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.
Highlights
Specialty section: This article was submitted to Exercise Physiology, a section of the journal Frontiers in Physiology
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)
Eccentric contractions allow the dissipation of mechanical energy during body deceleration (Konow and Roberts, 2015; e.g., descending stairs/walking downhill, in which the quadriceps and plantar flexors muscles generate force while lengthening, to exert a breaking action against downward movement and to maintain balance), but they allow the conversion of kinetic energy into elastic energy of tendons (Hoppeler, 2014)
Summary
Specialty section: This article was submitted to Exercise Physiology, a section of the journal Frontiers in Physiology. We conclude that, when matched for either maximum load or work, similar increase in muscle size is found between ECC and CON RT 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. Eccentric contractions allow the dissipation of mechanical energy during body deceleration (Konow and Roberts, 2015; e.g., descending stairs/walking downhill, in which the quadriceps and plantar flexors muscles generate force while lengthening, to exert a breaking action against downward movement and to maintain balance), but they allow the conversion of kinetic energy into elastic energy of tendons (Hoppeler, 2014) Such energy is regained during limb support, resulting in less muscle work and energy required in locomotion. When velocity of movement approaches 0, will a larger number of cross-bridges be attached and myosin S2 complexes will be fully stretched and able to pull onto the actin filaments to produce bigger values of force (Figure 1B; Huxley and Simmons, 1971; Jones et al, 2004)
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