# Effects of Arm Swing in Lower Limb Muscle Activation and Coordination During Treadmill Walking
## 1 Introduction  
Locomotion, defined as a purposeful and goal-directed behavior, is a fundamental aspect of human mobility involving cyclic and rhythmic movements of the limbs. When a person is walking, there are rhythmic automatic processes involving coordination between the central nervous system and the musculoskeletal system that enable smooth and efficient movements [1]. This coordination is orchestrated by locomotor networks known as central pattern generators [2]. While the biomechanics and functions of the lower limbs (LLs) during gait have been widely studied [3; 4], there is a growing interest in understanding the role of the rhythmic swinging movement of upper limbs (ULs) [5; 6]. This natural swinging movement of the ULs is referred to as arm swing and has been suggested to be an integral component of human bipedal locomotion [7; 8]. For instance, when a disturbance is introduced during gait, such as tripping, the arm swing is prolonged and even accelerated, which has a beneficial effect on balance recovery and helps to avoid falling [9]. Likewise, arm swing contributes to energetic efficiency [10; 11; 12] and has been linked to decreased ground reaction moments [8]. Additionally, recent research suggests that the kinematics of arm swing may differ between genders during locomotor tasks like running [13]. Furthermore, arm swing during gait is not a passive pendular action, as it requires active muscle contractions in the ULs [14; 15; 16]. A study that investigated coupling between UL and LL muscles during gait revealed that arm swing during gait is an active contributor to efficient activation of LL muscles [16]. This contributes to the evidence supporting a connection between the neural pathways controlling arm movement and those controlling LL muscles [1; 6; 17; 18]. Altogether, these findings support the importance of arm swing in human walking. Regarding its impact in the neurorehabilitation field, adding arm swing to robotic therapy improved functional outcomes compared to conventional training with arm restraint in stroke survivors [19]. Some possible explanations are that arm swing allows maintenance of trunk and body stability with a reduced center of mass lateral displacement [10], or that active arm swing influences LL muscle activation and modulates corticospinal excitability [19]. Yet, these hypotheses have not been tested. Although some groups have tried to design robotic devices that allow natural arm swing [20; 21; 22], current treadmill rehabilitation assisted by commercial robots is mostly performed with constrained arm movements due to safety reasons and the lateral encumbrance of the robot [23]. In the past decades, several studies have suggested that the central nervous system modulates neural commands by activating groups of muscles rather than individual muscles to execute a given task [24; 25; 26; 27; 28; 29; 30]. Under this hypothesis, each group of co-activated muscles acts synergistically. Analysis of muscle synergies, which can be used to characterize muscle coordination [31], also reduces the high-dimensional space of multiple muscle activation recordings and degrees of freedom [25; 30] to a low-dimensional space of a few muscle synergies. Muscle synergies have been studied in a wide range of rhythmic tasks, such as walking [32], running [33], and cycling [24; 34; 35; 36], by measuring surface electromyography (sEMG). The analysis of muscle synergies can be used, for example, to compare muscle coordination between different tasks [33; 37; 38; 24]. Regarding human locomotion, some groups have studied the effect of walking speed, body weight support, or walking in the water on LL muscle synergies [38; 39; 40]. [PERSON] et al. [37] compared UL and LL muscle synergies during treadmill walking and Nordic treadmill walking, an activity that requires precise UL movements while walking.