MariposasEccentric (Lowering) Phase
The eccentric phase is the portion of any lift where the muscle lengthens under load. In a bicep curl, that's the controlled lowering of the dumbbell back to your hip. In a squat, it's the descent. Your muscles are still actively contracting during this phase, just fighting gravity rather than overcoming it, which is why it's sometimes called 'negative' work. The nuance most people miss is that muscles are actually stronger eccentrically than concentrically, meaning you can handle more load on the way down than on the way up. That extra tension is also a primary driver of muscle damage and soreness, which is why walking down stairs after a hard leg day hurts more than walking up. Deliberately slowing the eccentric, say a 3 to 4 second lower rather than letting the weight drop, is one of the most effective and underused tools for building strength and hypertrophy without adding more weight to the bar.
Example
A lifter doing a bench press controls the barbell down to their chest over three full seconds before pressing back up. That slow descent is the eccentric phase, and it places the pecs and triceps under sustained tension even though no concentric effort has started yet. Done consistently, this kind of tempo work often reveals weaknesses in a lift that fast, sloppy reps completely mask.