# Effects of a Dual-Task Paradigm and Gait Velocity on Dynamic Gait Stability during Stair Descent
## 1 Introduction  
Stair negotiation is an activity of daily living with the potential for falls. Falls on stairs account for 26% of all self-reported falls [1] and are one of the leading causes of accidental death [2]. Compared with stair ascent and level walking, stair descent accounts for 75% of falls on stairs [3] and demands greater lower-limb joint range of motion [4] and muscle strength [5]. Stair descent imposes significant challenges to dynamic gait stability (DGS) in people of all ages [6]. Dual-tasking is essential to daily living activities, including during stair descent [7]. The simultaneous performance of additional tasks often results in changes in gait patterns that are associated with falls [8]. A dual-task involves performing a physical task (e.g., movement or walking) while performing aconcurrent cognitive (CT) or manual (MT) task. Gait changes and concurrent dual-tasking are strongly associated when the gait task is challenging, like during stair descent [9]. Previous studies revealed different dual-task effects on DGS. Some reported that CT or MT adversely affected stability [10; 11], while others showed that it might help stabilize gait [12; 13]. A possible explanation for the inconsistent results is the different variables used to measure DGS, including center of mass (COM) displacement, center of pressure (COP) displacement, or COM-COP separation. COM displacement represents the degree of body sway in different directions [14]. COP displacement, calculated by the position changes of the point of ground reaction forces [15], represents gait strategy and body stability [16]. COM-COP separation represents the ability during gait to control COM motion from the COP beneath the feet within appropriate limits [17]. Increased COM, COP displacement, and COM-COP separation were all interpreted as a decrease of DGS during stair descent [14; 16; 17]. COM-COP inclination angle (IA), formed by the vertical line and the projection of the vector onto the sagittal and frontal planes [18], was proposed to provide a more comprehensive assessment than the examination of COM or COP [19]. IA can better represent instability than COM or COP measures [19] and better include the influence of leg length or body height than COM-COP horizontal separation [20]. IA considers both COM and COP, thereby providing a useful description of the relationship between the body and base of support [19]. Greater IA leads to an increase in moment arm of force, which further results in an increase in inertia and momentum of the body segments for balance recovery [19]. The greater the IA, the further the COM diverges from the COP, and the more difficult it is to bring the COM above the COP [21]. If the COM and COP become separated extensively and consequently prevent the lower-limb joint moments from supporting upright posture, a fall can occur [22]. IA has been extensively used to measure the DGS during level walking [23; 24], treadmill walking [20; 24], narrow-heeled walking [21], obstacle crossing [23; 25], and golf swing [26], and was claimed as an effective representation of DGS [20]. However, it has rarely been used to examine DGS during one of the most hazardous locations for fall accidents [27], stair descent. Changes in gait velocity caused by a dual-task paradigm may be another reason for the reported DGS differences. Some considered velocity when they tried to confirm the influence of a dual-task on DGS; others did not. Even the ones that considered velocity reported conflicting results. [PERSON] et al. (2017) reported that the velocity increase associated with young adults could cause a significant increase in IA [18], while [PERSON] et al. (2013) considered that gait velocity did not impose a limitation on the interpretation of the IA [21]. Although IA may change with changes in gait velocity and dual-task performance, few studies examined the association among gait velocity, dual-task performance, and IA.