Knee-inspired continuum robots using antagonistic-blocking strategy for torsional and bending stiffness enhancement

Continuum robots often exhibit positioning errors due to insufficient constraints and underactuation. Inspired by the three-dimensional tension network of biological knee joints, here we report a hierarchical stiffness-decoupling strategy to regulate torsional stability and bending stiffness. A knee-inspired joint with bilateral cross-cable constraints and stepped surfaces improves structural stability: cross-cable constraints increase reaction torque by 73%–90%, whereas hybrid constraints increase it by 171% at 45° and by at least 189% at 90°, while reducing radial perturbation by 64%. To mitigate load-induced displacement in constant-total-tendon-length systems, we further introduce a hybrid enhanced stiffness (HES) strategy combining a blocking mechanism with antagonistic actuation. HES improves apparent stiffness by 51%–131% under tested force-displacement conditions and exceeds the baseline (BN)-corrected arithmetic sum of blocking-enhanced stiffness (BES) and antagonistic-enhanced stiffness (AES) by 4%–35%, indicating nonlinear synergy. This bioinspired approach reduces coupling among compliance, torsional stability, and load-bearing capacity.