Applying ice packs to the upper back may stimulate brown fat activation by signaling cold to the hypothalamus.

“If you put ice right there where your spinal cord is close to the surface, that's where you're going to hit the vertebral arteries. So you're essentially putting a cold source into the brain, to the hypothalamus. The hypothalamus is you're too cold. So it is going to turn on shivering and brown fat so.”