The Journal of the Human Factors and Ergonomics Society has published a study on anti-fatigue mats, and their potential to relieve standing related soreness.
In jobs where standing up for long periods is a requirement, muscle fatigue can become a serious problem. Working as a cashier or bartender are two examples of professional roles that require long periods of standing. Many people work in retail and hospitality, so there's a big potential for anti-fatigue mats to help people.
The study, as reported on the Kelby Ergo Design blog, measured the effects of four different types of anti-fatigue mat. Subjects stood on each mat for a four-hour period. Three out of four anti-fatigue mats reduced test subject's discomfort.
Between the three effective mats, there was no tangible difference in the level of comfort they provided. Weight sensors under the test mats indicated that subjects shifted their weight less while standing on them. Shifting body weight is an indicator of muscle and joint discomfort.
The mat which produced no effect was the softest in the test. In conclusion, the study proved that soft mats are ineffective because they can "bottom out" with pressure over time, creating a surface as hard as the floor.