# Kinematic Analysis of Dynamic Coactivation During Arm Swing at the Shoulder and Elbow Joints
## 4 Discussion  
Most kinematic variables of the shoulder and elbow exhibited an increasing trend as walking speed increased. As shown in Table 1, the maximum joint angle of the shoulder remained consistent across different walking speeds. However, the minimum joint angle of the shoulder significantly decreased with increasing walking speed, indicating that the arm swing becomes more pronounced in the posterior direction relative to the anterior movement at higher speeds. Angular velocity and acceleration demonstrated a consistent increase of approximately 10% as walking speed increased, reflecting a natural augmentation of passive movement as arm swing intensifies [27]. Arm kinematic variables showed a proportional relationship with walking speed increments [15]. Adequate arm swing is essential for maintaining stability and reducing energy expenditure as walking speed increases [28; 29; 30; 31]. [PERSON] et al. [32] found that arm and trunk movements adjust to changes in walking speed to facilitate coordinated limb motion. Similarly, [PERSON] and [PERSON] [33] observed a strong association between arm movement and walking speed, noting that arm swing corresponds to changes in walking speed at the shoulder's center of rotation. Moreover, [PERSON] and [PERSON] [13] and [PERSON] et al. [14] evaluated that arm swing is driven by the active contribution of arm muscles during walking. Thus, in the design of exoskeletal devices, it is crucial to account for the rhythmic synchronization of arm movement in relation to walking speed. The walking speed of 7.2 km/h was designated as the fastest walking speed in this study. At this speed on the treadmill, the duration of the stance and swing phases during a single gait cycle (heel strike-toe off) is significantly reduced. Consequently, the range of motion required for adequate arm swing was constrained [33]. Analyzing upper-limb kinematics under such high-speed conditions is essential for understanding the limits of natural arm-leg coordination and for informing the ergonomic design of assistive technologies, particularly in applications where gait speed may be intentionally increased, such as in exoskeletons or advanced rehabilitation protocols. As a result, kinematic variables were analyzed at their peak values at the 7.2 km/h walking speed. As shown in Figure 8, maximum angular acceleration increased with walking speed, reflecting a rise in the instantaneous velocity of the shoulder joint. This suggests that the mechanical load on the shoulder joint intensifies as walking speed increases. Based on the findings of this study, walking at 7.2 km/h may result in a relatively higher biomechanical load and discomfort in the shoulder joint compared to lower walking speeds. [PERSON] and [PERSON] [34] reported that as walking speed increases, upper-limb movement also intensifies due to passive muscle recoil, thereby decreasing walking efficiency. The arm functions as a mass damper during walking, leading to higher instantaneous acceleration of the shoulder and elbow joints as walking speed increases, which elevates the risk of injury [35]. In this study, the kinematic variables of the shoulder and elbow were most prominently affected at a walking speed of 7.2 km/h. Furthermore, the observed increase in standard deviation at higher walking speeds (e.g., 7.2 km/h) may reflect a shift toward greater reliance on active motor control rather than passive arm swing dynamics. This transition highlights the critical need to integrate sensor-based feedback and compliant actuation mechanisms into the design of exoskeletal systems to facilitate safe, responsive, and user-centered interaction. Accommodating natural variability in shoulder kinematics is therefore essential not only for enhancing user comfort but also for minimizing neuromuscular fatigue and improving the long-term usability of the assistive device. Based on these findings, we highlight the critical role of adaptive control strategies in exoskeletons that dynamically adjust to variations in gait speed and upper-limb coordination.