The minimal displacement principle

  1. Abstract
    The ‘minimal displacement’ principle (MDP) holds that obstacles are avoided with minimal foot displacement during walking. According to the MDP, obstacles are avoided using either a long stride strategy (LSS; extending the stride and placing the foot over the obstacle) or short stride strategy (SSS; shortening the stride placing the foot before of the obstacle), depending on the obstacle location relative to the planned stepping location. However, it is still unknown whether obstacle avoidance is performed according to the MDP and whether or not the MDP is influenced by age and attentional interference. This study examined the effect of age and attentional interference on stride adjustment strategies (SAS; LSS or SSS) when obstacles were suddenly presented during walking at various obstacle locations relative to the predicted foot placement position (pFP; that was predicted to occur in the absence of a stride adjustment). Fourteen healthy young (24.3 ?? 2.0 years) and thirteen healthy elderly adults (65.3 ?? 3.4 years) participated. The experiment was performed using the Interactive Walkway (IWW) that projected visual obstacles on the walkway and recorded the participant’s movements. Two conditions (with and without attentional interference, as induced by counting backwards) were performed, both in which obstacles were suddenly presented at seven locations relative to pFP (one per trial). No differences in success rates and secondary-task performance were found between groups and conditions. ??2-tests revealed that LSS was the preferred strategy, suggesting that during walking obstacles are not always avoided according to the MDP. Age and attentional interference both negatively affected adherence to MDP, showing an increased likelihood for performing LSS when SSS was expected. Future research should focus on other factors than MDP to unveil what underlies the choice of a particular SAS for obstacle avoidance during walking.
    Keywords: minimal displacement, obstacle avoidance, dual-task, ageing, walking
    1. Introduction
    The goal of fall prevention programs for people who are prone to falling (e.g. due to ageing, previous falls, a stroke, Alzheimer’s or Parkinson’s disease) is to improve their walking abilities and to decrease their fall risk. The latter may benefit from training obstacle avoidance skills, for example by practicing daily life situations (e.g. crossing a door step) or by practicing obstacle-avoidance tasks (Weerdesteyn et al., 2006, 2008; Van Ooijen et al., submitted). Previous studies found that tripping over obstacles during walking in both young and elderly adults is dependent on the available response time (time from obstacle appearance until time of predicted foot placement; ART). Less time to respond has been demonstrated to result in higher obstacle avoidance failures (Chen et al., 1994; Weerdesteyn et al., 2005). When an obstacle has to be avoided, one can choose between two strategies: the long stride strategy (LSS) or short stride strategy (SSS) (Chen et al., 1994). The LSS means making one’s stride larger than in a situation without the obstacle. Making a shorter stride and crossing the obstacle with the contralateral leg is called the SSS. Previous studies showed that LSS is the favored strategy during walking (Chen et al., 1994; Moraes et al., 2004; Weerdesteyn et al., 2004, 2005; Roerdink et al., 2009; Bank et al., 2011). Overall, however, elderly adults were less successful in crossing obstacles when using LSS than young adults (Chen et al., 1994; Weerdesteyn et al., 2005).
    The studies of Chen et al. (1994) and Weerdesteyn et al. (2005) investigated the influence of different ARTs on the stride adjustment strategies (SAS), but their experimental approaches were different (see intermezzo). Weerdesteyn et al. (2005) suggested that the avoidance strategy of young adults is more in line with the ‘minimal displacement’ principle (MDP; Patla et al., 1999). According to the MDP, the foot is minimally displaced when avoiding an obstacle, which means that an LSS is expected when the obstacle is located closer by the person’s predicted foot placement (i.e. in absence of an obstacle; pFP), and SSS when the obstacle is located farther away than pFP. However, for each age group Weerdesteyn et al. (2005) applied the MDP to the performed strategies of all trials of all ART conditions, thereby varying obstacle locations with respect to pFP (due to their experimental approach), without taking the ART and obstacle locations into account. Because conclusions in terms of MDP critically depend on the relation between pFP and actual obstacle location, the interpretation of SAS-results in this respect is questionable. Therefore, it is important to clarify whether SAS are in line with the MDP, taking both ART and obstacle location with respect to pFP into account.
    It is of great interest to determine the SAS in elderly because most falls in elderly occur during walking (Markle-Reid et al. 2010) and 25% of the falls are due to tripping over an obstacle in the environment (Tinetti et al., 1988). A factor that increases the risk of falling is an impairment in avoiding obstacles while walking under dual-task conditions, which is suggested to be due to a decreased attentional capacity and difficulties in switching attention (Bock, 2008; Siu et al., 2008; Chen et al., 1996). A secondary task is often implemented to increase the attentional demand, and is comparable to daily life situations, e.g. talking on the phone while walking. Bock (2008) and Beurskens & Bock (2013) showed that walking and avoiding obstacles, while performing a visual secondary task resulted in a significant decrease in dual-task performance compared to situations in which other, non-visual secondary tasks were to be performed. Nonetheless, other studies that investigated the effects of non-visual secondary tasks during walking and/or avoiding obstacles found decreased dual-task performances, especially in elderly and patient groups (Chen et al., 1996; Kim & Brunt, 2007; Hausdorff et al., 2008; Hegeman et al., 2012; Smulders et al., 2012). Thus, identifying the effects of a secondary task on the SAS may provide additional information about the obstacle-avoidance skills.
    The current study aimed to examine which SAS are used when obstacles are suddenly presented with a fixed ART (0.5 s) at varying obstacle locations with respect to pFP, and whether these responses differ when walking with a simultaneous secondary cognitive task (counting backwards in steps of three; dual-task condition) or without (single-task condition). Both young and elderly adults were included in the experiment to get more insight into age differences in SAS, since elderly have been shown to (1) use the avoidance strategies in a different manner (Chen et al., 1994; Weerdesteyn et al., 2005) and (2) have lower dual-task performance than young adults (Chen et al., 1996; Kim & Brunt, 2007). Virtual obstacles were presented at different locations (viz., multiple locations at shorter and longer distances than pFP position) to determine the influence of obstacle location on SAS and avoidance success rates. A maximum of one obstacle was presented per trial. The type of response (LSS/SSS), success rate, and secondary-task performance were determined.
    To this end, the Interactive Walkway (IWW) was used. This is a new set-up that measures 3D-human-kinematics with Kinect’ cameras (Microsoft) during overground walking. Thus it allows participants to adapt their gait speed and walk as naturally as possible. This is an advantage over treadmill walking. Nagano et al. (2013) found that during treadmill walking especially elderly have increased gait variability compared to ove

Posted in Uncategorized