# Characteristics of Postural Muscle Activity in Response to A Motor-Motor Task in Elderly
## 4 Discussion  
The current study investigated how older adults utilize postural control to push a cart when standing on a moveable sliding board (that induced translation perturbations) and the role of the weight of the cart is pushed. All these temporal events occurred prior to cart movement. The onset time of muscles and segment movements were not affected by standing on fixed or sliding board and pushing weight, which did not support our first hypothesis. The magnitudes of muscle activity in ES and TA were significantly higher in the sliding conditions compared to the fixed condition from \(-750\) ms to \(+450\) ms in relation to T0. Furthermore, muscle co-contraction of the three segments were observed through Epoch 1 to Epoch 6. The trunk and shank segments showed significantly higher \(C\) values in the sliding conditions than in the fixed conditions, which confirmed our second hypothesis. The sequence of muscle activation onset was TA\(\rightarrow\)MG\(\rightarrow\)RFBF\(\rightarrow\)ES\(\rightarrow\)RA (Figure 2). The sequence of muscle activation observed in the elderly was very different from that observed in young adults (TA\(\rightarrow\)RF\(\rightarrow\)RA\(\rightarrow\)ESRFMG) performing the same dual-task. When young adults were exposed to upper and lower limb perturbations, they initiated ventral muscles of all three segments, then the dorsal muscles [19]. This sequential activation of distal-to-proximal muscles is usually observed during translation perturbation from the lower limbs [34]. The earlier onset of either ventral or dorsal muscle activation depends on the direction of translation perturbation [20]. The function of this adjustment is to ensure that the body moves as an inverted pendulum to coordinate the reciprocal strategy [19; 34], which was observed in young adults. Although older adults initiated the same distal-to-proximal sequence as young adults, the onsets of ventral and dorsal muscles of the three segments were very close and all occurred before the cart movement (Figure 2). Similar onset of ventral and dorsal muscle activation was associated with co-contraction strategy [35; 36], which was also revealed by the \(C\)-\(R\) analysis in the current study. Only ES and TA were significantly affected by the factor of board and larger magnitudes of muscle activities in the sliding conditions (Figure 3). All other muscles maintained similar magnitudes of muscle activities in all conditions. Hence, co-contraction patterns were observed in all three segments, and the corresponding \(C\) values were higher in the sliding conditions than in the fixed conditions on the trunk and shank segments (Figure 4). Older adults have shown to be utilizing less efficient postural control with the co-contraction strategy when performing the pushing task [12]. Together with the early onset time, co-contraction patterns indicated that older adults not only prepared for the pushing task  
Figure 4: \(C\) values of the trunk, thigh, and shank segments for sliding conditions (white) and fixed conditions (black) are shown for each epoch. \({}^{*}\) represents statistical significance (\(p<0.05\)) of the factor of sliding board and \({}^{**}\) for \(p<0.001\). but also enhanced the segment stiffness in preparation for potential translation perturbation [12; 37; 38]. Furthermore, the co-contraction strategy has been associated with increasing the available time to overcome the forthcoming perturbations [38; 39] and for further motor responses [40]. Therefore, it might explain that older adults utilized co-contraction strategy much earlier than anticipatory postural adjustment attended to earn more time for postural stabilization. Additionally, the CNS increased significant muscle activation for the lower limb perturbation in the sliding condition. In the current study, the trunk and board movements were observed with the comparable period in the sliding condition (Figure 2), which was different from young adults who coordinate shank segment for the moving surface while pushing [19].