There are mechanical systems that mainly consist of masses; that do not particularly suffer from effects of compliance and damping (except when it comes to instability). The response of these systems to effort-command changes could be called “inertial response.” Effort-actuated inertial loads—as opposed to the kinematically constrained ones—are discussed regarding their acceleration capability upon sudden command change and their velocity matching to the actuator particulars, together with the characteristics and selection of fast response drives, hydraulic and electric.