Mechanical stability of the interaction between a lower-limb rehabilitation exoskeleton and the user's body during functional tasks.

Introduction: Lower-limb exoskeletons support gait restoration in individuals with locomotor impairments. Mechanical stability in human-exoskeleton interaction is essential for safety. This pilot study evaluated the interaction stability between TWIN exoskeleton and user by analyzing relative displacements between the exoskeleton cuffs and the corresponding user's thigh and shank during walking (WA) and sit-to-stand (STS). A second parameter quantified the discrepancy between exoskeleton and user joint angles. Methods: Five healthy adults performed STS and WA while wearing TWIN. An optoelectronic system tracked markers placed on anatomical landmarks and exoskeleton segments. The standard deviation of the relative displacement (RDstd) between the exoskeleton and the underlying anatomical segment quantified mechanical stability. Human-exoskeleton kinematic discrepancy (HEKD) was determined as root mean square of the difference between encoder- and motion capture-derived joint angles. Results: During STS, RDstd was 4.6 ± 1.5 mm (thigh) and 1.4 ± 0.1 mm (shank). During WA, values were 3.3 ± 0.4 mm (thigh) and 2.7 ± 0.2 mm (shank). HEKD during WA was 1.7 ± 0.6 deg (hip) and 1.9 ± 0.4 deg (knee), whereas 2.2 ± 0.8 deg (hip) and 3.3 ± 1.3 deg (knee) during STS. No participants reported discomfort or pain during tests. Discussion: All relative displacement values were below 11.7 mm (safety value for the possible onset of skin damages). Larger relative displacements at thigh level during STS than during WA were likely due to larger hip and knee joint excursions associated to STS. Conversely, larger relative displacements at shank level during WA than during STS can be attributed to foot impact forces occurring during WA. Compared to the results from a previous study on a different Hip Active Orthosis, which reported RDstd values at the thigh between 4.0 and 6.3 mm during walking, TWIN showed smaller values, confirming a greater mechanical stability at that level. The small values of discrepancy between exoskeleton and anatomical joint angles during both tasks (below 2 deg during WA and 3.3 deg during STS) indicated strong human-exoskeleton kinematic coupling. Taken together, these findings suggest that TWIN provided high stability during both motor tasks without compromising comfort or safety.