grammar - How is the subject of this subclause made clear?

For full context: http://www3.nhk.or.jp/news/easy/k10011229751000/k10011229751000.html

The sentence in question: このほか、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機械もあります。

=> "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.

The main issue still stays that 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 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?^^ :D 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 wether 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.

Answer

Japanese relative clauses lack relative pronouns such as "which", "that", etc. So before you get fully used to them, they can look fairly ambiguous. What you're seeing here is an "adverbial head" type relative clause, which is discussed here.

Here are some simpler examples. Haven't you seen similar expressions?

• 寿司が楽しめるレストラン
  a restaurant {at which / where} people can enjoy sushi
  _{(× a restaurant that can enjoy sushi)}


• ビーチが見える部屋
  a room from which you can see the beach
  a room which allows you to see the beach
  _{(× a room which can see the beach)}


• 手紙を書くペン
  a pen with which you write a letter
  a pen to write a letter with
  
  _{(× a pen which is writing a letter)}


• 食べると頭が良くなるパン
  bread with which you get smarter upon eating
  bread which makes you smarter when you eat it
  _{(× bread that eats something and gets smarter)}

As you can see, in each example, the subject of the relative clause is omitted ("you", "I", "he", "people" or whatever depending on the context). The modified noun at the end works neither as the subject nor the object of the relative clause; instead, it plays an adverbial role (レストランで寿司が楽しめる, 部屋からビーチが見える, ペンで字を書く). The interpretations marked with × are "technically" possible, but we know they are wrong because we have common sense.

体につけると重い物を持つことができる機械 is almost the same as the last example above. A literal translation is "a machine with which one can carry heavy things upon wearing", but I think "a wearable machine which enables you to carry heavy things" is one of the good translation approaches.

Of course this sentence is referring to a so-called powered exoskeleton.