Coaches Danny Wilson, Ioan Cunningham and Byron Hayward will put the players through the gruelling sessions before finalising the 30-man squad selection at the end of next week.
Players have the extra incentive of putting their hand up for selection for the World Cup squad which will take part in June's Junior World Cup to be staged in South Africa.
And with 10 players missing due to other commitments - including Tom Habberfield, Harry Robinson and Rhys Patchell who are on international Sevens duty - a number of players have been drafted in to be given an unexpected opportunity to stake their own claim.
Wales Under 20 Team Manager, Mark Taylor, said: "The 10 players excluded will be available for the Junior World Cup, therefore it is important that those who are selected for the Six Nations take their opportunity to push their case."
Wales Under 20s Extended Training Squad
Props: Rob Evans (Scarlets, Tom Davies (Blues), Samson Lee (Scarlets), Dan Suter (Ospreys), WillGriff John (Blues).
Hookers: Kirby Myhill (Scarlets), Jack Dando (Blues ), Darran Harris (Blues), Jac Hopkins (Cardiff), Jamie Sollis (Dragons).
Second Row: Matthew Screech (Blues), Jack Jones (Ospreys), Rhodri Hughes (Ospreys), Jonathan Gardiner (Leicester).
Back Row: Ellis Jenkins (Blues), Daniel Thomas (Scarlets), Dan Crimmins (Cardiff), Ieuan Jones (Dragons), Rueben Tucker (Blues), Dan Baker (Ospreys).
Scrum Half: Jonathan Evans (Dragons), Joshua Davies (Dragons), Jack Moates (London Irish).
Outside Half: Jordan Williams (Scarlets), Sam Davies (Ospreys), Lewis Williams (Blues).
Centres: Rheon James (Scarlets), Thomas Pascoe (Blues), Cory Allen (Blues), Nathan Edwards (Bridgend), Luke Williams (Dragons).
Wing/Full Backs: Luke Morgan (Ospreys), Iolo Evans (Scarlets), Jake Randall (Scarlets), Tom Prydie (Ospreys), Ross Jones (Ospreys).
Not considered/unavailable for selection
Second Row: Cory Hill (Blues).
Back Row: Thomas Young (Blues), Luke Hamilton (Blues).
Scrum Half: Rhodri Williams (Scarlets), Lewis Jones (Blues), Tom Habberfield (Ospreys).
Outside Half: Matthew Morgan (Ospreys), Rhys Patchell (Blues).
Wings: Eli Walker (Ospreys), Harry Robinson (Blues)