Correction for gravity in isokinetic dynamometry of knee extensors in below knee amputees

In isokinetic dynamometry of the knee extensors the influence of gravity on the lower limb is often neglected. The aim of this study was to assess the shape, the quantity and reproducibility of the gravity correction curve (GCC). Eight amputees participated in this study. The GCC was produced on an isokinetic dynamometer (QD) throughout the range of motion on the amputated side with prosthesis and on the non-amputated side. The results were compared with the GCC calculated according to the Cybex data reduction computer procedure. The isokinetic torque registration of the QD is, in a mathematical way, a straight line (Pearson correlation coefficient greater than 0.99). The other GCC is shaped like a cosinus curve. The intra-individual reproducibility is high, the inter-individual variability substantial. An explanation for the difference between the two methods is discussed. For an ideal measurement of the muscular torque, one should not only correct for the effect of gravity but for all factors counteracting the effect of the knee extensors. In that case an individual GCC should be made directly with the dynamometer.