This is a four-part series paper, where the hardware design, mathematical model and the adaptive controller design has been explained for amputee’s who are not good candidates for the available prosthetic legs and need caretaker to make any physical movement. The proposed design aims to help patients with two medical conditions - impaired mobility and impaired decision making, that is, amputee and only with dementia, paralysis,  monoplegia, hemiplegia, paraplegia, cancer, infection or from induced virus/parasites, where decision making is impaired. In these conditions, it is challanging, very difficult with discomfort and even unsafe to use the existing expensive prosthetic legs due to the nature of medical conditions, and thus impossible to use any of the available transtibial legs. The design is intended to help such patients,allowing them to wishful control movements with natural feeling and with indication of dangerous paths including stairs detection, obstacles detection, uneven slopes, which can be even sent remotely to the caretaker. This transtibial is attempted to be imitated as real leg movements of climbing and descending ability, up and down, back and forth, at three joints - knee, ankle and metatarsophalaneal joints, based on sensorless SMO which in turn has feedback from NMPC,Kalman and particle filter, designed with 9 DoF and 4 rigid linkages.The simulation has shown to be a promising concept and is expected to be successful for use in practical clinical purposes but the model has not been clinically tested. This Part-I of the paper explains the mechanical considerations, design calculations and sensors.