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It's fairly common knowledge that the eccentric phase is more important for strength, hypertrophy and power production. The human body can be up to 1.75 times stronger during the eccentric phase in comparison to the concentric. During an eccentric contraction, fewer motor units are involved therefore, there is more mechanical load per motor unit within the phase. As a result, eccentric training is can generate up to 1.3 times tension than concentric. Greater tension provides increases stimulus to the muscle fibers, which encourages greater adaptations. Controlled lengthening of the muscle triggers the most hypertrophy. By using intensities of 65-80% 1RM, the eccentric phase causes micro trauma to the myofibers and preferentially recruits fast twitch (type IIa/x) muscle fibers inducing muscular growth.

Performing heavy eccentrics (negatives) is known to have an anabolic effect but also trigger a response in protein synthesis rates. Recommended intensities have been known to be anything from 25-50% post maximal concentric loads. This type of training allows you to train at higher intensities and therefore, creating a greater adaptation. Studies show that heavy negatives produce greater motor unit recruitment/rate coding, an increase in protein synthesis and the release of insulin growth factor.

Fast eccentric training can lead to greater concentric power. Studies have shown that eccentric enhanced training will allow for the production of more power. Results showed that heavier concentric loads (60,70,80kg) allowed athletes to produce greater concentric power than a 40kg load. The greatest concentric acceleration was caused using the 80kg, the heaviest load. Researchers appear to believe that the athletes were able to produce more because more power due to increased muscular tension and cross bridging of the muscle fibres.

Fast eccentrics recruit powerful type type IIa/x muscle fibers which are essential for muscular growth. These actions will also help activate, satellite cells, multipotent cells found in muscle that proliferate following muscle trauma and form new myofibers through a process similar to fetal muscle development. An 8 week study compared a typical power training program using a 30-60% 1RM with a traditional strength program using 70-85% 1RM. Results showed that the power group had a greater increase in type II muscle fibers further highlighting the importance of high velocity eccentrics.

Concentric and eccentric muscle actions are movements that are performed a number of times, whether it be in the gym or carrying out everyday duties. Whilst both phases have their benefits, eccentric training is shown to be more effective in promoting increases in strength and power. In addition, eccentric training also shows a trend towards increased muscle cross-sectional area and mass. The specialised neural pattern of eccentric actions possibly explains the high specificity of strength gains after eccentric training.

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References
Chen, T & Chen, H. (2012). Attenuation of eccentric exercise induced muscle damage by preconditioning exercises. Med and Science in Sports and Ex.
Hather, B et al (1991). Influence of eccentric actions on skeletal muscle adaptations to resistance training. Acta Physoligica Scandinavia, 143(2) 177-185.
Schoenfeld, B. (2011). The use of specialised training techniques to maximise muscle hypertrophy. Strength Con J, 33(4), 60-65.
Schoenfeld, B. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. J Strength and Con Res, 24(10), 2857-2872.
Sheppard, J & Young, K. (2010). Using additional eccentric loads to increase concentric performance in the bench press throw. J Strength Cond Res, 24 (10), 2853-2856.

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