Throughout history, engineers have used actuators to move objects providing rotary or linear motion.  A rotary actuator is simply a gearing system that either increases or decreases the rotational speed of a prime mover, typically a hydraulic motor, an internal combustion engine, a turbine engine, or an electric motor, to provide a desired level of rotational speed and torque at an output.  Examples of rotary actuators include: gearboxes, transmissions, differentials, Rotac® actuators, and rotary electro-mechanical actuators.  Linear actuators are machines designed to provide force and linear displacement to an object.  Some examples of linear actuators include: rack & pinion actuators, hydraulic rams, ball screw actuators, and crank arm actuators. 
Historically, hydraulic/pneumatic motors and hydraulic/pneumatic rams have been the primary source of power for both linear and rotary actuators.  Hydraulic systems offer many advantages to the designer including: high power density, accurate position control, low inertia (for high frequency response), and overload protection (via pressure relief valves). 
More recently, engineers have replaced hydraulic/pneumatic actuation systems with electro-mechanical actuation systems.  Electro-mechanical actuators (“EMA”), which typically include a motor, a gear box and an actuator, offer increased efficiency over their hydraulic and pneumatic counterparts and are less prone to leakage. 
When designing small, high power density EMAs, a designer is faced with a problem caused by the rotational inertia associated with the EMAs electric motor.  In order to create an EMA with a large force capability, the designer must create an electric motor that is capable of producing a large torque, or must create a gear train that reduces the motor's output torque requirement.  If the designer chooses to create a motor with a large torque capability, its rotor will contain a significant amount of rotational inertia.  If the designer chooses to utilize a gear reduction system to decrease the motor's output torque requirement, thereby reducing the motor's physical size and rotational inertia, the motor will be required to operate at a faster speed.  The inertia of the motor, as felt by the output of the actuator, will be proportional to the motor's inertia multiplied by the gear reduction ratio squared. 
The inertia of the EMA motor becomes extremely important when sizing the gear train and/or the actuator structure if, for instance, the actuator hits an internal stop at full speed, or if the actuated structure hits a stop at the end of its travel at full speed.  In this scenario, the rotational inertia of the motor will tend to cause the actuator to continue driving through its stop, or through the structure's end stop, causing significant damage to the EMA, or its supporting structure.  If the stops and structures are strong enough to maintain their integrity, the next weakest link, most likely the actuator or the gear train driving the actuator will be damaged. 
Historically, the gear train and the EMA's stops are overbuilt to handle an intense torque spike associated with the rapid deceleration of the EMA's motor as the actuator hits its stops, and the internal shafting flexes as the motor spins down.  This design approach tends to cause the actuator to become significantly larger and heavier than it would otherwise have to be. 
Another method to handle the scenario described above is to incorporate a slip clutch in the driveline between the EMA's motor and the EMA's output.  Incorporating a slip clutch in the driveline allows the EMA's output to nearly instantaneously stop, while the motor decelerates, with the stored energy of the rotating motor rotor being absorbed by the slip clutch's friction material.  This type of system works well, however, it again adds components to the EMA that add size, cost, weight, and reduce the actuator's overall reliability.