The word contract literally means to draw together or to shorten. This can be extremely confusing. A muscular contraction, as physiologists describe it, is whenever the muscle fibres generate tension in themselves no matter how large or small. So it becomes obvious a muscle is contracted whether it is actually shortening, remaining the same length or lengthening. In the human body most movements occur around a joint axis and very few occur purely linearly.

Concentric

Concentric, when literally translated, means towards the middle. This type of contraction occurs when the tension generated within the muscle 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 versa. 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.

Eccentric

Eccentric, when literally translated, means away from the middle. Whenever a muscle lengthens it generates a contractile force meaning all movements in the body occur with some muscular activity (this is absolute). The term lengthening is actually misleading as in most instances the muscle does not actually lengthen. In reality it returns from its shortened condition to its normal resting length. In most instances in which muscles contract eccentrically they actually act as a brake or resistive force against the moving force of gravity or other external force (like a weight). This work is often referred to as negative work for reasons I do not understand. Eccentric actions produce greater loading of the elastic musculoskeletal components and are used during many dynamic movements (like walking down stairs or hitting a ball). As the majority of muscle tears are thought to occur during eccentric motions improvements in this performance may be beneficial for injury prevention (Bennett and Stauber 1986). However, eccentric motions produced by active dynamometers are not considered to be like those seen during functional activities (they are described by Chan and Maffulli 1996 as ‘unnatural’). Short and rapid eccentric motions are normally produced during daily and sporting activities, isokinetically this is not seen as the movements are usually long and through full range of motion.

Isometric

Isometric, when literally translated, means equal length. When a muscle is contracted without any appreciable change in length this is referred to as isometric contraction. This term is abused by researchers, exercise scientists and physiotherapists to describe many different situations in which a muscle is contracted but cannot be said to be acting either concentrically or eccentrically.