Musculoskeletal disorders, including lower back pain, denote health problems of the locomotor apparatus and are the leading cause of disability, causing an increasing health care expenditure. To reduce the risk of suffering lower back pain, physical activity remains the best treatment. However, exercising requires awareness, understanding, and commitment to achieve the intended results. Current practices rely on illustrations, videos, and more recently, motion capture animation to convey how the exercises should be performed. In this paper, we present the development of a mobile app prototype to demonstrate proper lumbar-spine exercising. The interface is comprised of a physical interaction component with a humanoid-shaped avatar and a picture frame to compare postures.