In addition, there were also machines which could lift/hold heavy things with about 30% of the regular strength, when you wear them on your body.
My main issue with this sentence is that my translation ACTUALLY wants to say "robots which can MAKE (it so that) you use only 30% of the strength you usually need...".
However, "to make X to Y" and "strength (case X) compared to strength (default)" are components which aren't represented by anything I could identify as a suitable equivalent in the Japanese original sentence. Well, at least 普通の could hint at an implicit reference to the default case, but I'm not sure about it.
体につけると普通の３０％ぐらいの力で重い物を持つことができる seems to modify 機械. But then, the reference point of ３０％ぐらいの力で seems to be the machine itself, at least from a syntactical perspective. And this wouldn't make much sense, as a machine can't lift something with only 30% of the power required to lift it. It can help YOU to use only 30% of your strength, because the machine takes on the other 70%. But it can't meddle with physics itself, can it?
Since 体につける definitely must refer to the person wearing the machine, there is indeed something hinting that the subject of 持つことができる is a person. But there isn't much besides that, and I still wonder whether there any other explicit syntactical elements showing us that the subject of 持つことができる is a person, and not the machine to which the full block 体につけると普通の３０％ぐらいの力で重い物を持つことができる is attributed.