Physiological and Neural Adaptations to Eccentric Exercise: Mechanisms and Considerations for Training

By: Nosratollah Hedayatpour and Deborah Falla

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Its an age old question in the world of strength training: “What method works best?” When boiled down to its most basic level, resistance training includes three styles of muscle contraction; isometric, concentric and eccentric. Of course training goals and many other factors should be considered when designing a program, but it’s accepted by many professionals that eccentric loading should be performed to yield the greatest strength gains. A lot of research has outlined propositions for how different styles of training may yield specific results. There has also been numerous investigations into the varying physiological and biomechanical processes as a result of different loading principles. In this article, Hedayatpour and Falla review the mechanisms of tissue adaptation resulting from eccentric exercise, and emphasize its importance for improving strength.

Facts from the Article:

Physiological adaptations following eccentric exercise have been attributed to the increased mechanical tension compared to concentric exercise. It is thought that eccentric exercise specifically induces a more rapid addition of sarcomeres in series and in parallel, as inferred from the increase in muscle cross-sectional area and pennation angle

  • Histochemical Adaptations: Combining force production from muscle contraction and stretch (which only occurs during eccentric contractions) increases mechanical tension. This unique increase in mechanical tension can cause:
    • Increased stimulation/regulation of the rate of protein synthesis through changes in binding of ribosomes to the mRNA and/or by modifications in methylguanosine, which in turn encodes proteins that are central to the growth process
      Changes in muscle fiber membrane permeability to calcium ions, thus increasing protein synthesis and altering sarcomere recruitment during contraction
    • Mitogen-activated protein kinase is a master signaling pathway for gene expression and muscle hypertrophy, and is most responsive to mechanical tension and sub cellular muscle damage (Again, eccentric loa d = more tension = more mitogen-activated protein kinase)
    • High intensity eccentric exercise-induced elevation in testosterone is an important contributor to muscle hypertrophy
  • Metabolic Adaptations: Extramyofibrillar elements, especially collagen content in the extracellular matrix, contributes to an increased acidic environment. Such a state within the muscle can contribute to increased fiber degradation and increased sympathetic nerve activity, facilitating an adaptive hypertrophic response

Neural Adaptations: Strength gains have been attributed to a variety of mechanisms. Neural adaptations to strengthening exercises include increased central motor drive, elevated motor neuron excitability, and reduced presynaptic inhibition. More specifically, eccentric exercise has been found to elicit the following changes:

  • Cortical Activity: 
    • There is preferential recruitment of fast-twitch motor units and different activation levels among synergistic muscles during eccentric contractions
    • Cortical activities for movement preparation and execution were greater during eccentric than concentric tasks, including earlier onset of activity prior to the task
    • Cortical activities associated with the processing of feedback signals are larger during eccentric than concentric actions
  • Motor Unit Behavior: 
    • The CNS controls the production of increased muscle force by either increasing motor firing rates and/or the recruitment of additional motor units. Furthermore, the change in motor unit firing rate has been found to be dependent on the type of muscle contraction. Specifically, Dartnell et al. showed ~40% decline in biceps brachii motor unit recruitment thresholds (the amount of input needed for the muscle to “work”) and 11% increase in minimum motor unit discharge rates immediately after and 24 h after eccentric exercise
    • Down regulation of inhibitory pathways during eccentric muscle activation may also serve as an explanation for increased muscle force observed following eccentric resistance training
  • Muscle Force:
    • “Farthing and Chilibeck reported that 8 weeks of eccentric resistance training resulted in greater muscle hypertrophy and muscle force than training with concentric contractions”
    • “Kaminski et al. observed greater improvements in peak torque following eccentric (29%) compared to concentric (19%) training”

Summary:

As with all types of strengthening methods, there are always inherent risks specific to eccentric exercise that should be considered regardless of exerciser or setting. As identified by Hedayatpour and Falla, nonuniform changes in muscle activation, alternative muscle synergies, differing effects on various body regions, muscle micro-lesions, pain, reduced fiber excitability and compromised joint stability are among the potential deleterious effects of eccentric strengthening. However, it appears to be the opinion of the authors, as well as my own, that eccentric strengthening has tremendous value when done safely. Time is valuable to everyone, so the more time we spend strengthening muscles with the most efficient methods the better!

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