Participants in this arm will receive approximately 15 bouts of acute intermittent hypoxia. This intervention involves breathing lowered levels of oxygen (similar to being on a tall mountain) for 60 seconds at a time, followed by 60 seconds of room air. Participants will be assessed following the intervention.

This intervention involves breathing lowered levels of oxygen (similar to those experienced on a tall mountain) for 60 seconds at a time. This is immediately followed by 60 seconds of breathing normal room air. These bouts are repeated approximately 15 times. For the intervention, the participant is fitted with a non-rebreathing mask which supplies the two different air levels

The KAIST Upper Limb Synergy Investigation System (KULSIS) will be utilized. KULSIS was developed in collaboration with Neuro-

Figure 1: Design of the KULSIS Rehabilitation Engineering Lab at Korea Advanced Institute of Science and Technology, South Korea (Figure 1). It comprises a modular hand and wrist attachment, which connects to a linear actuator. To construct the linear actuator, ball-screw type linear motion module (RS-075N-Z05PR, Robostar, South Korea) was combined with a 100 W AC servo motor (SGM7J-01AFD2C, Yaskawa, Japan). The linear actuator has stroke range of 400 mm, which is ample for adjusting the position of modular hand or wrist attachment to the participant's upper arm before the outcome assessment. Once adjusted, the power of linear actuator will be turned off to fix the position.

A six degrees of freedom (DOF) Force/Torque sensor (Series Delta, ATI-Inc. Apex, NC, USA) is positioned between the module and the linear actuator to measure the force and torque applie

Stretch reflex thresholds will be assessed using a custom-designed tendon tapper system (Figure 2.A) based on a Linmot® linear motor (Linmot S.A., Spreitenbach, Switzerland). This system includes a stator and slider with integrated position sensing via a Hall effect sensor, ensuring precise control of tendon indentations and taps. The motor is mounted on a micrometer-driven positioner (Velmex, Bloomfield, NY, USA) for reliable placement of the tapper on the biceps brachii tendon. A multiaxial load cell (Series Nano17, ATI Industrial Automation, Apex, NC, USA) is attached at the tapping end to record applied forces and the resultant reflex responses. Additionally, the tendon tapper integrates a wireless ultrasound device(Figure 2.B) at its tip to provide real-time transverse imaging of the tissue composition surrounding the distal biceps tendon. The ultrasound device features an image resolution of 384 pixels (width) by 400-600 pixels (depth), corresponding to a physical size of 38.4 m