Go up in joyous extremes as one exercises (4)

'one exercises' is the definition.
The definition and answer can be both acts as well as being plural nouns.
Perhaps they are linked in a way I don't understand?

'go up in joyous extremes' is the wordplay.
'up' indicates an anagram.
'in' means one lot of letters goes inside another.
'extremes' means to remove the middle letters.
'joyous' with its middle removed is 'js'.
'go' is an anagram of 'og'.
'og' put into 'js' is 'JOGS'.

'as' is the link.

Can you help me to learn more?