Virgil Van Dijk, Trent Alexander-Arnold and Joel Matip were involved as Liverpool had their first day of pre-season training in Austria on Monday.
The trio of defenders were among the players undertaking lactate testing, a drill measuring endurance levels with a series of runs, on day one of the pre-season camp on the outskirts of Salzburg, the club said.
Van Dijk and Matip were sidelined last season after sustaining injuries in October and January respectively, while Alexander-Arnold sat out England's Euro 2020 campaign with a thigh problem.
Joe Gomez, another defender who has been on Liverpool's long-term injured list, is also part of the group that have travelled to Austria.
The Reds have confirmed they are set to have four matches during the camp.
On July 20 they are to play 30-minute 'mini-games' behind closed doors against FC Wacker Innsbruck and Stuttgart.
Three days later they will take on boss Jurgen Klopp's old club Mainz in the Greisbergers Betten-Arena, with a 3,500 capacity.
Then on July 29 they will play Hertha Berlin at the Tivoli Stadium, with a 15,000 capacity.
