These supposedly healthy exercises could be hindering your fitness goals. Here's why you should ditch three common culprits for more helpful exercise habits.

You put in a lot of effort at the gym and want your hard work to pay off. But some exercise practices could actually be sabotaging your fitness goals. We spoke to fitness expert Brent Bishop about three common things people do to get fit, how they can backfire and what to do instead.

1. Sit-ups
Many people who want flat stomachs and strong abs turn to sit-ups, but Bishop says most of us should eschew this abdominal exercise. "It's an exercise that puts you in excessive flexion, which most of us are already in all day while sitting at work," says Bishop. "Your hip flexors are already tight and short, so why tighten them and shorten them more? It puts a lot of strain on the discs over time."

And since the sit-up mainly engages the rectus abdominis (the top layer of abdominal muscles) and hip flexors, it doesn't help tighten or strengthen your core the way other exercises might.

Instead: Try planks. Variations of the plank activate your entire core, including your transversus abdominis (the innermost abdominal muscles), obliques and lower back. Not only will they help you chisel your waist, Bishop says planks promote proper posture, help alleviate back discomfort and minimize risk of injury down the line.

2. Boot camps
Not all boot camps are bad, says Bishop, but there's a troubling trend in which these exercise programs urge large groups of people to do as many burpees, pushups or squats as they can, as fast as they can. "It's very competitive. If you can do them fast and do them correctly, that's great. But if you can't do them properly, you need to back off on the reps and tailor your form," says Bishop.

A more-is-better mentality makes injuries more likely because there is little focus on performing the exercises well, and the lack of emphasis on engaging muscles properly makes the moves less effective.

Instead: Focus on doing exercises slowly and properly. Once you can complete them through the full range of motion with perfect form, feel free to speed it up or add weights while maintaining effective posture throughout.

3. Monotonous cardio
Many people who are focused on losing weight spend hours running each week or use the elliptical nearly every day because they think cardio is the best way to burn calories. "People who are putting in a lot of mileage are probably putting more stress on their joints than they need to," says Bishop. "If they're not doing strength training, not only are they not going to lose weight as effectively, but they're losing lean mass, too."

Instead: Replace about half of your cardio with strength training. "You're going to increase muscle a bit, so your metabolism is going to elevate and, over the long term, you're going to end up burning more calories," explains Bishop. "Not only that, but if you do high-intensity strength training, there's an after-effect in which your metabolism is elevated eight to 12 hours afterward, so you burn more calories after that workout."

Another mistake you could be making? Not stretching. Learn about the right way to stretch, and why it's so important.