Infini RF to lower back, thighs and legs. The skin darkened and isn't going away. Is it normal? (Photos)

Thank you for your question and photos, it is very helpful. I also notice some hyperpigmentation post procedure in your pictures. I would make sure to keep those areas covered and out of the sun and return to the physician who treated you to start you on a lightening regimen. Hope this helps

Hyperpigmentation occurs mainly on all areas of the body other than the face. The face has the best blood supply so dark areas occur infrequently. Treatment for this problem is time, sunscreens, and lightening agents. Please talk with your doctor about this issue.

These reaction is totally normal and expected when treating the skin on the arms or trunk. It is called post-inflammatory hyper pigmentation and goes away slowly on its own.  It will happen almost every single time. Its something you have to go through to get after stretch marks!  Rest assured, it is not a "burn" in the traditional sense. 
-CR

I first don't know exactly why these areas were treated. Was it for stretch marks? Stretch marks are hard to treat and are best treated with a combination of therapies including Infini, PRP, microneedling, and maybe Sculptra. Now you have what looks like PIH. You must protect from the sun, use SPF, Retin A, and hydroquinone. It will take weeks to get better. If not completely improved, you might need a light laser treatment with a 1064 laser. PIH is rare with Infini and is most likely the result of an error with too high an energy level at a too superficial depth. Other reasons are that the needles were pulled out too soon and the tips were still hot. This is much more likely to occur with Infini competitors which don't have insulation. Lasers are much more likely to cause this than Infini. I wouldn't treat again until the problem resolves. Make sure this is actually Infini you are being treat with as well because this is fairly rare.

Even though Lutronic says no incidence of PIH with Infini, there still is a chance. This will settle down. I assume you saw a Specialist, so they will have their own method of HQ, SPF and other methods to settle this down. It will fade, it does take 6 to 12 weeks. This can occur because 1. Treatment was superficial 2. Insulation was loss on the needles. 3. Power and PD or both too high 4. Shot fired as needles retracted. Regardless, I warn all ethnic patients that this can occur, and the PIH is less than with lasers, either way it will settle down. Davin Lim. Brisbane. Australia.