I watched a guy leg press the entire stack of weights at my local gym.
Since I've made poor progress from my own strength training, at least after the first few days, he seemed like a good person to ask. I asked him a few questions, and he was happy to show off his knowledge.
He says that what books have advised me, not to do strength training two days in a row, was over-cautious. He just uses exercises on alternate days that strengthen different muscles.
He suggests doing 3 - 6 sets/day of each exercise. The upper end is for people who want to get strong in a hurry, and is not what he does. But he uses a lot of machines, to do a lot of different exercises, on a lot of different muscle groups - about 17 exercises. He also says that it is his experience that he can do strength and aerobics training the same day without running into problems.
BTW, a set is a strength training exercises, with or without a machine, that you do as many times as you can until you are (usually) too tired or sore to continue. Typically, 10 - 15 repetitions form a set. If you can't do about 10, you are possibly using too few repetitions to benefit. If you can do more than about 15, you possibly aren't using enough strength to benefit, though at some point you get an aerobics benefit. People used to say you should do a set, and rest until you can do another. Now people mostly do "circuits" - after doing what they can of one exercise, they do another, using different muscles. This uses your training time much more efficiently. This is sort of like what a video had suggested, that we do alternate strength and flexibility training exercises.
I think a lot of the constraints that the ACE Manual and similar sources suggest, like exercise frequency, are designed with the intent that NO ONE will be hurt, with very cautious liability concerns in mind. The problem is that making sure that a 500 pound individual who hasn't left the couch in 10 years won't be hurt is different from making sure that an athlete won't be hurt, or that those of us who are in the middle won't be hurt. I'm beginning to think we need to figure out our own guidelines, and throw the books away. I've already given up on the aerobics videos - they don't address my particular issues specifically enough, and they do the same exercises every day you watch them.
Likewise, you can often figure out which parts of your body need to be stretched yourself, by trying to do things, and feeling your body to see where the tension is. Of course, we mostly can't feel muscles and ligaments that aren't next to the skin, unless we make them sore. The person I was speaking to said he occasionally exercises things that limit his motion until it gets quite sore, so he can sense and figure out what needs to be stretched. Probably dangerous, if you do it too much, but maybe OK once in a while.
Oh- he says different machines of exactly the same type and model are very different, for reasons of manufacturing quality, maintenance and lubrication, equipment centering, etc. It doesn't trouble him that he uses a quite different amount of weight using the exact same machine in another gym.
He trains for about an hour every day. Some days he spends another hour on a machine resembling a recumbent bicycle. I was surprised to find I could spend that much time on the bicycle-like machine. I've never been able to spend anywhere near that much time on treadmills, or on machines resembling upright bicycles.
For most people, that is a lot of time to spend on training. But I'm trying to incorporate some of what he said.
If you limit yourself to body weight exercises, that is sometimes uses too little and sometimes too much strength to be much benefit. Yes, it has the advantage of using more muscles, because machines limit you to motions along very few directions, but if you aren't in the right strength zone, body weight just isn't always right. E.G., I'm not nearly strong enough to do a pull-up. Trying with no success doesn't give me much benefit. But pull-down weight machines are very slowly increasing similar muscle strength.
Likewise, I'm not quite strong enough to do good deep single leg knee bend and straightening exercises, which is very relevant to skating skills. When I try, my knee doesn't track well, which a PT said could eventually cause knee problems. Perhaps if I can first use leg presses to strengthen myself to the point that there is a greater margin of strength, the problem will go away? The PT said to bring into use more muscles (e.g., obliques, gluts, etc.), but that doesn't seem to be enough in itself.
One thing is very clear. The operation of those stupid aerobics exercise machines, with all the fancy electronics, is not obvious. Why don't they put better directions on the machines? It's even worse than a voice mail menu. They should just dump all the fancy electronics and complicated menus (except maybe a timer), and just add an adjustable resistance lever or knob.
Another rant. Most of the weight machines in my local gym don't adjust for small people very well. No matter how I adjust them, there is a lot of unnecessary strain on the back, or other places the machine isn't supposed to stress. Is making a machine adjustable so hard to do? I mean, some women, and a lot of kids, are smaller than me.