Personally, I walk and pace. I spent many years as a bartender so that activity feels natural to me and actually is relaxing. As for strength, if I can open a jar, I feel pumped enough...

This already opened jar of gherkins became impossible to open. My daughter came up with the idea of putting boiling water on the lid and then it was easy to open. I was very impressed.

Just knock the jar (lid down) against a table. Works every time.

punching into the jar's bottom with a flat hand (90 degrees from the arm, palm facing outward) has always worked for me.

Wrap a thick elastic band around the cap. The greater friction from the rubber makes it much easier.