Sub-bandage dynamics: stiffness unravelled

The static stiffness index (SSI) is mathematical equation that results in a simple number when the sub-bandage pressure in the supine position is subtracted from the subbandage pressure in the standing weight-bearing position. When SSI data are reported, often a wide range of values is observed for similar materials. The aim of this study was to explore the strength and weakness of the SSI and its measurement. Pressure was recorded with bandaging materials with different resting pressures and properties. Measurements in the upright position were performed under weight and non-weight bearing conditions for up to 12 min of motionless stance. The measurements reveal that the SSI reveals more about the muscle forces of the person included in the system, rather than providing accurate information on the applied system or how well this system is applied. In addition, venous filling has a major effect on the final SSI. When performed under similar conditions, the SSI is able to differentiate between elastic and inelastic materials. The SSI gives us a rough estimate of the effectiveness of an applied system but interpretation is influenced by the muscle forces of the person being bandaged as well as the measured effects of venous filling and, because of that, the timing of the measurements. Future guidelines on measuring the SSI should include that the final standing pressure value should be taken when a stable recording over a certain period is observed.