By immobile I mean unable to perform activities of daily living, or maybe walk a quarter of a mile.
FWIW I’m not talking about the negative metabolic effects of obesity. I know those set in at a much lower weight than immobility sets in. Also I’m not talking about joint pain that doesn’t prevent activity. Lots of people have joint pain but are still active and independent.
I get the impression it depends heavily on factors such as age, gender, height and activity levels. A 500 pound 6’10 powerlifter in his 20s is going to be totally different than a sedentary female in her 60s who is 5’1 who is also 500 pounds.
From what I’ve seen, immobility seems to set in in the 400-500 pound range. But I don’t know if that is due to issues with muscle/joints/bones, or if it is more due to the fact that people who get that large are not really active. I once read for every 2-3 pounds of fat your body gains, you gain an additional 1 lb of skeletal muscle to carry it around. I don’t know if that remains true at extremely high weights (ie, I doubt you gain 100 lbs of skeletal muscle to move around 300 lbs of bodyfat, as the difference between normal people and professional bodybuilders is about 100 lbs of skeletal muscle. And the super obese are not as strong as bodybuilders).
Even on the show my 600 lb life, there are wild variances in how active people can be. But it seems to depend on age and gender (men are more mobile than women, likely due to more skeletal muscle).