 Isometric contractions do not change the length of the muscle. An example is pushing against a wall.  Concentric contractions shorten muscles. An example is the "up" phase of a bicep curl.  Eccentric contractions lengthen muscles. An example is the "down" phase as weights are lowered.

 Isometric exercise or "isometrics" are a type of strength training in which the joint angle and muscle length do not change during contraction  Isometric exercises can be done using the body's own weight, weight training equipment, resistance bands or everyday objects such as doorframes and towels.

 Isometrics are done in static positions, rather than being dynamic through a range of motion. The joint and muscle are either worked against an immovable force (overcoming isometric) or are held in a static position while opposed by resistance (yielding isometric).  While isometric training increases strength at the specific joint angles of the exercises performed and additional joint angles to a lesser extent, dynamic exercises increase strength throughout the full range of motion.

 Exercise featuring a heavy eccentric load can actually support a greater weight (muscles are approximately 10% stronger during eccentric contractions than during concentric contractions) and also results in greater muscular damage and delayed onset muscle soreness one to two days after training

 Use isometric exercise with caution with people with pressure response effects (high blood pressure, aneurysm)  DOMS ( delayed onset muscle soreness) is usually present hours after performing eccentric exercises, but moderate exercise should be continued through the recovery period

 Although ageing influences all muscle fibers, it has been reported that fast twitch fibers are more affected than slow twitch fibers (Lexell et al. 1983). It is obvious that a decrease in muscle function reduces quality of life and increases a risk of falls among the elderly.  In general resistance training, concentric (shortening muscle) actions are focused on more than eccentric (lengthening muscle) actions. However, it is known that fast twitch muscle fibers are more preferentially recruited during eccentric muscle actions (Nardone et al. 1989). The fast twitch muscle fibers are capable of producing higher forces, which are necessary for movements preventing falls  Therefore, resistance training consisting of fast eccentric muscle actions may be of greater benefit for old adults.

 Design: the effects of eccentric and concentric resistance exercise were compared during leg squats at a submaximal intensity known to increase muscle mass.  Subjects: 19 older persons (15 women/four men, age 65±4 years) and 19 young reference controls (10 women/nine men; age 25±2 years) were enrolled.  Methods: participants completed eccentric-only and concentric-only exercise bouts 5–7 days apart.  Conclusions: Eccentric resistance exercise produced less cardiopulmonary demands and may be better suited for older persons with low exercise tolerance and at risk of adverse cardiopulmonary events.

 In older relatively healthy men and women, Heart rate, blood pressure and cardiac index responses were less with eccentric compared with concentric resistance exercise.  Ventilation responses were less with eccentric compared with concentric resistance exercise.  Similar findings between eccentric and concentric exercise were observed in young controls.

 A resistance exercise protocol which includes eccentric as well as concentric exercise, particularly when the eccentric exercise is emphasized, appears to result in greater strength gains than concentric exercise alone

 A group of older adults were tested for their ability to perform activities of daily life (rising from a chair, stair climbing and descending, etc.). In the study, one group trained with concentric-only exercise while the other trained with eccentric and concentric exercise. The group that trained with the combined protocol showed greater improvements in function. Gur et al. (2002)

 One study looked at 6 weeks of eccentric vs. concentric training in women. Hortobagyi et.al. (1996). After the training, the concentric group improved strength 36% while the eccentric group had a 42% increase. This difference was significant (P<.05). The authors concluded that training eccentrically yielded greater strength adaptations faster than concentric training in women.