The neutral spine you’re supposed to maintain when you train starts in the neck’s cervical spine, says Matt Berenc, CSCS, director of education at the Equinox Fitness Training Institute in Beverly Hills. If you watch your reflection during any move in which the torso leans forward or goes completely horizontal (such as the deadlift or kettlebell swing), your neck becomes overextended.
This causes a host of problems that compromise your form, Berenc says. First, it leads to overextension in the low back, which reduces trunk stability. Second, it can force you to shrug your shoulders, placing more stress on those muscles and reducing their stability and strength. Third, it compresses the cervical spine, which puts pressure on the surrounding muscles, nerves, and other delicate structures.
The bottom line:
You should always maintain a straight line from the top of your head to your hips during exercises like kettlebell swings, squats, bent-over rows, deadlifts, and planks, to name a few. Instead of looking at the mirror, stare at a spot on the floor eight to 10 feet in front of you.