Ankle mechanics during sidestep cutting implicates need for 2-degrees of freedom powered ankle-foot prostheses.

INTRODUCTION Recent advances in powered prostheses promise to significantly improve the quality of life and well-being for individuals with impaired mobility. A better understanding of the complexities surrounding lower-limb prostheses, which are needed for walking and daily activities, will lead to increased health and well-being for the 1.7 million people with amputation in the United States, the majority of whom have lower-limb amputations [1-2]. The ankle joint of current commercially available lower-limb powered prostheses is capable of controlling only one degree of freedom (DOF) in the sagittal plane, focusing on improved mobility in straight walking. Turning, however, plays a major role in daily living activities and requires ankle control in both the sagittal and frontal planes. Additionally, even during walking on a straight path, the ankle functions in both the sagittal and frontal planes. This suggests that the next advancement in prosthetic ankles is to extend their design and control to the frontal plane. Agility describes the ability to alter the direction of the body efficiently and effectively. One definition of agility is proposed as "a rapid whole body movement with change of velocity or direction in response to a stimulus" [3]. Agility requires both maneuverability and speed, which are limited in people with lower-limb amputation who use passive prostheses. It is shown that individuals with unilateral below-knee amputation who use passive prostheses rely more on their hip joint and expend 20 to 30 percent more metabolic energy than nondisabled people at the same speed. As a result, their preferred speed of gait is 30 to 40 percent lower than that of nondisabled people [4-5]. Additionally, people with amputation use compensatory strategies that result in asymmetrical gait patterns that affect joints in both lower limbs, which may inadvertently lead to secondary complications such as back pain or knee or hip osteoarthritis of the intact limb [6-9]. In contrast, it has been shown that a powered ankle-foot prosthesis reduces the metabolic costs of people with unilateral transtibial amputation during straight walking by providing sufficient power during push-off [10-11]. However, studies of four representative daily activities show that turning steps may account for an average of 25 percent of steps, ranging from 8 to 50 percent of all steps depending on the activity [12], which people with amputation accomplish using different control strategies than nondisabled people. While a nondisabled person relies on hip movement in the coronal plane and moments generated at the ankle, a person with amputation using a passive prosthesis relies on hip extension in the sagittal plane [13-16]. During a turn, modulation of ankle impedance in the sagittal and frontal planes plays a major role in controlling lateral and propulsive ground reaction forces in order to accelerate the body's center of mass along the gait path; thus, during a turning step, lateral and propulsive impulses are larger than during a straight step [17]. This difference will result in different gait strategies between people with amputation and nondisabled people to compensate for the lack of propulsion from a passive prosthesis in order to increase maneuverability [13]. This suggests that an ankle-foot prosthesis capable of generating moments in two DOFs, i.e., dorsiflexion-plantar flexion (DP) and inversion-eversion (IE) directions, with impedance modulation similar to the human ankle will augment maneuverability and mobility that leads to a more agile gait. Additionally, design features that allow walking in arbitrary directions on slopes while conforming the foot to the uneven ground profile may result in a more efficient gait. Understanding the ankle's capability to modulate impedance while generating net positive work during the stance period of gait has influenced the design of new ankle-foot prostheses [18-21]. The ankle mechanical impedance is a dynamic operator that maps the time-history of angular displacements onto the corresponding time-history of ankle moments. …