“If you train regularly, you've done thousands of reps of the same movements over time,” says lead study author David Neumann, Ph.D., professor of performance and applied psychology at Griffith University in South East Queensland, Australia.
The result: Proper form and engagement are already ingrained in your memory. If you then think about your muscles (like the biceps during barbell curls), you make the exercise less automatic and more tiring.
That’s good for hypertrophy since it’s a sign that your muscles are being maximally taxed, which leads to growth. But if you’re doing more than 12 reps at a time or you’re taking a class in which you have to work for, say, 10 minutes straight before you get a rest, premature fatigue works against you.
The bottom line:
Unless your primary goal for a specific workout is to gain muscle mass, focus on the external weight or motion. During a curl, for example, think about where the barbell is in space throughout the movement.