This paper describes the design and the experimental testing of a knee exoskeleton with variable non-active interval (NAI) specifically designed to fit the human gait. This exoskeleton is designed to provide support at the knee joint by supplying assistive torque around the knee in order to reduce the maximum muscular effort required to perform daily activities such as walking and squat lifting, thus, enabling users like elderly people to continue those activities longer. The main mechanism of the exoskeleton behaves like a torsion spring with a variable NAI. The torque provided by the mechanism outside the NAI is quasi-linear with respect to the angle variation. The position of this NAI can be modified to disable the mechanism. Moreover, the angular stiffness of the mechanism can be modified to adapt the magnitude of the support to the user. Experimental evaluation was conducted in two phases: during the first one, the NAI was tested for following the walking pattern of the user, in order to avoid disturbing the user's gait. During the second one, the performance of the support provided by the device was measured by the muscle activity during squatting series. The results show the ability to modify the position of the NAI and a decrease of the maximum muscle activity during squatting while wearing the device, concluding that the exoskeleton could assist the knee joint although further evaluation is required to establish the whole support in the gait cycle.