Okay, but for example:
I am pretty overweight, but I'm in pretty good physical condition. I currently weigh 180 lbs. On this planet, I would weigh 315 lbs. That's like carrying 135 lbs of extra weight. If I'm backpacking, I carry anywhere from 25-35 lbs total, and I can "go" all day like that. I get pretty wiped out, but it's something I can adjust to, over time. I've hiked at 70 lbs, carrying equipment for other "less capable" people. That's really about my limit. This seriously cuts my hike range from about 15 miles in a day (max, really humping hard) to about 5-7. After a day like that, I'm fucking beat. And that is where I can take this pack off, and set up camp, cook, eat, sleep. I could pack 135 lbs, but I wouldn't get far, and I'd probably hurt myself trying.
When I train, I train with a pack that starts at about 10 lbs, and I ramp up over a few weeks to 40 lbs. And that is gradual enough, to avoid injuries, while building-up strength. And this increases my capacity and performance, but this takes weeks. (and as I get older, it gets much harder).
I see absolutely no way in hell I'm going to deal with an extra 135 lbs of weight, 24-hours a day. I'm not going to "build-skeletal tissue" or strengthen my body's structure. What will happen, physiologically, is I'll "survive" (minimally) maybe a day. My joints will get beat all to hell. I will be too sore to move for another few days of immobility. If I stayed on the surface, it's pretty likely I'd not recover.
I think that a "1.25 g" planet might be survivable for short periods. And this level MIGHT be enough for "physical toughness" to develop (over time), given a proper training/rest regimen, proper nutrition, and medical assistance with things like testosterone, HGH, and whatever other "black-magic" stuff that the pro athletes are taking.
I'd also tip my hat to probably the top-10% of the genetic bell-curve; those individuals who have rare, natural gifts of athletic ability, and while they are in their prime years of life, to MAYBE be able to adjust long-term to 1.5 g.