Muscleactionsaretypicallypidedinto3categories：concentric,isometric,andeccentric.Anactivemusclethatisshorteningandproducespositivemechanicalworkissaidtoworkconcentrically;anactivemusclethatisnotchangingitslength,andthusdoesnotproducenetwork,issaidtoworkisometrically;andanactivemusclethatiselongatedbyexternalforces,andthusabsorbswork,orproducesnegativework,issaidtoworkeccentrically．1
