Hi Juhasz,Context is key. The kind of people who "rough people up" or talk about "roughing people up" may not consider a broken arm to be a "serious injury," but you or I might.
In other words, there's no universally accepted meaning of "rough someone up" or of "serious injury," so it's difficult to say whether they're equivalent. Do you have an example sentence that we can look at?
I got it. Can't thank you enough for your excellent explanation.1. It's a vague threat. The violence could range from a punch in the gut to a broken knee cap.
2. Jails can range from relatively safe to very dangerous. In a dangerous jail, being "roughed up" could mean being beaten up, or being stabbed. If the "roughing up" was being done by the jailers, we would reserve this phrase for treatment milder than "torture" (but "torture" has different definitions as well).
In general, describing violence as "roughing someone up" diminishes the severity of the incident. For example, if I wanted to make the point that the guards at Prison X act brutally, inhumanly or illegally, I might describe their actions as "torture." On the other hand, if I wanted to make the counter argument, and say that those very same guards at Prison X are not actually seriously violent, I might say, "Sure they're not saints; they rough the prisoners up a bit. But what do you expect?"
In other words, the person who says "rough up" thinks the violence is not serious, or intends to suggest that the violence is not serious. But it is not at all an objective description.