Jürgen Klopp watched - and cheered - on from the side as Virgil van Dijk and Georginio Wijnaldum conducted their lactate test amid the sunshine at Melwood on Sunday.

The exercise involves running around a track at increasing speeds, with blood samples taken from the ear at intervals to test the heart rate and blood lactate, which is an indicator of endurance levels.

Having returned to begin pre-season today, Van Dijk and Wijnaldum completed the task in high temperatures at the training ground - and with the support of Klopp and his backroom staff nearby.

Take a look in our photo gallery below.

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Virgil van Dijk and Georginio Wijnaldum train at Melwood

Photos: Van Dijk and Wijnaldum return to Melwood