Pardon the drama, but it has been a rough morning...
Hot? HOT? HOT! FIRE!!!
When someone's using a relatively new heating pad (like for a sore back), one doesn't expect it to get really hot and BURST INTO FLAMES.
So, I'm sitting there -- ahh, this feels good. Hmmm, getting too hot, I'll turn it down. All of the sudden it gets really hot and HOTTER! Ouch! Then FIRE!
I ripped the cord out of the wall and it ran to the kitchen sink and watered it down. The whole pad arced and began burning the rubber enclosure and the nylon cover with lots of smoke and noxious fumes. If it was unattended, it could have started a significant fire.
Luckily I noticed the excessive heat and got up to look at it, so no burns.
In conclusion, the back strain: worse.