Exercise ring won’t close

The green, exercise ring won’t close in my Apple Watch (4th Generation). It is up to date, & I also “reset” it by holding in the power button as well as the crown until the Apple Logo appeared.

I read some other questions similar to this, that have been answered. It says that a person must participate in a “brisk walk or higher,” but doesn’t seem to go into specifics regarding that. What exactly would be considered “a brisk walk or higher?”

Would there be any way to change the BPM that an Apple Watch must detect in order for something to be considered a work out?

Thank you!