Six degree of freedom joint power in stair climbing

In the past, joint power has most often been calculated as the product of the sagittal plane joint moment and the sagittal plane joint angular velocity, thus modelling the joint as a simple one degree of freedom (DOF) hinge. More recently three DOF power has been calculated by taking the scalar product of the net joint moment and the angular velocity of the joint, thus modelling the joint as a ball and socket joint. We introduce a six DOF approach for calculating joint power, an approach which allows three degrees of rotational freedom, as well as three degrees of translational freedom, thus implementing a rigorous definition of true mechanical power. We established that for the hip joint during stair ascent, three DOF power was significantly greater than six DOF power (by as much as 60 W/kg), while for stair descent, one DOF power was significantly less than six DOF power (by up to 45 W/kg). On the basis of the total work done in raising the body up a set of stairs (weight × height), the six DOF approach provided more accurate results than either the one or three DOF models. We also showed that six DOF power data were as repeatable as joint moments, with variance ratios between 0.13 and 0.20. While these findings are certainly not the definitive word, they do offer some guidance regarding the effect that certain assumptions have when calculating joint power in three dimensions.