McKibben air muscles were invented for orthotics in the 1950s. They have the advantages of being lightweight, easy to fabricate, are self limiting (have a maximum contraction) and have load-length curves similar to human muscle. The muscles consist of an inflatable inner tube/bladder inside a braided mesh, clamped at the ends. When the inner bladder is pressurized and expands, the geometry of the mesh acts like a scissor linkage and translates this radial expansion into linear contraction.
Standard McKibbens contract in a linear motion up to a maximum of typically 25%, though different materials and construction may yield contractions around 40% . Though they can technically be designed to lengthen as well, this is not useful as the soft muscles buckle.
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