Wearable assistive robotics is a modern field where intelligent actuated systems work in collaboration with the body to replace a lost limb, to enhance performances (e.g. carrying load, walking, running, jumping) or to train or rehabilitate specific activities. Wearability (e.g. weight, bulkiness) and power are the two main competing characteristics of most assistive wearable devices. Energy efficient actuation systems with high power density per mass and volume are thus preferred. The biological muscle-tendon solution is very interesting for energy storage and generation of efficient cyclic patterns. The present study investigates on the capacity to replicate these biological properties using series elastic actuator (SEA) based on pneumatic cylinder technology. While classic SEA designs are composed of electric actuator in series with steel springs, pneumatic actuators are intrinsically compliant due to the compressibility of the fluid. The study starts by presenting an experimental approach for characterizing the elastic behavior of a pneumatic cylinder actuation targeting the assistance of the hip flexion. Results are then confronted to measurements of lower limb joints' stiffness observed during walking to identify the suitability of the solution with the targeted application..