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Lower‐limb exoskeletons and powered orthoses are external devices that assist patients with locomotive disorders to achieve correct limb movements. Current batteries cannot meet the long‐term power requirements for these devices, which operate for long periods of time. This issue has become a major challenge in the development of these portable robots. Conversely, legged locomotion in animals and humans is efficient; to emulate this behaviour, biomimetic actuation has been designed attempting to incorporate elements that resemble biological elements, such as tendons and muscles, in the mechanical systems. The purpose of this paper is to present a mechanism that resembles a human tendon to achieve and utilise the synergic actuation of the leg joints.

In this paper, we present a mechanism that resembles a human tendon to move the ankle joint and utilise the synergic actuation of hip and knee joints. Implementation of the proposed transmission system in the ATLAS active orthosis prototype allowed for a better ankle gait fit, which resulted in a more natural stride and, as expected, optimised energy consumption in the locomotion cycle and actuation energy requirements.

The fitted passive ankle motion provides toe‐off impulse, increases support force, and helps provide ground clearance.

A synergetic underactuated movement in the ankle joint, implemented by two cables in each leg, improves the functionality of the device without increasing the leg weight and while maintaining a reduced size. To achieve a correct and efficient motion in the ankle of an active orthosis, two steel cables were attached in the ATLAS orthosis. These cables act as a synergic biarticular linkage and transfer motion from the hip and knee joints. Synergic ankle motion provides impulse in the toe‐off, increases support force, and provides ground clearance. These goals are achieved with low energy expenditure because of synergical actuation, and high inertia is prevented in the more distal limb.