Years ago I read a book titled Healing is Voltage. Basic premise, the earth gets struck by lightening 8 million times a day, average strike of 15 million volts. Supposedly, barefoot contact with dirt,grass, untreated concrete ( wet surface is better ) increases a persons millivolts, which is supposed to be good for you? Easy and free so most of my training is barefoot out in the dirt. Heavy squats indoors? Been awhile, but would probably have some Chucks or Vans on.