Skeletal Muscle Remodeling in Response to Eccentric vs. Concentric Loading: Morphological, Molecular, and Metabolic Adaptations

Abstract

Skeletal muscle contracts either by shortening or lengthening (concentrically or eccentrically, respectively); however, the two contractions substantially differ from one another in terms of mechanisms of force generation, maximum force production and energy cost. It is generally known that eccentric actions generate greater force than isometric and concentric contractions and at a lower metabolic cost. Hence, by virtue of the greater mechanical loading involved in active lengthening, eccentric resistance training (ECC RT) is assumed to produce greater hypertrophy than concentric resistance training (CON RT). Nonetheless, prevalence of either ECC RT or CON RT in inducing gains in muscle mass is still an open issue, with some studies reporting greater hypertrophy with eccentric, some with concentric and some with similar hypertrophy within both training modes. Recent observations suggest that such hypertrophic responses to lengthening vs. shortening contractions are achieved by different adaptations in muscle architecture. Whilst the changes in muscle protein synthesis in response to acute and chronic concentric and eccentric exercise bouts seem very similar, the molecular mechanisms regulating the myogenic adaptations to the two distinct loading stimuli are still incompletely understood. Thus, the present review aims to, (a) critically discuss the literature on the contribution of eccentric vs. concentric loading to muscular hypertrophy and structural remodeling, and, (b) clarify the molecular mechanisms that may regulate such adaptations. We conclude that, when matched for either maximum load or work, similar increase in muscle size is found between ECC and CON RT. However, such hypertrophic changes appear to be achieved through distinct structural adaptations, which may be regulated by different myogenic and molecular responses observed between lengthening and shortening contractions.

Keywords: concentric exercise; eccentric contraction; eccentric exercise; mechanotransduction; muscle architecture; muscle hypertrophy; muscle remodeling; muscle signaling.

Figure 1

Schematic representation of myosin S1 and S2 segments behavior during different contractions and at different respective portions of the F-V curve. (A) During fast shortening muscle actions, S2 complex will not fully stretch, hence myosin will apply lower pulling force onto actin (i.e., the thinner straight line in the figure). (B) During slower shortening contractions, up to when the shortening velocity would be equal to 0 (i.e., isometric contractions), the S2 segment will be fully stretched and therefore myosin will be able to apply greater pulling force onto the thin filament. (C) During lengthening actions, myosin S2 complex will be able to stretch even further (figure adapted from Jones et al., 2004).

Figure 2

From Seger et al. (1998): preferential distal significant hypertrophy after ECC RT. If the two sites had been considered together as a sum of ACSA across consecutive axial scans, (as in Higbie et al., 1996), the differences between ECC and CON in terms of “whole” hypertrophic (and not regional) response might have shown a different outcome (ECC ~ CON hypertrophy). ^*P < 0.05.

Figure 3

From Franchi et al. (2014): VL Regional hypertophic adaptations to ECC vs. CON RT programs are shown, ECC resulting in greater distal hypertrophy while CON presents larger mid muscle belly increase of CSA.

Figure 4

Ultrasound scan of VL muscle: Lf (fascicle length) and PA (pennation angle) represent two of the major features of muscle architecture.

Figure 5

Adapted from Franchi et al. (2014). Contraction-dependent muscle growth in response to eccentric and concentric resistive training in young males. Similar hypertrophy is achieved through two different patterns of structural re-assembly (^*P < 0.05, ^**P < 0.001, ^***P < 0.0001). Y = 1 represent the baseline value, data are normalized to pre-exercise values (Vol, Volume; Lf, fascicle length; PA, pennation angle; MVC, maximum voluntary contraction).

Figure 6

From Reeves et al. (2009): conceptual diagram that illustrates what could theoretically happen when displacing the same external load (empty dots) between concentric and eccentric phases in Conventional RT. As a consequence, the eccentric contraction belongs to a different force-velocity curve of lower neural activation. Because a fundamental requirement of the force-velocity relationship is that all the point of force and velocity should belong to a curve of same neural activation, then the filled dot represent the level of external load that should be adopted to meet such requirement: something that is not occurring in conventional resistance training.

Figure 7

Schematic diagram illustrating the distinct contraction-specific hypertophic patterns in response to chronic ECC vs. CON RT in human vastus lateralis (image acquired by using extended field of view ultrasound technique): a similar increase in MT can indeed be reached either through a preferential addition of sarcomeres in-parallel (usually occurring after CON RT) with an increase in PA, or through a preferential addition of sarcomeres in-series (usually occurring after ECC RT), which is represented by an increase in Lf (Franchi et al., 2014, 2015). +++ = the preferential addition of either sarcomeres in-series or in-parallel (dependent on the contraction mode used) compared to + = likely to happen as a marginal response. The white lines highlight the initial pre-training scenario, whereas the red dotted lines represent a post-training hypertrophic state.