Why are muscles strong, and why do they require little energy in eccentric action?

It is well acknowledged that muscles that are elongated while activated(i.e.,eccentric muscle action)are stronger and require less energy(per unit of force)than muscles that are shortening(i.e.,concentric contraction)or that remain at a constant length(i.e.,isometric contraction).Although the cross-bridge theory of muscle contraction provides a good explanation for the increase in force in active muscle lengthening,it does not explain the residual increase in force following active lengthening(residual force enhancement),or except with additional assumptions,the reduced metabolic requirement of muscle during and following active stretch.Aside from the cross-bridge theory,2 other primary explanations for the mechanical properties of actively stretched muscles have emerged:(1)the so-called sarcomere length nonuniformity theory and(2)the engagement of a passive structural element theory.In this article,these theories are discussed,and it is shown that the last of these—the engagement of a passive structural element in eccentric muscle action—offers a simple and complete explanation for many hitherto unexplained observations in actively lengthening muscle.Although by no means fully proven,the theory has great appeal for its simplicity and beauty,and even if over time it is shown to be wrong,it nevertheless forms a useful framework for direct hypothesis testing.

Residual force enhancement in human skeletal muscles:A systematic review and meta-analysis

Objective:We reviewed and appraised the existing evidence of in vivo manifestations of residual force enhancement in human skeletal muscles and assessed,through a meta-analysis,the effect of an immediate history of eccentric contraction on the subsequent torque capacity of voluntary and electrically evoked muscle contractions.Methods:Our search was conducted from database inception to May 2020.Descriptive information was extracted from,and quality was assessed for,45 studies.Meta-analyses and metaregressions were used to analyze residual torque enhancement and its dependence on the angular amplitude of the preceding eccentric contraction.Results:Procedures varied across studies with regards to muscle group tested,angular stretch amplitude,randomization of contractions,time window analyzed,and verbal command.Torque capacity in isometric(constant muscle tendon unit length and joint angle)contractions preceded by an eccentric contraction was typically greater compared to purely isometric contractions,and this effect was greater for electrically evoked muscle contractions than voluntary contractions.Residual torque enhancement differed across muscle groups for the voluntary contractions,with a significant enhancement in torque observed for the adductor pollicis,ankle dorsiflexors,ankle plantar flexors,and knee extensors,but not for the elbow and knee flexors.Meta-regressions revealed that the angular amplitude of the eccentric contraction(normalized to the respective joints full range of motion)was not associated with the residual torque enhancement observed.Conclusion:There is evidence of residual torque enhancement for most,but not all,muscle groups,and residual torque enhancement is greater for electrically evoked than for voluntary contractions.Contrary to our hypothesis,and contrary to generally accepted findings on isolated muscle preparations,residual torque enhancement in voluntary and electrically evoked contractions does not seem to depend on the angular amplitude of the preceding eccentric contraction.