Rising from a chair by a spring-loaded flap seat: a biomechanical analysis.

UNLABELLED Nine healthy male subjects and 8 patients with unilateral knee osteoarthrosis were studied while rising from a chair with and without the aid of a spring-loaded flap seat. The seat force started at 216 N and decreased with increasing seat angle. Ground reaction forces and motion were recorded using a force plate and video. EMG of the left vastus lateralis muscle was recorded. The effort when rising was estimated by the Borg scale. The use of the flap seat reduced the mean peak knee moment for the healthy subjects from 73 to 41 Nm and for the patients from 55 to 33 Nm. The decreases were significant at a p < 0.001 level. The patients also reduced their hip moment significantly from 50 to 35 Nm (p < 0.005). The muscle activity in the vastus lateralis was also significantly lower when the flap seat was used (p < 0.005). When the theoretically maximal friction force between seat and subject was added, the effective seat force decreased more slowly, and the relation between its horizontal and vertical force components was more advantageous. The patients estimated greater effort reduction than the healthy subjects comparing the ordinary chair and the flap seat. CONCLUSION a spring-loaded flap seat can reduce knee and hip load and can thus be useful for people with knee osteoarthrosis.