Method-related variations in estimates of gravity correction values using electromechanical dynamometry: a knee extension study

Dynamometry is widely used to measure subject strength. The method employed to correct dynamometry scores for gravitational influences can result in differing correction estimates. This study investigates differences between mathematical estimates of correction values and directly measured passive forces. Using the Kin-Com dynamometer, passive force measurements from 90 degrees of knee flexion to full extension were collected for nine asymptomatic subjects. These measurements were then compared with correction estimates mathematically extrapolated from a force reading obtained at one point in the test range. Direct passive measurements obtained between 0 and 70 degrees of knee flexion and mathematical estimates of correction values differed by as much as 50 N. The equivalence of gravity correction values obtained using the two methods detailed cannot be assumed. Mathematical estimates of correction values for knee scores obtained between 0 and 90 degrees of flexion were found to be clinically identical to direct passive measurements when: 1) the limb was weighed close to 50 degrees of flexion and 2) the angular location of the lower limb mass relative to the horizontal was not assumed to be represented by the angular location of the lever arm, but rather 15 degrees further below the horizontal than the lever arm.