A.: Lifting too heavy.
You see it all the time in the gym—for example, guys on a bench (yes, it’s usually men) grunting away doing chest presses, with their back highly arched so that their feet and shoulders can help the pectoral muscles push the weight up. They may think their showing off and being cool, but they’re actually causing potential long-term damage. They’re “leveraging”: using other body parts to assist a particular muscle to do its work.
Here’s the problem with leveraging. First, the more assistance you get from another body part, the less the muscle that needs to do the work gets used. So with chest presses, your pecs are underworked, but there’s increased (and unnecessary) strain on your neck and shoulders. Definitely not the workout you want, and you won’t see the results you want from it. [Hint: put your feet on the bench so that your back is flat—that will force you to focus on using the pecs, and just the pecs, to do the chest press. Try it! You’ll notice the difference right away.]
The second (and more devastating) problem with “leveraging” is that you’re not just having an adverse impact on your muscles. Your joints come into play. Muscles can get stronger. Joints, however, don’t. In fact, they get weaker. So when you’re 22 and think you’re Superman lifting all that weight, the truth is that you’re doing damage to the joints—damage that can’t easily be repaired as you age.
Here’s a YouTube example of how NOT to weight train: