Let's start out with the machines that are good for you before we get into the time-wasters. First up -- the classic -- the treadmill.

With a range of different speed, elevation, and

difficulty settings, this machine allows you to replicate outdoor walking or running patterns without having to dodge strollers, dogs, and weather on your neighborhood sidewalks. Bonus points if you can work it out

on a treadmill like this guy.

Frank Underwood had one thing right in House of Cards -- the rowing machine is one of the best machines you can use for a full-body workout. The range of movement it

provides works your legs, arms, glutes, and back without putting a lot of wear and tear on your joints. All that makes it a great choice for those with sensitive knees.

Spin bikes get major points for being gentle on the joints, providing a kickass cardio workout and giving you the option to change your elevation and difficulty levels to mimic outdoor biking

conditions.

While climbing an endless set of stairs could sound a little like a Sisyphean nightmare, it turns out that this machine provides one of the best cardio workouts you can

get at the gym. Also, it's an incredible way to get the booty and calves of your dreams.

Now that we've talked about machines that are the most efficient to use for your cardio workouts, let's take a look at the ones that aren't worth your gym-time energy.

You may be

surprised to see the elliptical on this end, but the main reason it's in our "skip it" category is because of it's inability to help you sustain a consistent stride.

Also, for those with shorter legs, it can be difficult to get into a comfortable stride in the first place, thanks to the fixed stride length. 

Although these machines are great for people who are giving their lower bodies a rest, when it comes to calorie payoff, they're not the most efficient way to burn. The

more muscles you're working at once, the more calories you'll burn for your time, and this machine targets only the muscles in the upper body.

Choose the arm ergometer for the

times you want to isolate your upper back and arm muscles, if you're suffering from an injury to your lower body, or if your knees need a break.

If you have the option to go for the spin bike over this guy, always choose a spin bike. Although the upright bike works to strengthen the lower body and

can get you sweating for a heart-healthy workout, it just doesn't measure up to other similar machines that can do a lot more for you in the same amount of

time you'd spend on this bike.

Because of the limited range of motion this machine allows for, add the stair stepper to the list of ones to skip over when you're ready to hit your cardio.

If stairs are your thing, opt for the much more effective StepMill if your gym has one.

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