There are countless exercises that even fitness buffs perform incorrectly. Some mistakes are simply too subtle to even notice that they're wrong, while others are exercises that have been taught incorrectly from the start. Zoning in on even the smallest mistakes can mean improving your progress and, even more importantly, avoiding a serious injury.

We asked training expert Rob Knox of Navesink Fitness about a common mistake that drives him crazy. Here, he talks about the dangers of the "Chicken Head," something he sees all the time at the gym, while performing dumbbell curls. This tiny mistake can lead to back and neck injuries, and chances are, you didn't even notice that you do it.

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