Concnetric (n) con·cen·tric is a term used to describe a type of movement or exercise.
The word concentric is most commonly used in sports science and medicine. In these settings concentric defines a type of exercise or movement.
Translated literally concentric means towards the centre.
This type of contraction occurs when the tension generated within amuscle is sufficient to overcome a resistance (in most cases at least gravity) to move a body segment (or the attachment of the muscle on that body segment) towards another segment (or the origin of the muscle in question) or vice verse. This type of contraction is dependant on one end of the muscle having more stability than the opposite end. The term dynamic shortening seems to be a more appropriate way of describing concentric contractions.
In concentric contractions the muscle shortens using the sliding filament theory (shown below). Here the thick myosin filaments attach onto the thin actin filaments and pull. This has the effect of bringing the Z disks closer together which shortens the muscle and will most often change the angle of a joint (although this not always true think of the eye muscles here there are no joints).
A good example is a concentric contraction of the biceps muscles in the arm. Here the concentric contraction shortens the bicep which pulls on the elbow bending it. Hence a concentric contraction of the biceps bends the elbow.
Concentric contractions tend to be the weakest contractions we can produce with isotonic concentric contractions normally weaker (due to lever dis-advatage) than isokinetic concentric contractions.