Optimization of the biomechanical design of plate-loaded strength training machines: The free-weight lifting experience

We established a set of analytical equations that constitute the biomechanical framework for the optimal design of plateloaded strength training machines. Specifically, we assessed the effect on the exercise kinetics of a change in the distance dP between the site P of the resistance lever where the weight plates are loaded and the axis of rotation of the lever. To this end, the distance dP was increased, while keeping the value of the resistance torque tR unchanged by a simultaneous decrease in the mass mP of plates loaded on the lever (dPmP=const). A progressive increase in dP (under the condition dPmP=const) yielded a sharp decrease, followed by a steady increase, in the moment of inertia of the loaded resistance lever (relative to its axis of rotation). The impact of this change on the kinetic effects related to the inertia of the moving equipment masses (inertial effects) has been discussed for maximal and explosive exercises, and for sub-maximal exercises executed at controlled cadence. We also detected a specific value of dP for which the torque related to the inertial effects, expressed as a percentage of tR, turns out to be independent of the selected level of external resistance. This condition precisely reflects the linear-dynamic condition that occurs when lifting free weights.