I’m somewhat confused about that myself. I’m still searching for more information on that. Exertion can significantly increase the pounds of pressure but I don’t know how to calculate what I need to do to reach 4.2x with my body weight. I did find a supporting article in medical news today but it doesn’t give me the details I need to calculate.

It’s not a typo and yes it is a lot. The original poster, cabahagirl, mentioned OsteoStrong. You may want to check out their website—lots of useful info. There isn’t an OsteoStrong within 2 hrs of my home so I’m not able to sign up for their program but I think they are definitely on to something. I have visited ann OsteoStrong and, basically, they use 4 pieces of osteogenic loading equipment. Based on the goals OsteStrong sets for members my takeaway was that the weight needed for your upper body to build bone is different from the weight needed for you lower body to build bone. When you are using their leg equipment (similar to gym leg press equipment but there are no moving weights, you just apply the force) they try to get you to hit a higher number than when you are using the equipment that you push with you hands (hope that makes sense). Also with that 4.2 times your body weight remember that when you are standing your body is carrying 1 x your body weight and different activities can increase that amount without the need to actually add more weight. For instance, jumping creates more force on your bones and can help stimulate bone growth. Also some sports are “better” than others at building bone. Swimming will not help your bones directly. As far as bone health is concerned running is better than swimming or biking for increasing one density. Additionally, studies show that socccer players have greater bone density than runners—the thought is that running is super-repetitive so it doesn’t challenge your muscles/bones enough, whereas with soccer there are lots of short brisk movements and sharp turns and stops that cause muscles to “tug” on your bones more and thus, stimulate bone growth. You might find this link very interesting and helpful https://americanbonehealth.org/exercise/bonesense-on-osteogenic-loading/

So you weigh 123 pounds. Nice. Our 4.2 is right, but thank you for catching LIFT! With osteoporosis we do NOT LIFT that weight. Both yoga for osteoporosis and OSTEOstrong have us use safe PRESSURE to stimulate bones--with excellent back posture. Yoga has us hold a posture--with a straight back--that is a stretch or twist to pressure/stimulate bones. Always in control. OSTEOstrong has machines with handles or plates that we press on with our arms or legs at a precise angle. And the machine measures the pounds of pressure. My first visit, I pressed 630 pounds with my legs (4.8%+). My arms and core did only 1.5-2.6. Yes, do not lift.

That is incorrect. As soon as you move a force that is equivalent to 4.2 x BW you have exceeded 4.2 x your BW. To lift 520 lbs would require more force than 4.2 x your BW. How much more? It depends on how fast you're moving it (and what planet you're lifting it on but let's assume you've chosen Earth). Squat force calculators estimate the force to be 2834 Newtons which is approximately 637 pounds. So you've overshot and are actually lifting 4.9 x your body weight.
At your body weight you would only need to lift about 403 lbs to create a force equivalent to 4.2 x BW. However, this would not create the correct load because the direction of that force would not necessarily be ideal to achieve your goal (creating bone growth).
Osteogenic loading is based on isometric holding (not lifting) a force equivalent to 4.2 x your BW as measured at the hip. How do you get 4.2 x BW to vertically load into the hip? 1. You can't be bending the hip at the time because that would direct mass in the wrong direction and not simulate bone growth. 2. You can't be moving the weight because that causes more than 4.2 x BW force to be imparted to the hip. 3. Ideally you would stand still and have someone place that 520 lbs on your shoulders. Personally I would suggest finding a resistance machine that does isometric exercises. These are exercises where you push against a plate that doesn't move but the force plate measures how hard you are pushing. (FYI, there is a chain out there providing this type of exercise and physical therapy clinics sometimes have these isometric machines).
For context, I can easily generate a force greater than 8 times my body weight (134 lbs) at the hip but I cannot do a squat with a 45 lb bar. I am leg pressing 200-240 lbs for 8-10 reps after 6 weeks of isometric training. Before starting this training my leg press was limited to 140 lbs.