human: Given a node-centered hypergraph: <hypergraph>, where nodes represent authors and hyperedges represent groups of authors who co-authored the same paper.

Details of the hypergraph:
Center node: "Design and control of a robotic leg with braided pneumatic actuators"
  Abstract: A four-DOF planar robotic leg actuated with McKibben artificial muscles was designed, constructed, and controlled. Both position and passive stiffness were independently controllable at each joint. The tunable passive stiffness properties of the actuators provided stable, forward walking for the robot. A benefit of passive joint stiffness is energy efficiency. Results indicate that the leg may be capable of walking on a horizontal plane with its control valves off 90% of the time. The muscle-like properties of these actuators, including high strength-to-weight ratio, tunable passive stiffness, and self-limiting force output, make them well suited for legged robots.

Connected through hyperedge HE-1 (induced by source paper "Modeling of braided pneumatic actuators for robotic control"):
  with members:
    (none)

Question: Please tell me which research area the center author belongs to. The 4 classes are: Computer science, Mathematics, Engineering, Economics, Psychology, Business. Directly output the class name.

gpt: Computer science