Golf has increasingly moved toward firmer surfaces primarily for putting greens but also fairways. Increasing firmness makes golf more difficult for the better players. Firm greens for example tend to be fast, are not as receptive to golf shots (less holding characteristics). Firmness increases with a decrease in soil moisture. Related to that, organic matter or thatch can reduce firmness (make greens softer) so managing or removing organic matter is important. Measuring firmness is seen with the demonstration in the video below by Karl Danneberger:
In the athletic field area, measuring surface hardness has been around much longer than measuring green firmness. In the case of athletic field hardness, measurements are used to try to correlate the potential of head injuries (example: football players hitting an artificial or natural field). Here is Pam talking about the Gmax and its use of testing firmness at Ohio Stadium:
(information from Dr. Karl Danneberger and Pam Sherratt).