuals produce fewer weight transfers from one lower limb to the other during unconstrained standing. By weight transfer, we mean the act of transferring partially or completely the supporting forces acting on one lower limb to the other lower limb during a task. The affect of aging on weight transfer is not only important for understanding unconstrained standing but also for mobility because it is essential for the initiation and termination of any voluntary whole-body movement. Several studies have shown that the ability to transfer weight deteriorates with age during tasks such as gait initiation, sit-to-stand, and standing [5–8]. However, little is known about how weight transfer (specifically large-amplitude weight transfers such as observed during whole- body movement) during natural standing is affected by age. Our hypothesis is that elderly individuals perform fewer weight transfers of large amplitude compared to young adults during unconstrained standing. In turn, it is well known that there is a decrease in mobility (defined as the general ability to move) with ageing [9,10]. Hence, a decrease in the number of weight transfers Gait & Posture 33 (2011) 93–97 ARTICLE INFO Article history: Received 18 January 2010 Received in revised form 18 August 2010 Accepted 4 October 2010 Keywords: Aging Balance Posture Movement Natural standing ABSTRACT The ability to transfer weight from one lower limb to the other is essential for the execution of daily life activities and little is known about how weight transfer during unconstrained natural standing is affected by age. This study examined the weight transfer ability of elderly individuals during unconstrained standing (for 30 min) in comparison to young adults. The subjects (19 healthy elderly adults, range 65–80 years, and 19 healthy young adults, range 18–30 years) stood with each foot on a separate force plate and were allowed to change their posture freely at any time. The limits of stability and base of support width during standing, measures of mobility (using the timed up and go and the preferred walking speed tests), and fear of falling were also measured. In comparison to the young adults, during unconstrained standing the elderly adults produced four times fewer weight transfers of large amplitude (greater than half of their body weight). The limits of stability and base of support width were significantly smaller for the elderly adults but there were no significant differences in the measures of mobility and in the fear of falling score compared to young adults. The observed significant age-related decrease in the use of weight transfer during unconstrained standing, despite any difference in the measured mobility of the subjects, suggests that this task reveals unnoticed and subtle differences in postural control, which may help to better understand age related impairments in balance that the elderly population experiences. ß 2010 Elsevier B.V. All rights reserved. * Corresponding author at: Escola de Educac ¸a ˜o Fı ´sica e Esporte, Universidade de Sa ˜o Paulo, Av. Prof. Mello de Moraes, 65, Sa ˜o Paulo/SP, 05508-030, Brazil. Tel.: +55 11 30918735. E-mail address: mduarte@usp.br (M. Duarte). URL: http://www.lob.iv.org.br/ Contents lists available at ScienceDirect Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost 0966-6362/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2010.10.003