How have facial sign vehicles been measured?
Three important methods for measuring the rapid sign vehicles of the face exist:

• Ekman and Friesen's Facial Action Coding System (FACS) and other techniques like it,
• Electromyography (EMG),
• Computation.

Notice an important implicit fact for these methods. As later answers discuss, both FACS and EMG are intimately tied to muscular action, but the computational method lacks an inherent connection to muscular action. Therefore, this link must be built in during development of the computational method, if the objective is to measure rapid sign vehicles relevant to emotion expression or other similar messages. In other words, if you are developing a system that measures facial behaviors or their messages, you must apply validation procedures to show that your measurements are actually measuring the relevant sign vehicles.