Stroke, total knee replacement, and osteoarthritis are common causes that can result in temporary/permanent lower-limb motion impairments. Such patients require physiotherapy interventions to recover their lost motion capabilities. With the increasing number of such patients, there is also an ever-increasing demand on physiotherapists. Hence, there is a need for physiotherapy assistive devices that will reduce the burden on the physiotherapists and allow them to cater to a larger number of patients. One such requirement is to provide assistance for knee flexion and extension. Soft actuators present a novel solution for developing such devices. We propose a novel, low-profile (5 mm), contractile vacuum actuator that, is lightweight (2 g), can provide a high force-to-weight ratio (230), with an effective force of 4.5 N. This proposed actuator comprises an airtight pouch made of thermoplastic polyurethane (TPU) fabric with a helical spring skeleton. Once the air inside the pouch is evacuated, the actuator contracts longitudinally along the spring. This single actuator is combined to fabricate multi-actuator configurations of 3- and 5- actuators capable of producing 8 N and 14 N respectively. Both the single and multi-actuator configurations are experimentally evaluated to obtain isometric, isotonic, and isobaric performance characteristics. A numerical model is also developed to predict the multi-actuator isometric performance. The multi-actuator configurations were combined to evaluate the performance in a model leg extension test setup. The proposed actuators were able to lift a 0.8 kg leg model by 17° in extension at a vacuum pressure of 40 kPa (abs.)