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Using lower limb exoskeletons in healthcare sector like for rehabilitation is an important application. Lower limb exoskeletons can help in performing specific functions like gait assistance, and physical therapy support for patients who are lost their ability to walk again. Since active lower limb exoskeletons require more complicated control instrumentation and according to the limitations of the power/weight ratio that arises in such exoskeletons, many quasi-passive systems have developed and employed. This paper presents the design and testing of lightweight and adjustable two degree of freedom quasi-passive lower limb exoskeleton for improving gait rehabilitation. The exoskeleton consists of a high torque DC motor mounted on a metal plate above the hip joint, and a link that transmit assistance torque from the motor to the thigh. The knee joint is passively actuated with spring. The action of the passive component (spring) is combined with mechanical output of the motor to provide a good control on the designed exoskeleton during walking. The results show that muscles' efforts on both the front and the back sides of the user's leg were decreased when walking using the exoskeleton with the motor and spring.

One form of energy storage in spring is applying a bending moment and converting it into tilt at the head of the spring as strain energy. The relationship between them is the lateral stiffness of the spring. The aim is to find a mathematical equation for the lateral stiffness of the spring and the effect of the length of the spring on the behavior of stiffness. The mathematical model is created according to Castigliano’s second theorem. A simulated model of a conical spring is built using a Solid Work program. The theoretical results are compared with the mathematical model for the same conical spring. Results of both theoretical and simulated models evinced a linear behavior of lateral, while an exponential relationship between the length of the spring and the lateral stiffness is indicated. The difference between theoretical and simulated models is not exceeded 3.2%, which indicates the acceptability of results.

Four prosthetic knee joints (polycentric knee weight activating-4bar and friction, extension assist controlled),(single axis knee weight activating and friction, internal extension assist controlled), (single axis knee weight activating-4bar and hydraulically, controlled) and (polycentric knee geometric locking-6bar, hydraulically controlled) for a trans-femoral patient were tested. The tests were conducted to find the maximum velocity as well as discussing the most comfortable prosthetic forthe patient and walking stability for these prosthetic knees by examining the gait cycle and measuring the ground reaction force (GRF), using force a plate device. Also, the interface pressure was measured between socket and stump muscles by using F-socket device to get the stress distribution during walking with a prosthetic knee. Results manifested that the polycentric knee geometric locking - 6bar, hydraulically controlled is the best because of the good homogenous distribution of GRF between the healthy and prosthetic limb, during which the difference between both the healthy and prosthetic limb is with the least value (4%).And, K4 gives the minimum value of differences in contact pressure between the left and right limb with a value of (24%), it alsoimparts the maximum symmetry between the left and right limb according to the gait cycle parameters.The best results of the interface pressures and kinovea velocity are achieved whenK4 is used with (132.4KPa, 0.71m/s), respectively. Finally, the polycentric knee geometric locking - 6bar, hydraulically controlled is the best according to the ANSYS results during which it yields the minimum values of Von-Mises stress with 14.24MPa and a maximum factor of safety of 3.11.

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