Also, according to Nick Curson (coach of UFC Champion Rafael Dos Anjos) on the Joe Rogan podcast, heavy slow weight lifting is counterproductive for most sports [1] [2]
Please take a look at Gus, a woman in her 90s who has recently started to train at Rip's gym and has improved her quality of life such that she no longer requires her walker.
It's hard to hurt yourself if you truly read the book, because if you start with an empty bar and only add small amounts of weight using the proper form (as the book instructs) it's tough to hurt yourself.
If anyone needs some help with barbell form you can drop into the forums where coaches will gladly review a video for you, or if necessary find someone to help you in person.
And before I fall down the rabbit hole of repeating Rip's gospel word for word, what a coach says they do for an elite athlete has little bearing on the general population. Elite athelets are genetic freaks that could be trained in many ways and still achieve their results.
Lifting heavy is not going to slow you down or make you big, it's going to get you strong. Training for strength and practicing your sport of choice will just make you a stronger version of your previous self. If you train and eat like a power lifter you are not going to be able to maintain your 3:30 marathon time, but if you add in properly programmed strength training for the goals you are looking to achieve, it is petty hard to argue that being stronger is worse than being weaker all other things being equal.
Then again, too much strength competes with balance and flexibility.
If you can move through the full range of motion of the movement you are flexible enough.
Being able to squat above your body weight is going to make you a more useful human being than putting your leg behind your head.
I don't think strength is bad, but: you only have so many training hours and you might want to spend some of them on balance or flexibility instead. Further, it's possible to have such a volume of muscle that you cannot achieve certain positions (e.g. trying to touch your elbows together leaves an 8 inch gap). And it's also possible for your muscles to weigh so much that you can't move as gracefully because equilibrium is harder to maintain. That's all I meant. Ask any dancer, yogi, acrobat, gymnast, skier, martial artist, figure skater, etc. Any discipline where you need strength, balance, and flexibility in approximately equal measure, it's possible to overtrain or undertrain any one of them.