system
Given the following post, respond as if you're commenting on Reddit:
human
What is going on with heat creams such as Deep Heat?

How do the active ingredients generate that sensation, and if it is simply a trigger to the heat/pain receptors, how does it relieve muscle tension?

Does the cream penetrate the upper skin layers in any meaningful way? Are there any physiological differences that can change the effectiveness (for example, if you are hot and actively sweating would the effect of the cream be more or less?).  Just some questions that occurred to me last night.
gpt
They trigger receptors. 

Most rubs use menthol for cooling and methyl salicylate for heating. Menthol triggers TRPM8 receptors (cold sensitive ones) and methyl salicylate acts as a rubefacient and irritates the skin.

I have no clue how methyl salicylate irritates the skin though.

Edit- Also, menthol wouldn't cool you off. It would stop you from sweating, but your actual body temperature wouldn't decrease.