It's generally common knowledge that if you're not pushing yourself during a workout, you're not getting the kind of benefit you likely want from your exercise.

But how do you know that you're working hard enough? Heart rate monitors can help, but what if you don't have one?

We spoke recently with Shawn Arent, an exercise scientist at Rutgers University, and we asked him to give us tips on how to know you're working hard enough.

He recommends technology like good heart rate monitors, but he also has some other tricks.

Here's what he told us:

"I'm a big fan of technology ... like heart rate monitors because I think people do have a hard time judging how hard they're actually working. And in some cases there's been a couple studies that show people do one of two things when it comes to self selection of exercise: One, some of them will self select at an intensity that's too low to produce fitness improvement, so what happens is they don't progress, they get disappointed, and they quit.

"The others over select: They pick an exercise that's really hard, they don't enjoy it, they stop, and they quit. So one simple way [to see if you're working hard enough] is heart rate. It's a good measuring stick.

"The other one is simply perceived exertion. So they can use what's called the Borg scale, a rating of perceived exertion. It's a 6-20 scale. Six is basically 'I'm not doing anything I'm just sitting here,' and 20 is 'I'm pretty sure you're trying to kill me.' And right around at 11 or 12 would be what we consider to be moderate intensity exercise ... A 14 or 16 is right around lactate threshold [a good threshold for people seriously training] ...

"That's one easy way to do it, but I generally rely on heart rate whenever possible. In many cases, if you're working with somebody with a heart condition, RPE [rated perceived exertion] and heart rate [together] are even better because it might feel like they're working harder than they are ... Heart rate might not respond, and then you want to go by how they feel.

"...If somebody doesn’t have a heart rate monitor, and they want to know ‘how hard should I work out’: hard enough that you could still probably talk to somebody but it would be a broken conversation. In other words, you couldn’t just talk leisurely while you’re doing it. You’d have to take a breath every once in awhile to catch your breath. So it should be a difficult conversation, but maybe not so hard that you can’t talk at all — unless you want to start doing intervals [alternating short bursts of high-intensity exercise].

"The other way to judge resistance training too is if you set a repetition goal. So for example, let's say you do three sets of 10 on an exercise — high intensity would be taking each of those sets to momentary muscular failure. So in other words, 10 reps is all you can do. More moderate intensity would be you selected a weight where you do 10 reps, but probably could have done 12 or 13. Low intensity would be you stop at 10 reps and you probably could have done about 20.  So you can just intensity based on that as well."

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