Prolonged sitting has become an inherent characteristic of modern occupational and domestic activities, yet it is increasingly associated with musculoskeletal discomfort and chronic low back pain. Traditional seating ergonomics has long promoted the 90° hip–knee posture as an optimal sitting position; however, emerging evidence challenges its biomechanical suitability. This paper presents a theoretical and ergonomic review of seating and sitting postures, anchored on Mandal’s Homo Sedens concept. The review examines the biomechanical limitations of conventional sitting, the physiological implications of static postures, and the ergonomic significance of forward-inclined and dynamic seating configurations. In addition, the paper incorporates a comparative postural framework illustrated in Figure 1, which delineates three primary sitting alignments: the active ergonomic sit, the passive reclined sit (110° – 135° trunk–thigh angle), and the C-curve slump posture. The figure highlights their respective impacts on spinal curvature, pelvic orientation, and load distribution. While the neutral S-curve alignment promotes balanced muscular engagement and reduced disc pressure, the slumped C-curve posture is associated with thoracolumbar flexion, cervical strain, and increased mechanical stress on intervertebral structures. By synthesising theoretical principles, ergonomic modelling, and visual analysis, this study underscores the importance of posture variability, spinal alignment, and workstation integration in contemporary seating design. The paper concludes that ergonomically sound seating must prioritize movement, adaptability, and spinal health rather than rigid adherence to fixed postural angles.