EVALUATION OF THE GRAVITY-CORRECTION FEATURE OF A KIN-COM(R) ISOKINETIC DYNAMOMETER

Background and Purpose. Torques reflecting muscle performance must be corrected for the effects of gravity if error-free ratio-scaled data a re to be obtained. Ratio-scaled data are necessary, for example, to re port muscle performance characteristics in terms of ratios or percenta ges or in the form of ratios with another muscle (eg, ratio of hamstri ng to quadriceps femoris muscle peak torque). The purpose of this stud y was to determine the error associated with the gravity-correction pr ocedure of the Kin-Com(R) dynamometer when a weight or limb segment is weighed at different lever arm positions. Methods. A known weight was suspended from the lever arm of the Kin-Com(R) dynamometer. The rotat ional component of gravitational forces on the weight was measured at 11 different angular positions. We compared the force measurement obta ined from the Kin-Com(R) with the predicted rotational component at ea ch angular position. The gravitational forces on a subject's limb segm ent (leg) were then measured at 12 different angular positions in each of two trunk positions (sitting and supine). We compared the force me asurement obtained from the Kin-Com(R) with the predicted rotational c omponent for each position. Forces measured for the two trunk position s with the lever arm horizontal were compared. Forces measured with th e knee extended were compared with forces measured with the lever arm horizontal. Results. The dynamometer accurately recorded the rotationa l component of gravitational forces for the weight suspended from the lever arm. There was a progressively greater discrepancy between measu red forces and predicted forces for the leg as the lever arm was moved from horizontal to 50 degrees below the horizontal. The force recorde d with the Kin-Com(R) was greater when the subject's knee was in full extension than when the lever arm was horizontal, which placed the sub ject's knee in a flexed position. A small difference was noted between the measured forces with the lever arm horizontal when the subject wa s sitting compared with when the subject was positioned supine. Conclu sion and Discussion. When weighing a leg with this dynamometer, we rec ommend that the lever arm be as close to the horizontal position as po ssible. Positions of proximal joints should be chosen that do not put two joint muscles in stretched positions.