Evaluation of the Gravity-Correction Feature of a Kin-Com® Isokinetic Dynamometer

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Abstract
Background and Purpose. Torques reflecting muscle performance must be corrected for the effects of gravity if error-free ratio-scaled data are to be obtained. Ratio-scaled data are necessary, for example, to report muscle performance characteristics in terms of ratios or percentages or in the form of ratios with another muscle (eg, ratio of hamstring to quadriceps femoris muscle peak torque). The purpose of this study was to determine the error associated with the gravity-correction procedure of the Kin-Com® 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® dynamometer. The rotational component of gravitational forces on the weight was measured at 11 different angular positions. We compared the force measurement obtained from the Kin-Com® with the predicted rotational component at each angular position. The gravitational forces on a subject's limb segment (leg) were then measured at 12 different angular positions in each of two trunk positions (sitting and supine). We compared the force measurement obtained from the Kin-Com® with the predicted rotational component for each position. Forces measured for the two trunk positions with the lever arm horizontal were compared. Forces measured with the knee extended were compared with forces measured with the lever arm horizontal. Results. The dynamometer accurately recorded the rotational component of gravitational forces for the weight suspended from the lever arm. There was a progressively greater discrepancy between measured forces and predicted forces for the leg as the lever arm was moved from horizontal to 50 degrees below the horizontal. The force recorded with the Kin-Com® was greater when the subject's knee was in full extension than when the lever arm was horizontal, which placed the subject's knee in a flexed position. A small difference was noted between the measured forces with the lever arm horizontal when the subject was sitting compared with when the subject was positioned supine. Conclusion and Discussion. When weighing a leg with this dynamometer, we recommend that the lever arm be as close to the horizontal position as possible. Positions of proximal joints should be chosen that do not put two joint muscles in stretched positions.

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