The best replacement for exertion is activity.

The best alternatives to exertion are rated as following:
★★★★★  activity
★  heart rate

Your alternative for exertion is not listed? Suggest!
Speaking about exertion, one probably shouldn't forget about shortness of breath.

Other search terms related to exertion and their ratings are:
★★★★★ shortness of breath
★ chest pain
You have in mind a companion for exertion and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of exertion in all years. Click on graph for more.
exertion
activity
heart rate