Go up in joyous extremes as one exercises (4)

'one exercises' is the definition.
(I've seen this in another clue)

'go up in joyous extremes' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'in' means one lot of letters goes inside another.
'extremes' means to remove the middle letters.
'joyous' with its centre removed is 'js'.
'go' with letters rearranged gives 'og'.
'og' going within 'js' is 'JOGS'.

'as' acts as a link.