Currently there are an estimated 875,000 people with major lower limb loss in the United States, with numbers projected to increase 1.6-fold by 2050 due to increasing prevalence of diabetes, obesity, and related dysvascular conditions [1]. Lower limb amputation often leads to secondary conditions such as knee pain, knee osteoarthritis, osteopenia, back pain, postural changes, and general deconditioning [2]. For people with transtibial (below-knee) amputation, prevalence of knee osteoarthritis in the contralateral limb is 17x higher than in the general population, with 27% of people with unilateral amputation developing knee osteoarthritis [3]. This large increase in incidence is likely due to insufficient push-off power from the prosthesis and increased limb loading on the contralateral side [4].

This thesis presents an ankle-foot prosthesis which increases energy storage and return, increases peak power, and decreases contralateral limb loading in a low-mass, quasi-passive device. This is achieved by automatically adjusting prosthesis stiffness to maximize energy storage across walking speeds. A novel quasi-passive variable stiffness ankle-foot prosthesis is presented with high resolution stiffness adjustment from 352 - 479 Nm/radian, corresponding to biological ankle quasi-stiffness during level ground walking from 0.75 - 1.5 m/s for a 50th percentile male. This thesis presents the development of a novel mechanism for varying bending stiffness of leaf springs which utilizes independently controlled lockable linear actuators which constrain relative sliding of parallel leaf springs relative to a mechanical ground to control bending stiffness. The detailed device design and analysis of the variable stiffness ankle-foot prosthesis is described, including a parametric model for approximating device stiffness, contact stress analysis, fatigue life calculations, and bolted joint analysis. The benchtop testing results demonstrate that the device successfully achieves the targeted stiffness range, device mass, and structural integrity.

A study was conducted with 7 participants with unilateral transtibial amputation in order to evaluate the kinetic and kinematic effects of the variable stiffness prosthesis during walking compared to a passive energy storage and return foot. During the experiment, subjects walked on an instrumented treadmill at the speeds of 0.75 m/s, 1.0 m/s, 1.25 m/s, and 1.5 m/s while force and motion data was recorded. This thesis presents results from the clinical study which demonstrate a 15.5 - 19.3% greater peak ankle angle during walking across all speeds with the variable stiffness ankle compared to a passive control, 5.4 - 14.8% greater peak joint power, 10.5 - 23.7% greater energy return, and a 4.0 - 6.7% lower contralateral limb knee external adduction moment across walking speeds.

This thesis presents the first of its architecture variable stiffness ankle-foot prosthesis utilizing a novel locking parallel leaf spring mechanism for stiffness control. The prosthesis has a lower device mass compared to existing powered and quasi-passive devices, and increases biomimetic functionality beyond standard passive prostheses. This thesis presents significant clinical results demonstrating the benefits of such a device on the biomechanics and energetics of people with transtibial amputation while walking. This device has the potential to improve health outcomes in people with transtibial amputation by normalizing biomechanics and increasing energy storage and return, and decreasing contralateral limb loading and unwanted knee external adduction moment. This prosthesis has the potential to expand access to high performance prosthesis technology by creating a device that is low mass, low power, and lower cost compared to fully powered devices.