The effects of simulated knee-flexion contractures on standing balance.

The effects of simulated unilateral and bilateral knee-flexion contractures on standing balance were studied by testing 15 normal subjects on a Kistler force platform. Postural sway (mediolateral and anteroposterior travel) and the mean position of the center of pressure (as a percentage of the distance between the midlines of the feet and from heels to toes) were determined from 20 s of data. Unilateral and bilateral knee-flexion contractures of 15 degrees and 30 degrees were simulated for each subject by means of an adjustable line from the subject's waist to the sole of each foot. Paired t tests were used to compare balance parameters while standing with the simulated contractures with those during relaxed standing. Mediolateral travel increased by a mean difference of 3.6 cm with a 30 degree unilateral contracture (P less than 0.01) and by 5.0 cm with 30 degrees bilateral contractures (P less than 0.01). Anteroposterior travel increased by 4.7 cm (P less than 0.05) and 8.8 cm (P = 0.08) with 15 degrees and 30 degrees bilateral contractures, respectively. With a unilateral contracture of 30 degrees, the center of pressure shifted 15.6% (P less than 0.0005) toward the unflexed side, changes that were not eliminated by correction of the induced leg-length discrepancy. The center of pressure moved anteriorly by 8.3% with 30 degrees bilateral contractures (P less than 0.001). The results provide insight into how knee-flexion contractures alter standing balance, and underline the importance of preventing and treating this common disorder.