Core, core, core. Yes, we hear it ad nauseam: "You need to strengthen the core." Or the other favorite, "It's all about the core."
But what does it really mean?
There is no clear medical definition of core, but fitness trainers use the term to describe the muscles of the trunk -- front and back.
"Many people associate core with abs and maybe lower back, but from a more global perspective, it includes all the supportive muscles of the spine," says Robert Gillanders, an endurance athlete and physical therapist at Sports + Spinal Physical Therapy in Washington.
"And part of the reason the core muscles are so important is because the spine is not a fundamentally stable structure," he says.
Core muscles are important for athletic performance, in particular, because they ensure efficient transfer of energy from the powerful lower body to the less bulky upper body, says Stephen Burgett, a personal trainer certified by the National Strength and Conditioning Association.
In other words, if you are a golfer or baseball player, you know that your swing doesn't originate in the arms -- it comes all the way from the ground up. And if you have a strong core, that power is more likely to stay intact all the way up from your toes through your fingertips, generating a powerful swing.
"Same thing for a runner. If they don't have good core strength and can't stay upright at the end of a race, they will lose a lot of efficiency and force," Burgett says.
Todd Miller, a professor in the Department of Exercise Science at George Washington University, calls the core the "mechanical link between the upper and lower body."
So, how to train the core? All three experts agree that traditional crunches are not the way to go.
"They are overrated," says Burgett. "And the way many people do them, there is way too much range of motion and they end up complaining about back pain."
And, he says, the key to functional fitness is to find a way to build muscle that supports daily activity -- of which crunching in a fetal position probably is not one.
Instead, Burgett works on exercises such as side-lying planks to help build the lateral muscles of the trunk, and single-leg bridges with hip extensions to work the back and glutes.
It's important to comprehend the many dimensions and layers of core muscles, he says. If you were doing only crunches, for example, you would be neglecting those lateral muscles of the core as well as muscles of the back.
In the trunk, like elsewhere in the body, there are big, small, front, back, side, surface and deep muscles -- and for the best outcome for fitness and wellness, he says, all of them need to be worked to "fire appropriately."
For example, there are the rectus abdominis (the six-pack) and underneath them are the transverse abdominis (equally important but not visible to the naked eye). Then there are the big back muscles, such as the erector spinae, and the much smaller and local muscles, such as the sectional muscles between the vertebrae.
It's a lot to keep track of, but it's important to do so, Gillanders says, because if you overtrain certain muscles and underwork others, the body becomes less adept at moving efficiently.
Gillanders points out that the smaller, more local muscles often require subtle movements -- like "bringing the navel toward the spine" or "bracing with the pelvic floor" -- that don't necessarily fit the idea of "workout exercises."
"Connecting with those movements is a form of mental gymnastics," Gillanders says.
Miller says that doing "abs" in isolation is completely unnecessary if you are looking to improve core strength.
"If you have a properly designed resistance training program with lunges and squats you don't need to do abs," Miller says.
Think of exercises such as lunging and squatting with overhead weights. This type of work strengthens the core because it requires the core to stabilize the body.
But let's face it, it's almost swimsuit season, and some people want to go for the six-pack look. How to get there?
"Eat less," says Miller half-jokingly, adding that everyone has a six-pack -- it's part of our anatomy. It's just a matter of how much fat is layered on top.
There is no such thing as spot-reducing fat, he says, so it's overall dietary -- and exercise -- changes that create a calorie deficit that ultimately will reveal the six-pack. Working the abs to create more bulk will also help reveal them.
And here is where Miller has an admission to make.
"I do crunches," he says. "But I do them to look good. And if the goal is to have good-looking abs, then sure, by all means do the crunch machine. But it's for show more than anything else."