ConcentricConcentric phase · Lifting phase
The phase of a lift where the muscle shortens under load, the 'lifting' part.
The concentric phase is the part of a rep where the muscle shortens while producing force: standing up out of a squat, pressing the bar away, curling the weight up. It is the phase people mean by lifting, and the one that fails first when a set gets hard.
Muscles are weaker concentrically than eccentrically, typically by a meaningful margin, which is why you can lower a weight you cannot lift and why a failed rep stalls on the way up rather than on the way down.
It matters for training because the concentric phase is where intent shows up. Trying to move the bar quickly, even when the bar itself moves slowly because it is heavy, recruits motor units more forcefully and is the basis of compensatory acceleration and of power training generally.
Comparisons between concentric and eccentric emphasis are one of the tidier questions in training research. A meta-analysis of healthy adults found eccentric training produced greater gains in strength and mass, particularly at high intensity, which argues for controlling the lowering rather than for neglecting the lifting.
Concentric only work has its uses. Sled pushes, prowler work and some machine setups minimise the eccentric, which means far less muscle damage and soreness, which makes them useful for conditioning, for training around an injury, and for adding work without adding recovery cost.
The practical rule for most people is unremarkable: lift with intent, lower with control, and do not let the concentric turn into a heave that recruits everything except the muscle you were training.
In practice, our one rep max calculator sets loads from a hard set, and the double progression planner decides when the concentric has got easy enough to add weight.