The first is the requirement that in any force-displacement measurement platform the stiffness of the machine (formula_63) must approximately match the stiffness of the sample (formula_64), at least in order of magnitude. If formula_63 is too high, then the indenter probe will simply run through the sample without being able to measure the force. On the other hand, if formula_63 is too low, then the probe simply will not indent into the sample, and no reading of the probe displacement can be made. For samples that are very soft, the first of these two possibilities is likely.