Copyright © 2015 by ASME. The standard recovery treatment for ankle and lower leg injuries consists of using underarm crutches. Hands-free crutches have recently emerged as a more comfortable, natural and energy efficient alternative. However in the currently available devices such as the iWalk-Free (iWALKFree, Inc., USA) the lack of a knee joint results in abnormal motion pattern at the hip and pelvic joints to ensure foot clearance during the swing phase of the gait. To address this shortcoming, the paper describes the kinematic synthesis of a planar passive four-bar linkage that can be used as a mechanical knee in lower limb exoskeletons and other wearable devices. The knee design is based on anthropomorphic foot walking trajectory obtained from optical motion capture system. The task geometry at the foot, related to the contact and curvature constraints between the foot and the ground at two critical positions 'heel strike' and 'toe off' is scaled to the knee level. Velocity and acceleration specifications compatible with the contact and curvature constraints assist in defining the synthesis equations for the knee design. A working prototype of a passive wearable crutch substitute that incorporates the mechanical knee shows the applicability of the proposed technique.