Posture and postural support are therefore extremely important factors when considering rider comfort. The spinal column supports the upper extremities and compresses under these loads. The loads are transmitted through the spine to the gluteus maximus where continuous pressure can lead to decubitus ulcers. In addition, these compressive loads induce moments, and these moments increase as postural deviation increases. These moments can cause spinal deformities.
Seat supports can reduce and redistribute pressure as well as minimize postural deviation.
Dynamic wheelchair applications incorporate accelerations and cyclic loading compounding the existing static loads. For example, wheelchair motion as the result of user propulsion, or propulsion from external forces such as automobile transportation, are illustrations of dynamic situations. During an acceleration, the spinal column acts as a shock absorber, an energy absorber, and a transmission couple for vertical forces. Vibration, especially vibration that is near the first human resonance frequency of 5 Hz, can lead to spinal deformities, herniated discs, and chronic back pain over time. It is important to design wheelchair stiffness and damping characteristics to minimize vibration transmission.