This gym-class standard needs an update: According to research from San Diego State University, the traditional crunch is the least effective strengthener for both the rectus abdominus (6-pack muscles) and the obliques (waist muscles). What's more, because sit-ups require more strength from front ab muscles than obliques, this move can create a strength imbalance in the core -- setting you up for back problems.

Not only does the plank require more muscle activation in the obliques than the traditional crunch, according to research, but researchers have also found that practicing this position can actually help lower the risk of developing lower back pain later in life, according to Matthews. Why? "The plank targets your abdominal muscles, helps to build muscular endurance and spares your spine in the process," she says.

While research has found that the pec deck machine is a great way to build chest muscle, the fact is that this machine puts your shoulder joint in an extremely vulnerable position, as it simultaneously rotates and abducts, according to Matthews. While this is especially dangerous for those with shoulder problems, using this machine can also cause injury in otherwise healthy exercisers.

Engaging the same muscles as the Pec Deck, the Bent-Forward Cable Crossover allows you to build strength quickly and effectively -- without unnecessary risk to your joints.

Though research has shown this staple gym exercise to be excellent for building chest and triceps strength, some bench press-related shoulder injuries are common enough to have earned the nickname "bench-presser's shoulder." Also, the bench itself limits natural movement in the shoulder blades, putting tremendous stress on rotator cuffs. As such, some fitness experts have deemed bench press unsafe.

Push-ups are a safe and (happily) equipment-free exercise that builds muscle in the pecs, triceps and shoulders while developing core strength. What's more, it can be modified dozens of ways to work different muscles or increase the intensity and complexity of this tried-and-true, basic movement. There's a reason it was the foundation of fitness icon Jack LaLanne's routine.