# Effects of Arm Swing in Lower Limb Muscle Activation and Coordination During Treadmill Walking
## 2 Materials and Methods  
Ten healthy young participants (seven men and three women, mean age: 24.3 \(\pm\) 2.26 years old) volunteered for this study. All participants were physically active in daily life, though none were engaged in any formal athletic training. They were informed about the procedures involved in the study. The protocol was approved by an institutional Ethics Committee (the Ethical Committee for Clinical Research of the Hospital Universitario Severo Ochoa de Leganes) under the code CYCLING-AL in October 2022 and was conducted in accordance with the Declaration of Helsinki at the Center for Clinical Neuroscience of Hospital Los Madronos. Prior to data collection, informed consent was obtained from the participants. First, participants walked on a sensorized Y-Mill treadmill (Motek Medical, B.V., Houten, The Netherlands) for 2 min, so they could familiarize themselves with the treadmill and select a speed. During this initial trial, experimenters determined the desired walking cadence by manually fitting a metronome to the participants' natural cadence. Then, participants walked for an additional minute on the treadmill to familiarize themselves with the selected metronome. The sensorized treadmill is part of the Bestable testbed [43] of the EUROBENCH project ([[https://cordis.europa.eu/project/id/779963/es](https://cordis.europa.eu/project/id/779963/es)]([https://cordis.europa.eu/project/id/779963/es](https://cordis.europa.eu/project/id/779963/es)) (accessed on 1 October 2022)), and allows: (1) measurement of ground reaction force; (2) recording of foot strike and foot off events; and (3) reading of an external analog signal for synchronization by means of a digital acquisition board (NI USB-6210, National Instruments Corp, Austin, Texas, USA). Next, surface electrodes (Ambu WhiteSensor WSP30, Ambu A/S, Ballerup, Denmark) were placed on each participant's dominant leg to measure the muscle activity of eight muscles--the Gluteus Medius (GLM), Rectus Femoris (RF), Vastus Lateralis (VL), Biceps femoris (BF), Semitendinosus (ST), Gastrocnemius Medialis (GaM), Soleus (SOL), and Tibialis Anterior (TA)--following SENIAM guidelines [44]. Additionally, one reference electrode was placed on the surface of the lateral malleolus of the same leg. sEMG was recorded with a sampling frequency of 2000 Hz (Sessantaquattro+, OTBioelettronica SRL, Torino, Italy). Then, participants performed 2 min of treadmill walking with arm swing (walking with arms, WWA) and 2 min of treadmill walking without arm swing (walking with no arms, WNA). In both tasks, participants were asked to synchronize their walking cadence with the metronome. We used a metronome to control walking cadence and ensure that any differences in muscle activation and coordination were not due to variations in gait speed, allowing for more accurate comparison between walking with and without arm swing. During WWA, subjects were asked to swing the arms naturally, which resulted in a 1:1 arm-leg coordination that was visually verified [45]. The two recording systems (the sensorized treadmill and the EMG recorder) were synchronized by means of an analog signal pulse of 100 ms and an amplitude of around 3V delivered in parallel by an analog function generator (HDG2102B, Hantek, Asturias, Spain). The experimental setup and protocol are shown in Figures 1a and 1b, respectively. Figure 1: (**a**) Setup of the experimental protocol. From left to right: participants walked for 2 min on a sensorized treadmill to record comfortable speed and cadence; electrode placement on each participant’s dominant leg: synchronization between devices by means of an analog signal delivered in parallel to the treadmill computer and the Sessataquattro+ EMG recorder. (**b**) Experimental protocol: participants walked for 2 min on a sensorized treadmill under two conditions: walking with arms (WWA) and walking with no arms (WNA). (**c**) Example of the electromyographic (EMG) analysis and extraction of muscle synergies.