Design and Functionality Assessment of Crutch-Integrated Variable Grounding Flexible Limb for Sit-to-Stand and Stand-to-Sit Transfer Assistance

Our previous work introduced motor-driven Variable Grounding Flexible Limbs (VGFLs) that assist users' sit-to-stand (STS) and stand-to-sit (SIT) transfers by dynamically adjusting their ground contact positions. Building upon this concept, this study proposes crutch-integrated VGFLs actuated by a fully passive mechanism, eliminating the need for powered actuators or onboard batteries, and assesses the functionality of the VGFLs through verification experiments involving healthy subjects simulating a forward-leaning posture. The proposed VGFLs incorporate a passive power unit consisting of constant-force spiral springs, which transmit mechanical energy to the VGFLs via braided polymer lines. This configuration produces bidirectional coiling and uncoiling motions, generating a vertical assistive force during STS and SIT transfers. A functionality assessment of the VGFLs was conducted by comparing users' STS and SIT transfers under the VGFL-assisted condition with those under three other conditions (no support, short crutches, and long crutches), based on the root mean square (RMS) values of the center of mass (CoM) acceleration and the peak-to-peak amplitudes of surface electromyography (sEMG) signals. The STS and SIT transfer assistance provided by the VGFLs resulted in statistically significant reductions in CoM acceleration RMS and quadriceps sEMG activity, specifically in one or more of the rectus femoris, vastus medialis, and vastus lateralis muscles, in several subjects compared to the control conditions. These findings suggest that crutch-integrated VGFLs can enhance postural stability and reduce lower limb muscle loading during STS and SIT transfers, potentially improving transfer performance in daily living.