Biological tendons aren't passive cables — they're springs under pre-tension. They store energy during muscle contraction and release it simultaneously, amplifying output speed beyond what the muscle alone could produce.
Transmits force AND stores elastic energy. The spring in the system.
Pure force transmission. No stretch. High-load joints.
Ligaments prevent dislocation, limit range of motion, maintain joint geometry — exactly like biological ligaments. They never actuate. Pure Kevlar, woven into flat panels anchored bone-to-bone.
Rectangular panel — basic constraint, both sides of joint
X-cross internal — resists valgus + rotation
Figure-8 wrap — limits rotation range at a joint axis
Loop + cinch — prevents pull-apart on compression joints
Rubber elastic cord creeps under sustained pre-tension — it permanently elongates over hours/days of load. TPU (thermoplastic polyurethane) is more fatigue-resistant under continuous load, holds calibration over time. Better recovery, more consistent spring behavior. TPU is stiffer than rubber but durability wins for a robotic arm that stays pre-tensioned.