# Kinematic Analysis of Dynamic Coactivation During Arm Swing at the Shoulder and Elbow Joints
## 4 Discussion
The observed modulation of arm movement patterns with increasing walking speed supports the use of impedance control models, which can flexibly modulate assistance by interpreting user intent and biomechanical state. Such models reduce resistance during natural arm swings while delivering targeted support during periods of increased load or fatigue. Furthermore, exoskeletal systems intended for dynamic walking environments should integrate multisensory inputs and sophisticated control algorithms capable of real-time assistance adjustment based on walking speed and trunk-arm coordination. By promoting seamless coordination between the upper and lower limbs, these designs can improve gait stability and overall biomechanical efficiency across diverse locomotor conditions. These insights provide valuable guidance for the development of next-generation upper-limb exoskeletons that synchronize effectively with whole-body movement, ultimately enhancing user safety, comfort, and functional performance. As walking speed increases, the amplitude of the arm swing increases proportionally [29]. However, the ROM of shoulder was slightly lower at PWS (4.2 km/h) compared to the slowest walking speed of 3.6 km/h. This is attributed to an unnatural walking pattern that occurs at deliberately slow walking speeds. [PERSON] et al. [36] and [PERSON] et al. [37] have reported a close relationship between arm swing and gait stability at slow walking speeds. [PERSON] and [PERSON] [19] found instability due to changes in arm swing patterns at slow walking speeds ranging from 0.3 to 0.7 m/s. The arm swing has been shown to reduce trunk movement and improve walking stability [18; 38; 39]. However, excessively slow walking speed could lead to unnatural arm swing, which may decrease walking stability. Therefore, the preferred walking speed induces natural walking patterns of the limbs and improves walking stability. The kinematic variables of the elbow demonstrated a positive correlation with increasing walking speed. The elbow joint operates as part of a linked segment system, moving in synchrony with the shoulder joint during arm motion [40; 41]. Both joints exhibit dynamic covariation in their torque patterns, reflecting the body's natural biomechanics and generating a linear synergy effect [27; 42]. This synergy minimizes the degrees of freedom that the central nervous system must independently control, optimizing coordination and efficiency in limb movement [43]. In addition, a significant increase in the standard deviation of elbow joint angular velocity and acceleration was observed with increasing walking speed. This increased variability reflects not only the heightened neuromuscular demands but also the adaptive strategies employed by the central nervous system to maintain postural stability and dynamic coordination under varying locomotor conditions. From an ergonomics perspective, such variability may be interpreted as an indicator of functional complexity, wherein the arm swing mechanism continuously adjusts to gait perturbations and shifting task demands. The observed increase in kinematic variables at the elbow joint with rising walking speed indicates a growing demand for linear inter-joint synergy, emphasizing the need for enhanced neuromuscular coordination. This finding carries significant implications for the design of wearable robotic systems and upper-limb assistive devices. In particular, the increased variability suggests that exoskeletal systems should employ adaptive control algorithms capable of accounting for both inter-individual and intra-individual differences in upper-limb dynamics across varying gait speeds. Rather than imposing rigid or pre-defined motion trajectories, ergonomically optimized systems should support controlled flexibility and real-time responsiveness, aligning with the natural variability and complexity inherent in human joint behavior.