Case Study: A Bio-Inspired Control Algorithm for a Robotic Foot-Ankle Prosthesis Provides Adaptive Control of Level Walking and Stair Ascent

Abstract

Powered ankle-foot prostheses assist users through plantarflexion during stance and dorsiflexion during swing. Provision of motor power permits faster preferred walking speeds than passive devices, but use of active motor power raises the issue of control. While several commercially available algorithms provide torque control for many intended activities and variations of terrain, control approaches typically exhibit no inherent adaptation. In contrast, muscles adapt instantaneously to changes in load without sensory feedback due to the intrinsic property that their stiffness changes with length and velocity. We previously developed a "winding filament" hypothesis (WFH) for muscle contraction that accounts for intrinsic muscle properties by incorporating the giant titin protein. The goals of this study were to develop a WFH-based control algorithm for a powered prosthesis and to test its robustness during level walking and stair ascent in a case study of two subjects with 4-5 years of experience using a powered prosthesis. In the WFH algorithm, ankle moments produced by virtual muscles are calculated based on muscle length and activation. Net ankle moment determines the current applied to the motor. Using this algorithm implemented in a BiOM T2 prosthesis, we tested subjects during level walking and stair ascent. During level walking at variable speeds, the WFH algorithm produced plantarflexion angles (range = −8 to −19 •) and ankle moments (range = 1 to 1.5 Nm/kg) similar to those produced by the BiOM T2 stock controller and to people with no amputation. During stair ascent, the WFH algorithm produced plantarflexion angles (range −15 to −19 •) that were similar to persons with no amputation and were ∼5 times larger on average at 80 steps/min than those produced by the stock controller. This case study provides proof-of-concept that, by emulating muscle properties, the WFH algorithm provides robust, adaptive control of level walking at variable speed and stair ascent with minimal sensing and no change in parameters.

Prosthetic push-off power in trans-tibial amputee level ground walking: A systematic review

“…To mimic human gait, PWR ankle-foot units adjust push-off power based on walking speed, i.e. push-off power increases with walking speed [22–24]. This has already been proven in the literature and can also been seen in Fig 2 (similar slopes of the PWR and NA regression lines) and Fig 3 (no significant differences of peak push-off power for slower and faster walking speeds).…”

“…In healthy non-amputee subjects, the level of ankle push-off power increases with walking speed [21]. Similar to non-amputee control subjects, unilateral lower-limb amputees exhibit increased push-off ankle power with increasing walking speed [22–24]. For example, when walking with the iWalk’s Power Foot BiOM (the only type of commercial powered prosthetic foot in the market at this time) peak ankle power increases from about 1.3 W/kg at 0.75 m/s walking speed to about 4.2 W/kg at 1.75 m/s [23].…”

Power Amplification Increases With Contraction Velocity During Stretch-Shortening Cycles of Skinned Muscle Fibers

“…The separation of XB- and non-XB structures is of primary importance to give a more detailed understanding of the potential involvement of viscoelastic elements, such as titin, working as an energy-storing spring during lengthening contractions and SSCs. This information is required for the improvement of muscle models ( Heidlauf et al, 2016 ; Tahir et al, 2018 ; Seydewitz et al, 2019 ) as well as for improved predictions by multi-body models ( Röhrle et al, 2017 ; Haeufle et al, 2020 ) concerning, e.g., movement control and efficiency of locomotion.…”

Antagonistic Co-contraction Can Minimize Muscular Effort in Systems with Uncertainty

“…This model was chosen because the PEE will not be active when there is co-contraction, because its slack length is equal to at least the optimal fiber length. Recently, a winding filament model was introduced as alternative to a Hill-type muscle model [35]. It is expected that with a winding filament model there would be more advantages of co-contraction, since a contraction of the contractile element would then also lengthen the titan spring, further stiffening the muscle.…”