## 1 Introduction
Since it is not possible to establish safe thresholds for soft tissue mechanical loading at the pHMI based on interfacial pressure measurements alone [31], this pilot study investigates tissue oxygenation as an additional physiological metric, thus focusing on the user safety in assessing the human-exoskeleton interaction. Near-infrared spectroscopy (NIRS) is a well-tolerated, non-invasive, and widely used technique for determining oxygen saturation and blood flow in the capillary bed of soft tissue. In terms of occupational exoskeleton evaluation, NIRS has mostly been used to detect muscle fatigue [32; 33; 34]. Another application of NIRS in recent exoskeleton studies is on human-machine interaction, prefrontal cortex activity, and muscle activation patterns, e.g., in exoskeleton-based rehabilitation after stroke [35; 36]. To our knowledge, only two studies used NIRS concerning the evaluation of physical human-machine interfaces of exoskeletons [37; 38]. [PERSON] et al. aimed to identify the impact of circumferential compression levels and different cuff sizes of a soft exoskeleton on the lower extremities during walking and directly related it to the occurrence of discomfort and pain. With regard to tissue oxygenation, (1) a negative correlation between inflation pressure and deep tissue oxygenation was found, (2) oxygen deprivation was differently tolerated at different sites of the lower extremity, and (3) the increase in oxygenation after cuff release differed at different sites in the lower extremity, which was further enhanced by a larger cuff. To the authors' knowledge, there is no study to date that has utilized NIRS to measure tissue oxygenation in the upper extremity in the context of exoskeleton interface pressure. In a previous study, it was shown that pressure within shoulder exoskeleton pHMIs with a circular enclosure at the upper extremity exceeded the recommended pressure of 20 mmHg [39] by an average of \(\sim\)23% in resting position (with the arm hanging) and by an average of ~70% in dynamic arm movements [40]. However, to date, it has not been investigated how tissue oxygen saturation underneath the interface behaves during normal use of shoulder exoskeletons and whether exceeding the recommended pressures has a negative effect on tissue oxygenation. In addition, it is unclear whether the different designs of circular exoskeleton interfaces affect pressure and oxygen level. Therefore, this pilot study investigates possible changes in muscle oxygenation saturation in the resting position and during dynamic overhead arm movements underneath the pHMI of three shoulder exoskeletons which were selected with respect to their pHMI design. This study investigates the following hypotheses:  
1. pHMIs of shoulder exoskeletons restrict the blood supply underneath the pHMI and result in a decreased oxygen saturation in the soft tissue in resting posture. 2. Dynamic arm movements with the shoulder exoskeletons have a neutralizing effect on the tissue oxygen saturation underneath the interface compared to resting conditions. 3. The design of the pHMI influences the circumferential pressure and the tissue oxygenation underneath the pHMI.