Border Collies, like many larger breeds of dogs, are prone to hip dysplasia. This is a painful and sometimes crippling problem in which the bones in the hips don't move together properly. There is some evidence that it is genetic; there is apparently more and more evidence that environment plays a significant part in whether the dog develops the problem. There is corrective surgery that can sometimes be done, but we don't want to go there.
The real challenge is that there's no way to tell whether a dog is going to get it until either they get it or until fairly definitive x-rays at 2 years old reveal a problem. Earlier than that, and the dogs' bones are still growing and changing, and it's impossible to tell reliably what the future holds.
At two years of age, when the dog can be certified by the Orthopedic Foundation of America (OFA), it receives an OFA rating of excellent, good, or fair for nondysplastic joints, borderline for iffy joints, and mild, moderate, or severe for dysplastic joints. Boost's mother is rated Excellent; her father Fair. But, since environment is believed to be such a large part of it, this is only a foundation on which to build.
This article talks about some of the research and evidence for the causes of hip dysplasia in border collies. There's lots more info out there, too. Everyone I've talked to and everything I've read (in the last month or so--never was relevant to me before) emphasizes keeping the puppy thin but not scrawny; this is OK because that's what I try to do with my other dogs, anyway, like you'd do with any running and jumping athlete. It also emphasizes not overexercising the dogs. Like, don't do any serious athletic training until the dog is at least a year old. Like, if you play tug-of-war, do it gently, don't let the dog really put all its energy into it.
Because I've always bought breed-specific books when I've had a breed-specific dog, I bought a very recently published book on Border Collies. This recommends, among other things, that you not let your puppy play with your adult dogs until it's at least a year old so that it doesnt' run or wrestle or play too hard, and--here's where I'm thinking I'm drawing the line--don't let your puppy go up and down stairs until it's at least a year old! Ha! We have to go up and down stairs to get to the yard, to get to the car, to get to the bedroom, to get between my office and the kitchen, which are the main places the puppy is allowed.
The puppy has put on 2.5 pounds in a week and is now 17 pounds--they've got to be kidding that I should be carrying this dog up and down staircases several times a day from now until next January! You know this crate that I borrowed from a friend 2 days ago in which she could easily sit up and also lie down spread out--I could swear that she's already starting to stoop when sitting up and her feet are hitting the far side of the crate when she lies down!
I've read that it has been demonstrated that human teens and preteens really do indeed grow an inch or two literally overnight--everyone always joked about it, but apparently it's true, accounting for why teenagers often feel so gangly and uncoordinated as if they haven't gotten used to their body: they haven't! If Boost is really supposed to double in size in the next month, she's got a lot of growing to do every day.