Classification of Voluntary Muscular Contractions
Voluntary muscular contractions can be classified several ways. One of these categorizes them as either eccentric or concentric.
Concentric contraction occurs while the muscle is going from a state of rest into a shortened position. An example of this would be your Biceps brachii muscle while you are raising a glass to your mouth to drink.
Eccentric contraction occurs as the shortened muscle returns to a lengthened state under control (some fibers remain in contraction until the muscle returns to rest to prevent abrupt motion that could damage tissue). An example of this would be when you are lowering the glass back to the table. If your Biceps brachii muscle just completely relaxed back to resting you would probably spill the contents of the glass as it comes flying back down. So your Biceps brachii muscle stays in a contracted state while lengthening so that you have control over how fast the muscle lengthens.
Alternatively, muscle contractions can be categorized as isometric or isotonic.
Isometric contraction occurs when muscles contract and does not produce movement of bones. Isometric contraction occurs when a muscle contracts but does not bring its bony attachments closer together as in carrying a box. Again your Biceps brachii must maintain its contraction to bear the weight of the load placed on it, but it is not actually moving any bones.
Isotonic contraction occurs when muscles contract to produce movement of bones. Isotonic contractions can be either concentric or eccentric, although these two typically happen one after the other.