We all know we should go to the gym (or otherwise work out) but the question for beginners is always the same: What am I supposed to do when I get there?

The truth is, many people don't know what the best option is for their health and fitness. Often they'll default to running on the treadmill every day or just doing a half hour on the elliptical or stationary bike.

Yet we know that the key to a successful workout program is variety, and a huge part of that is resistance training — better known as weight lifting.

We recently chatted with Shawn Arent, an exercise scientist at Rutgers University, and we asked him what kind of exercise people aren't doing enough that they should be doing more. Here's what he told us:

"I’d have to say lifting weights. Still, so many people shy away from it. Because, one, maybe they don’t know how to do it, and two, maybe it’s accessibility and things like that ... but the number of health benefits to be gained from lifting weights especially as we age are incredible …

"As you age, there’s a natural process for losing muscle mass. Resistance training can prevent that to a degree, or at least slow it down dramatically. It increases bone density, it increases balance and control, and helps prevent falls later on ...

"If you control for [the length of variety of the workout], resistance training has the same fat-burning effects as cardiovascular exercise. In addition, you burn fat for longer after a high intensity resistance training bout because of what we call excess post-exercise oxygen consumption or 'EPOC.' So your metabolism is still active for a period of time afterwards ... So it actually has a calorie burning effect to it.

"Not only that, but if you add lean muscle you burn more calories in a day because that influences your metabolic rate. It takes energy to support muscles; it doesn’t take energy to support fat ... So that makes a difference as well. [Weight lifting] changes body composition, it improves insulin sensitivity, it improves cardiovascular function, it improves walking speed, it improves balance, it improves cognitive function, it improves hormonal regulation. It has effects on depression, anxiety ...

"So what should everybody be doing? They should lift! Now, you don’t have to be the next Arnold Schwarzenegger. I’m just saying get in there, do some stuff that’s going to challenge the muscles, make you a bit tired, maybe a little bit sore the next day ... something that makes you work, that makes you get stronger from workout to workout."