# Characteristics of Postural Muscle Activity in Response to A Motor-Motor Task in Elderly
## 1 Introduction  
Aging-related changes in postural control are associated with balance maintenance regardless of different types of perturbations. In response to perturbations, the central nervous system (CNS) reacts to fast, directional arm movement or reaching by employing feedforward as anticipatory [1; 2; 3] and feedback mechanisms as compensatory postural adjustments [4; 5] to maintain and restore equilibrium. Older adults accommodate both anticipatory and compensatory postural adjustments related to the perturbations compared to young adults [6; 7; 8; 9; 10; 11]. Specifically, for a perturbation from upper limb movement, such as pushing an object, older adults utilize a less efficient strategy such as muscle co-activation [12]. Alternatively, for a translation perturbation from lower limb induced by standing on an unstable or moving surface, older adults employ corresponding postural adjustments with increasing magnitudes of muscle activities and center of pressure displacement [13; 14].  
Effects of a single perturbation on postural control have been widely studied and well documented. Meanwhile, studies of aging effects on dual-tasking commonly focus on combinations of a cognitive task with a motor task. These studies have shown declined performance in the elderly during thecombination of cognitive and motor tasks [15; 16; 17]. When dual-tasking involved walking with a visual cue, older adults showed more difficulties in making corrective step adjustments [18]. However, activities of daily living involve multiple perturbations to balance, such as walking while holding the cellphone, or pushing a shopping cart while walking around in the supermarket. Both activities engage upper and lower limb movements involved in two motor tasks.  
Postural control in response to perturbations from upper and lower limb movements simultaneously, such as pushing an object while standing on the sliding board, has been studied in young adults [19]. Our previous findings indicated that when the surface was movable, the onset times of tibialis anterior and rectus femoris were delayed and the magnitudes of muscle activation were decreased. In addition, the ventral muscles (tibialis anterior, rectus femoris, and rectus abdominis) and dorsal muscles (medial gastrocnemius, biceps femoris, and erector spinae) initiated before and after the pushing movement respectively, which suggested that the reciprocal muscle activation pattern was utilized. It also revealed that the CNS of young adults handling these dual-motor tasks as prioritizing upper limb perturbation, pushing movement [19; 20] or griping reactions [21], along with maintaining vertical posture. Subsequently, their postural control responded to lower limb perturbations, such as sliding translation [19; 20; 21].  
For young adults, the CNS prioritizes motor tasks over postural maintenance because their balance is not in imminent danger during the task performance [22; 23]. Contrarily, the CNS prioritizes postural maintenance over motor tasks when one or two tasks involve postural control [24] or threat to balance [25]. However, the organization of postural control is not well understood when older adults perform a dual motor-motor task rather than a cognitive-motor task. Thus, the objective of the present study was to investigate how older adults handle a combination of upper limb activity and a translational perturbation, and how that affect characteristics of muscular strategies used in balance maintenance and restoration. The experimental paradigm involved two body perturbations: the upper extremities performing the pushing of a cart, and the translation perturbation of standing on the sliding board. Hence, we hypothesized that the onset time of muscle activities would be affected by the presence of these perturbations. Additionally, the second hypothesis was that older adults would utilize the strategy of postural muscle co-contraction to maintain vertical posture when exposed to upper and lower limb perturbations.