Lowering the dumbbell is the eccentric component
I think it was Jane Fonda who, decades ago, popularized the phrase, “No pain, no gain.”
Did you know that the muscle soreness that follows a weight training session is caused only by the eccentric component of the exercise you do? “Eccentric” in this context isn’t easy for me to explain.
But here’s a simple try. You lift a barbell over your head with your arms; gradually lowering the barbell to its starting position is the eccentric component.
Learn more about exercise-related muscle soreness from Mark Rippetoe:
“Productive training entails some soreness, and everybody that trains gets used to the idea that getting stronger over time is accompanied by soreness – not the debilitating, crippling kind that makes normal movement difficult, but the mild soreness that accompanies a PR squat. To the extent that PRs are enjoyable, this soreness is welcome. It is possible to train for months and double your squat without being terribly sore at any point in the process.
But doing stupid workouts that cannot make you stronger and have not made you anything but sore indicates that you either don’t know what you’re doing, or that your priorities are other than getting stronger. If I were you, I’d reevaluate my priorities.”
Source: Soreness | Mark Rippetoe
Advanced Mediterranean Diet 