This is where legal concepts (in the US) such as &#x27;reasonable person&#x27; come into play.  Courts usually hold that any use of a product that a &#x27;reasonable person&#x27; would do is an acceptable use (regardless of what the instructions say - a reasonable person, it would seem, doesn&#x27;t read the instructions :P). However, a company doesn&#x27;t need to be held responsible for an un-reasonable use.<p>In this case, would courts find that a reasonable person would assume that a heart rate monitor can accurately monitor their heart rate?  My guess is, yes.