A McKibben actuator is a rubber bladder sleeved in an inextensible braided mesh (the same weave as a finger-trap toy). Pressurised air pushes the bladder to expand radially, but the fixed-length braid fibers can only trade that radial growth for axial shortening — so the whole actuator contracts like a real muscle, with no rigid parts.
L = Lf·cosθ r = Lf·sinθ / (2πn)
F(θ) = (P·π·D₀²/4)·(3cos²θ − 1)
- θ (braid angle) — angle of the mesh fibers to the muscle axis; starts small (~15°) at rest and grows as the muscle inflates.
- Pull force — maximal at low contraction, and drops to zero at the "magic angle" (54.7°), the real non-linear force-contraction curve of a PAM.
- Compliance — because force fades as contraction grows, a PAM behaves like a soft spring rather than a stiff motor, which is why bio-inspired limbs use pairs of them instead of rigid gearmotors.
- Agonist–antagonist pair — biological muscles only pull, never push, so joints are driven by two opposing muscles; the finger demo pressurises a flexor while the extensor relaxes, exactly like a real limb.