The big difference between free weights and machines is the activation of the zillion little supporting muscles that one gets with free weights. Like in the gym I'm using now, there are no squat racks, just Smith machines. They put a sort of similar stress on the main movers, but it's just not the same. Oddly, I can't squat as much on a Smith machine as I can with a free barbell. The machine somehow limits what I can do, the muscles I can use, something. For curls, rather than a curl machine, I stand, well bent over, legs spread, knees bent, holding on to something for balance, and use dumbbells. Lots more going on there than with a curl machine. Similar thing just doing dumbbell benches. Isolation exercises just don't work as well, unless one is competing and is trying for some particular look or having symmetry problems.
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