Research Paper (Lit. Review) *FOR THE NERDS
The Effects of Eccentric Resistance Training on Muscle Hypertrophy and Strength
From a general standpoint, we understand that there are clear benefits to the muscle hypertrophy, specifically the shortening and lengthening actions in muscles; this means that there are two modes that contribute to this effect. Although there is a vast amount of research being unveiled with respect to conventional training protocols done by bodybuilders and powerlifters in pursuit of muscle strength and hypertrophy, there is a lack of emphasis that involves more eccentric-focused resistance training. It is generally known that eccentric actions substantially differ from concentric actions, and the literature is vast within the last decade. To name a few, eccentric training outcomes appear to be superior to concentric training outcomes when compared to one another in regard to force generation, energy cost, and body composition. This review is not to diminish the return of investment in concentric contractions but is instead aimed at placing more attention on the eccentric portion in exercise programs, and how to manipulate conventional training protocols for not only bodybuilders and fitness enthusiasts but for those who wish to adopt as an alternative for rehabilitative purposes due to injury or chronic disease.
Daily life activities for humans require a shortening of a muscle and a subsequent lengthening or vice versa. The two actions combined with repetition against a load or gravity can lead to greater muscle strength and hypertrophy. We focus largely on the concentric portion of a muscle action as a primary contributor to muscle gain, and the literature is clear as to how concentric actions that are put under mechanical load leads to gains in hypertrophy. Conventional studies continued to focus on work done by shortening muscles as an essential to strength training and muscle hypertrophy, but little was observed in regard to the properties of lengthening muscle contractions and its’ contribution to overall hypertrophy and strength.
As a consequence, much less is known about both the mechanics and energetics of muscle lengthening than muscle shortening. To emphasize, “so little is known that the late muscle biomechanist Tom McMahon and his student Jason Harry characterized lengthening contractions as ‘the dark side of the force-velocity curve’; a reference to the relative lack of knowledge about this region of the classic model of Hill that describes the relationship between a muscle's shortening velocity and its force production” (Lindstedt, Lastayo, & Reich, 2001).
Inadequate data in comparison of the two dynamics of muscle action creates controversy for which resistance training approach causes greater hypertrophic responses. For whatever reason – possibly due to the majority of studies simply overlooking the effects of the entire range of motion on hypertrophy – health professionals and trainers are not incorporating enough exercises that involve eccentric-focused movements. Recent studies suggest that lengthening contractions lead to different adaptations in muscle architecture when compared to concentric contractions. (Hedayatpour & Falla 2015; Hortobagyi et al. 1996; Hyldahl, & Hubal 2013).
Eccentric Training on Hypertrophy and Strength
For hypertrophy to take place, stress by a load is required to challenge the muscle being loaded — specifically the tearing of muscle. No, not the tearing you hear from 40-year old gym rats who live and breathe their egotistic deadlifts and bicep curls in the squat rack. I’m referring to micro-tears that happen all the time when we lift. It’s well established now that muscle tearing primarily takes place during higher mechanical stresses. When contracting concentrically, or positively, you are overcoming the load placed on the muscle and there is very little mechanical stress. If we were to only focus on picking up the weights and letting them drop to the floor, that muscle being used is already well-adapt, and no further growth of that muscle is necessary. In contrast with eccentric contractions, the load is overcoming the muscle, causing it to go the reverse direction of the positive direction, and the muscle is “defeated.” This means that the muscle is challenged to a greater degree and should adapt by growing if we apply Wolff’s law. It is well established that mechanical stress is a primary drive to anabolic signaling and adaptive response (Schoenfeld, 2013). This means that eccentric, or negative muscle contractions should provide more than just returning the weight to its starting position. Although correlation is not always causation, we should still look at the literature we have so far.
It is widely accepted that concentric contractions have muscle damaging elements, but studies have shown how eccentric actions can cause greater damage in muscle (Enoka, 1996). Schoenfeld et al. claims that these heightened force demands on fewer active fibers are responsible for this phenomenon due to higher susceptibility to muscle fiber tear during lengthening exercises (2017). In an aim to focus on muscle hypertrophy and strength, eccentric actions may produce greater hypertrophy as a result of increased muscle damage (Farthing & Chilibeck, 2003; Friedmann et al., 2004; Higbic, Curelon, Warren, & Prior, 1994; Norrbrand, Fluckey, Pozzo, & Tesch, 2007). Schoenfeld et al. also performed a systematic review and meta- analysis that comprised of 30 treatment groups rom 15 studies, and they found that, “eccentric promote superior increases in muscle mass” (2017). In one study comparing fiber size across eccentric-focused groups and concentric-focused groups, type II fiber area increased approximately 10 times more in the eccentric group than in the concentric group (Hortobagyi et al. 1996). It was even shown how maximal hypertrophy cannot be obtained without the eccentric in resistance training (Hather, Tesch, Buchanan, & Dudley, 1991). From this, we can gather more reason to incorporate more eccentric training.
In addition, studies also show greater forces being generated in eccentric actions through different mechanisms of force when compared to concentric actions (Franchi, Reeves, & Narici 2017), showing that eccentric strength is approximately 20–50% greater than concentric strength (Schoenfeld, 2017). These higher intensities of load used during eccentric training is believed to be a confounding factor when comparing adaptations associated with the 2 actions. In one study, “eccentric training increased eccentric strength 3.5 times more than concentric training increased concentric strength. Eccentric training increased concentric strength and concentric training increased eccentric strength by about the same magnitude. Hortobagyi et al. found that, “Eccentric training increased EMG activity seven times more during eccentric testing than concentric training increased EMG activity during concentric testing (1996). Since greater forces are seen generated from muscle lengthening, heavy eccentric-focused resistance training may be a large piece missing from strength training programs. Nardone, Romanò, & Schieppati argue that “Eccentric exercise may preferentially recruit fast twitch muscle fibers and perhaps the recruitment of previously inactive motor units” (1989).
With this in mind, muscle hypertrophy is more complex than the concentric portion of an exercise by itself. Hedayatpour & Falla 2015 argue that eccentric action “induces a more rapid addition of sarcomeres in series and in parallel” as well as adding how “Previous studies reported an increase of fiber length in muscles subjected to chronic eccentric work, whereas a decrease or a lack of change of fiber length was shown in muscles worked concentrically. Greater muscle hypertrophy following high intensity eccentric exercise was also associated with larger fiber pennation angle” (2015). With this, can see that there is some clear evidence that shows how much eccentrics play a role in muscle hypertrophy and strength. Overall, it is probable that the reason eccentric training is superior to concentric training, in regard to muscle hypertrophy, is because of the greater capacity for force generation that eccentric
Eccentric Actions Are Cost Efficient When Compared to Concentric Actions
Eccentric Training Is More Efficient in Comparison to Concentric Training. In a sense, the muscle absorbs energy during the eccentric work and can further be described as “the mechanical energy absorbed by the work conducted on a muscle when the force on the muscle is greater than the force produced” (Lindstedt, Lastayo, & Reich, 2001). But if timing is optimal, that elastic stored energy in the muscle can be used as a “compressible shock absorber” which can then be utilized as a muscle spring (Lindstedt, Lastayo, & Reich, 2001). During an eccentric action, not only can more force be generated than concentric actions, but less oxygen is required in eccentrics; Bigland-Ritchie & Woods were the first to document this observation, reporting that “the oxygen requirement of submaximal eccentric cycling is only 1/6–1/7 of that for concentric cycling at the same workload” (1976). To say the least, one study revealed “A significant increase in REE is also observed in healthy young women after chronic ECC [eccentric] exercise compared with chronic CON [concentric] exercise performed at the same power output” together with “increasing post-exercise fat oxidation by 13% and reducing glucose oxidation” in ECC exercise whereas the CON exercise showed “no significant modification in metabolic substrates after exercise” (Paschalis et al., 2011).
Because more volume of exercise can be done for less metabolic cost, eccentric-emphasized strength-training programs can provide greater benefit for populations that have higher chances of injury (i.e. older individuals) (Hyldahl, & Hubal 2013). Lindstedt, Lastayo, & Reich suggest that eccentric training is profoundly beneficial and can be applied to patient populations and/or to those interested in enhancing sport performance because much greater force can be produced eccentrically than concentrically (2001). It’s increasingly apparent now that eccentric exercise is currently being used as a form of rehabilitation for sport injuries and is an alternative form of exercise for the elderly, neurological disorders, COPD, cardiopulmonary disorders, and cancer (Robertson, 2005). Low oxygen consumption of eccentric exercise studies have been observed on patients with severe COPD. In this study, an eccentric cycling exercise workout was created for these patients and was followed up with “no side effects, minimal muscle soreness that had no effect on power, and a high compliance” (Abbott, Bigland, & Ritchie, 1952). Additionally, other cycling studies found that patients can perform high intensity work with lower cost using eccentric cycling and was a safe alternative for those with COPD (Rooyackers, Berkeljon, & Folgering, 2003). The takeaway here is just to provide a safer alternative to strength or resistance training whilst having greater cost efficiency.
Dynamic resistance training has been adopted by many for purposes none other than for hypertrophy, and the benefits that coincide these results. When creating a program for a specific population, there tends to be variation across each one in regard to the type of exercise prescribed rather than the dynamics of muscle shortening and lengthening. We've have already established the elements along with the mechanisms in which concentric or muscle shortening
contractions require for optimal motor unit recruitment. However, although the literature is long and vast, we have yet to fully understand how eccentric or muscle lengthening can be optimally adopted across populations that are muscle hypertrophy-directed. It appears that we can manipulate training programs to create greater hypertrophic responses, muscular strength, improve body composition, all while becoming more cost efficient within exercise.
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