In the Halo universe it has never been clear to me how strong a SPARTAN-II truly is, going from not killing a human by punching it in game, to flipping a 66 ton tank, or simply having gun recoil, so, how strong is a Spartan lore-wise?
I can't manage to find numerical data trustworthy enough to do precise calculations.
Anyway, the soldiers involved in the SPARTAN-II program were originally 75 children selected based on genetic predispositions. A genetic enhancement process was then conducted on them, increasing their physical and mental abilities even more. The 35 of them who survived the program were given Mjolnir armors, which provide an additional mechanical increase in strength.
To sum it up, the 5 remaining post-war remaining SPARTAN-II are stronger than usual people that were genetically modified and intensively trained during most of their childhood and teen years before being stuck in assisted armors.
So they are as strong as can a GMO ubersoldier with robot limbs can get.
Numerical data is more than welcome, even though I consider the canon fact that a SPARTAN-II can single-handedly flip a Warthog as relevant enough.
Champion squat lifters can lift in excess of 1000 pounds with the current real world record at 1,268 pounds. Considering the modifications done to Spartans bodies they can probably lift as much or more with less effort. However their build is considerably average looking, not as heavily built as professional lifters, especially the female Spartans. Never the less, with augmented muscles added to their bodies at childhood and fortified bones, it's safe to say a Spartan can probably bench press well over pounds or more without tearing ligaments or herniating something. John-117 was seen lifting 495 lbs by age 14 after his augmentations.