To develop an effective quasi-passive (QP) exoskeleton, maximizing its range of variable assistive torques while minimizing the energy required for this variation is crucial. However, achieving this goal has proven challenging so far owing to the common trade-off between the range of variable assistive torques and the energy required for torque variation. Additionally, the shoulder's standby posture complicates the search for viable solutions. To tackle this issue, we derived design principles based on elastic potential energy field. Utilizing these principles, we developed a QP shoulder exoskeleton called adjustable shoulder exoskeleton (AD Exo), which successfully achieved a wide range of variable assistive torques with significantly reduced required energy for variation. Benchtop tests demonstrated a remarkable enhancement in variability, with a range of assistance spanning 6.37 Nm
achieved with a variation energy of 0.9 J. In human trials, AD Exo significantly minimized the average percentage of maximum voluntary contraction in shoulder muscles. Compared to the condition without the exoskeleton (NE), the average muscle activation was reduced by 25% at the adjusted assistance (ADJ), 7.5% at the low assistance (LOW), and 6.7% at the high assistance (HIGH), respectively. Considering repetitive and long-term tasks, this reduction in muscle activity can accumulate, making AD Exo highly effective for alleviating shoulder muscle burden and fatigue. Furthermore, kinematic motions of wearers and actual assistive torque delivered to wearers were examined to analyze the underlying the assistive effect of the exoskeleton.



Funding Information
* The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article
* This research was supported by the Industrial Strategic Technology Development Program (No. 20007058) funded by the Ministry of Trade, Industry,
  and Energy (MOTIE, Korea).
* Additional funding was provided by the Korea Medical Device Development Fund grant, funded by the Korean government (the Ministry of Science
  and ICT, the Ministry of Trade, Industry and Energy, the Ministry of Health & Welfare, and the Ministry of Food and Drug Safety)(RS-2022-00140621). 
  The research of Sangeui Lee was supported by the Chung-Ang University Graduate Research Scholarship in 2024.