Green Exercise ring

To earn exercise or workout credit you must move at the speed of a brisk walk.

To earn 1 minute of exercise credit(green ring);

Travel a full minute at a brisk walking pace 3.3 mph or 18 minutes per mile approx.or 5.3kph or 11.3 minutes per kilometer.

Raise your heart rate to approx 75% of your maximum.

When you're at around or above this heart rate and speed for the entire minute you'll receive 1 exercise minute credit.

If you pause or slow down during this minute then you won't get any exercise credit for that minute,it will still measure your steps and calories burnt but will not give you any credit towards the exercise ring.

Source;Successful phone call I had with apple support.

Here is a better idea for you;

Choose "Other workout type.With this workout you earn the credit equivalent of a "brisk walk" which means 1 minute credit for every 1 minute in the workout regardless of how fast or how slow you walk,this is particulary useful for anyone that can't walk at the brisk walking pace of 3.3 mph or are recovering from injury or are an older person who just can't reach the "brisk walking" speed.

So if you do 30 minutes in the "other" workout then you will close your green exercise ring.Active calories are also recorded and estimated at a rate equivalent to a "brisk walk" or based on data recorded by the heart rate sensor in the watch, whichever is higher.