The forces and moments at the ankle, knee and hip joints of the human lower limbs are divided into static and inertial components. They are calculated for various activities ranging from slow walking to running. The relative roles of these two components in the ‘total’ joint loads are studied, and the limitations of using a quasi-static analysis approach for joint load approximation are discussed. The results indicate that the static loads only reflect the gravitational and external reactions between the body and the environment, whereas the inertial loads provide dynamic information on each body segment involved. The effect of the inertial forces and moments becomes more important as the speed of locomotion increases; where the more proximal joints in the human lower extremity are concerned; and where the shear components of the force and moment are of interest. On the other hand, it seems that most of the joint moments in the lower extremity during walking and even running could reasonably be approximated by static components.