Steven V. Fisher

Sitting forces and wheelchair mechanics

The effects of back angle and leg height on sitting forces in a wheelchair were studied, using a force plate mounted on a wheelchair seat. Readings of both normal force (perpendicular to the seat) and shear force were measured while the chairs back angle and footrest height were changed. Pressure under the ischial tuberosities was also measured during the footrest height adjustments. Five normal subjects sat directly on the plate as well as upon ROHO and Jay cushions placed on the force plate. Returning the back to the upright position after a recline caused the normal force (+/- SD) to increase 5.4 +/- 2.5, 9.5 +/- 4.0, and 10.0 +/- 2.3 kg for the hard surface, Jay cushion, and ROHO cushion respectively, while shear at the plate increased to 5.1 +/- 2.2, 11.6 +/- 2.6, and 12.3 +/- 2.7 kg for the hard surface, Jay cushion, and ROHO cushion respectively. Leaning forward (away from the back) caused all the forces to return to measurements close to the starting values. The results suggest that the wheelchair user should momentarily lean forward after a recline to reduce undesired forces. If a cushion with firm thigh support is used, ischial tuberosity pressure can be reduced by lowering the leg height as much as possible, which causes a levering action by lifting the pelvis.

Thirty-minute continuous sitting force measurements with different support surfaces in the spinal cord injured and able-bodied.

Able-bodied, paraplegic, and quadriplegic subjects sat for 30-minute intervals on various surfaces in a wheelchair with a forceplate mounted on the seat in order to determine factors that could contribute to the formation of decubitus ulcers. All three groups of subjects sat on ROHO and Jay cushions; in addition to sitting on the two cushions, the able-bodied subjects sat on a hard surface. Factors studied were: normal and shear seat forces, the location of the center of mass, and armrest force. The forceplate was under the cushions; therefore, the values reflect average forces over the buttocks and posterior thighs. These factors were compared between disability levels as well as between surface types. Larger, normal, and forward shear forces and a more anterior position of the center of mass were observed with the ROHO cushion. More frequent and larger lateral weight shifts occurred with the Jay cushion. The armrests tended to support from 5 percent of the body weight for quadriplegics to 9 percent for paraplegics. The results suggest that armrests reduce seat forces by carrying some of the body weight.

The Accuracy of Electrical Transducers for the Measurement of Pressure Applied to the Skin

The accuracies of five commercially available small flat pressure transducers (~5 mm ×~1 mm) were evaluated for their ability to measure pressures applied to the body surface. The diaphragms of the transducer were loaded with rubber and cloth surfaces in an in vitro test and under a blood pressure cuff over the gastrocnemius muscle, 2 cm on the medial and lateral sides of the tibia and on the tibial crest of the leg. These areas offer varying amounts of underlying tissue compliance. The cuff was pumped up to 200 mmHg in 20 mmHg increments. The average error obtained with the transducers zeroed in air before placement on the leg varied from 20.4 to 64.9 mmHg depending on the hardness of the underlying surface. The averaged error obtained with the transducer zeroed after placing on the leg ranged from 9.5 to 47.9 mmHg.