Joshua M. Caputo

Prosthetic ankle push-off work reduces metabolic rate but not collision work in non-amputee walking

Individuals with unilateral below-knee amputation expend more energy than non-amputees during walking and exhibit reduced push-off work and increased hip work in the affected limb. Simple dynamic models of walking suggest a possible solution, predicting that increasing prosthetic ankle push-off should decrease leading limb collision, thereby reducing overall energy requirements. We conducted a rigorous experimental test of this idea wherein ankle-foot prosthesis push-off work was incrementally varied in isolation from one-half to two-times normal levels while subjects with simulated amputation walked on a treadmill at 1.25 m·s−1. Increased prosthesis push-off significantly reduced metabolic energy expenditure, with a 14% reduction at maximum prosthesis work. In contrast to model predictions, however, collision losses were unchanged, while hip work during swing initiation was decreased. This suggests that powered ankle push-off reduces walking effort primarily through other mechanisms, such as assisting leg swing, which would be better understood using more complete neuromuscular models.

An experimental robotic testbed for accelerated development of ankle prostheses

Biomechatronic devices show promise for restoring human performance, but development has been made inefficient by the need for specialized autonomous devices prior to testing benefits of proposed functionalities. This has severely limited exploration within and across intervention strategies. We have developed a laboratory testbed suitable for emulating and rapidly assessing wearable robot designs. The testbed is comprised of powerful off-board motor and control hardware, a flexible tether, and lightweight instrumented end-effectors worn by a person. We performed a series of benchtop tests to gauge mechatronic performance, and found significant improvements over prior candidate testbed platforms. In particular, this system has an unusual combination of low worn mass (less than 1 kg), high closed-loop torque bandwidth (17 Hz), and high peak torque (175 N·m), key to emulating specialized devices. We also performed walking trials to gauge dynamic torque control and versatility. Walking trials with a prosthesis end-effector demonstrated precise torque tracking (4 N·m RMS error), both in time and joint-angle space, and versatile mechanical behavior through systematic changes in high-level control law parameters. For example, we widely varied net ankle work (from -3 J to 9 J per step) using an impedance law relating joint angle and velocity to desired torque. These results suggest such testbeds could be used to emulate and evaluate novel assistive robot concepts prior to laborious product design.

Increasing ankle push-off work with a powered prosthesis does not necessarily reduce metabolic rate for transtibial amputees

Amputees using passive ankle-foot prostheses tend to expend more metabolic energy during walking than non-amputees, and reducing this cost has been a central motivation for the development of active ankle-foot prostheses. Increased push-off work at the end of stance has been proposed as a way to reduce metabolic energy use, but the effects of push-off work have not been tested in isolation. In this experiment, participants with unilateral transtibial amputation (N=6) walked on a treadmill at a constant speed while wearing a powered prosthesis emulator. The prosthesis delivered different levels of ankle push-off work across conditions, ranging from the value for passive prostheses to double the value for non-amputee walking, while all other prosthesis mechanics were held constant. Participants completed six acclimation sessions prior to a data collection in which metabolic rate, kinematics, kinetics, muscle activity and user satisfaction were recorded. Metabolic rate was not affected by net prosthesis work rate (p=0.5; R2=0.007). Metabolic rate, gait mechanics and muscle activity varied widely across participants, but no participant had lower metabolic rate with higher levels of push-off work. User satisfaction was affected by push-off work (p=0.002), with participants preferring values of ankle push-off slightly higher than in non-amputee walking, possibly indicating other benefits. Restoring or augmenting ankle push-off work is not sufficient to improve energy economy for lower-limb amputees. Additional necessary conditions might include alternate timing or control, individualized tuning, or particular subject characteristics.

Emulating prosthetic feet during the prescription process to improve outcomes and justifications

The current process of prescribing prosthetic feet is hampered by imprecise classifications based on self-assessment, recommendations based on subjective prediction, burdensome justification requirements, and slow, costly testing of devices. These problems have been exacerbated by the introduction of robotic prostheses, which can improve gait performance for some individuals, but are very expensive. We propose an alternative process, in which a versatile robotic emulator is used to preview patient interactions with a range of prostheses, while objective data related to effort, stability, speed and preference are collected, all prior to prescription. Results from pilot testing with a prototype emulator system demonstrate accurate haptic rendering of a wide range of prosthesis classes and differentiation of user performance across these classes. Eventually, emulation-based prescription could reduce bias, cost and waste in the prescription process, while simultaneously improving patient outcomes.