THE POSTERIOR SHIFT ANTICIPATORY POSTURAL ADJUSTMENT IN CHOICE REACTION STEP INITIATION
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2014-03
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Falls are a major source of injury and death among the elderly population (Melzer, 2014, St George, 2007, Cohen, 2011, Sparto, 2013, Sparto, 2014). Rapid postural adjustment to restore center of mass (COM) equilibrium after loss of balance (slip, trips, etc) is critical for the prevention of falls (St George, 2007, Cohen, 2011, Melzer, 2007, Melzer, 2007). The most common approach for compensation of a postural perturbation is to initiate a step to increase the base of support as well as decelerate the speed of COM movement (Mickelborough, van der Linden, Tallis, & Ennos, 2004). The speed of voluntary step initiation has been shown as an important predictor for fall detection among the elderly population (Lord & Fitzpatrick, 2001; Itshak Melzer, Shtilman, Rosenblatt, & Oddsson, 2007; Itshak Melzer et al., 2007; St George, Ritzpatrick, Rogers, & Lord, 2007). To minimize the risk of falling, the cognitive processing, weight transfer, and locomotion components of a step must be timed and executed appropriately (Rogers, Kukulka, Brunt, Cain, & Hanke, 2001; St George et al., 2007; Uemura, Oya, & Uchiyama, 2013). Thus, the aforementioned components during step initiation were studied to investigate the cognitive processing to prepare and execute a step.
Step initiation can be divided into 5 stages (see Fig. 1): 1) the RT lasts from the stimulus delivery to the onset of center of pressure (COP) deviation; 2) the release phase lasts from the COP onset until the furthest point of postero-lateral COP (MaxCOP); 3) the unloading phase lasts from the MaxCOP to the swing foot toe off the ground (SWTO); 4) the single support phase (SSP) or swing phase lasts from the SWTO to the swing foot initial contact (SWIC); 5) the double support phase (DSP) lasts from the SWIC to the stance foot toe off ground (STTO). Stages 2 and 3 combined are defined as the APA phase, while stages 4 and 5 combined are defined as the stepping phase (Cohen, Nutt, & Horak, 2011; Dalton, Bishop, Tillman, & Hass, 2011; MacKinnon et al., 2007; I Melzer, Kurz, Shahar, Levi, & Oddsson, 2007; Itshak Melzer & Oddsson, 2004; Itshak Melzer et al., 2007; Mickelborough et al., 2004; Sparto et al., 2013; Uemura et al., 2013).
Previous studies have identified Anticipatory Postural Adjustment (APA) as a marker of preparatory movement during step initiation (Cohen et al., 2011; Uemura et al., 2013). The APA is defined as lateral weight transfer towards the swing foot preceding a step. It is believed to stabilize posture and generate the initial momentum needed to begin walking (Dalton et al., 2011). Multiple studies report that errors (Cohen et al., 2011; Sparto et al., 2013; Uemura et al., 2013) in the initial weight transfer, which is defined as APA error, lead to slow choice step execution due to additional time for error correction before a step can safely take place.
The present study examined the effects of choice on the temporal characteristic of step initiation. The study also aimed to investigate the impact of APA on step characteristics. Previous studies on APA error were limited to a binary choice situation in which APAs were either correct or incorrect based on the shift direction in weight transfer (Cohen et al., 2011; Sparto et al., 2013; Sparto, Jennings, Furman, & Redfern, 2014; Uemura et al., 2013). According to decision theory (Busemeyer & Townsend, 1993; Roe, Busemeyer, & Townsend, 2001; Simonson, 1989), human behavior tends to reflect a compromise effect in which subjects add an intermediate alternative that lies between the two competing extreme options in the original choice set. Therefore, we hypothesized that by analyzing the COP trajectory, we could identify the compromise effect during the reaction and APA phase of step initiation.
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