A concentric contraction is the lifting phase of a rep: the muscle shortens while producing force. Standing up out of a squat, curling a dumbbell toward your shoulder, pressing a bar overhead. It's the half of every rep people picture when they think of lifting, and it's the phase where the weight moves against gravity.
Why it matters
Concentric and eccentric work behave differently in three ways worth knowing. Force: you're weaker concentrically, by 20 to 50 percent, which is why the lift up fails long before the lowering does. Energy: concentric contractions cost more metabolically per rep, so they drive more of the huffing and puffing. Soreness: this is the big one. Concentric-only work produces very little muscle soreness, while eccentric work produces most of it. That's why pushing a sled (almost purely concentric) leaves your legs tired but rarely wrecked the next day.
How to use it in training
Lift with intent. Even when the bar moves slowly because it's heavy, trying to move it fast recruits more muscle fibers and builds more power. Slow lowering, fast lifting is a solid default for most strength work.
And concentric-dominant exercises have a specific job: hard work with minimal soreness. Sled pushes, kettlebell swings, cycling. They're ideal when you need to train legs two days before a game, a race, or anything else where waking up sore would be a problem.
Related terms
Go deeper
For the full picture of how lifting and lowering phases compare in the research, read: Eccentric training benefits.