I previously used a groin protector in kickboxing but that still hurt when you take a kick.
Most groin protectors will not work in Krav Maga, which is very heavy groin-kick oriented. What makes a good groin protector?
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I've been training in (and recently teaching) Krav Maga for a while, and groin kicks are a regular occurrence. From my experience, it really doesn't matter all that much which cup you use, as long as you check it regularly, it shouldn't break spontaneously. Cups will usually crack first. Even if you wear a cup, it's still gonna hurt, you just have to get used to it. The difference is whether or not there is any real damage being done.
There are a few things to make sure of to eliminate the risk of injury:
Besides that, just make sure that whatever you buy has good reviews. And get used to the idea of getting kicked hard in the groin. It's gonna happen a lot, but as long as you are wearing proper groin protection, and you check it regularly, it shouldn't do any real damage.
I use Shock Jock compression shorts with the added cup. How would it handle a full force knee to the groin? No idea, just as I have no idea how well my mouth guard would handle a full force elbow to my chin. It does handle the level of force used in my training, and I have caught some pretty good shots.
Muay Thai steel cups are the only way to go. Anything else has the potential (small, but still potential) of breaking when hit hard enough, and worst case scenario you get your foreskin ripped along with other such unpleasantness. They're also almost always too narrow, so it's easy to get one of your testicles caught between the cup and your thigh, which is actually more painful than just being hit.
If you don't like the 'g-string' aspect of Thai cups, Ring to Cage makes a supporter that goes with a steel cup that should also work fine (I haven't tried it, I wear my cup when I expect the intensity to be high so I don't notice any minor discomfort, so the traditional design doesn't bother me).