G5 massage machines have been sold and used successfully for over 50 years. They provide very deep penetrating massage using frequencies from 15 to 55 CPS. G5 massage machines are the only massagers with Directional Stroking, not vibration. They are reliable, durable and quiet. Using G5 massage machines over a hot pack or cold pack seems to drive in the heat or cold. They can be equally-effective when used over toweling or clothing, and won't tangle in hair.