The long and short of residual force enhancement non-responders
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EDITORIAL
The long and short of residual force enhancement non‑responders Geoffrey A. Power1 · Avery Hinks1 · Parastoo Mashouri1 · Vincenzo S. Contento1 · Jackey Chen1
© Springer-Verlag GmbH Germany, part of Springer Nature 2020
Residual force enhancement (rFE) refers to the ‘extra’ force that is achieved in an isometric contraction following active muscle lengthening as compared to a strictly isometric contraction (i.e., fixed end contraction) performed at the same muscle length and level of neuromuscular activation. As an intrinsic property of muscle contractility, rFE is present from the single sarcomere, to whole muscles of humans during electrical stimulation and voluntary activation (Chapman et al. 2018; Seiberl et al. 2015). In the current issue of EJAP, Bakenecker et al. (2020) investigated the effect of muscle–tendon unit length on transient force enhancement (tFE; force production during an eccentric contraction) and rFE (steady-state isometric force following active lengthening). They found that tFE was indeed present at short and long muscle–tendon unit lengths, while rFE was blunted at shorter lengths. This finding is consistent with the phenomenological mechanisms of rFE, which shows greater magnitudes of rFE on the descending limb of the length–tension relationship as compared with the plateau or ascending limb (Rassier and Herzog 2004). However, it is possible that significant rFE was not observed at short muscle lengths, because 3–4 of the 12 participants displayed no rFE. Conversely, all participants showed rFE at long muscle lengths (see Fig. 3). This offers an important discussion point, from reduced muscle preparations we would expect rFE across all operating muscle lengths, just less at shorter than at longer lengths (Rassier and Herzog 2004). It does not necessarily need to be stated that humans performing voluntary contractions are not isolated muscles in a bath of chemicals, thus a myriad Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00421-020-04511-5) contains supplementary material, which is available to authorized users. * Geoffrey A. Power [email protected] 1
Neuromechanical Performance Research Laboratory, Department of Human Health and Nutritional Sciences, College of Biological Sciences, University of Guelph, Guelph, ON N1G 2W1, Canada
of neuromuscular factors could contribute to blunting rFE. With this, an important consideration in the rFE field is the phenomenon of non-responders. Non-responders have been identified as human research participants exhibiting negligible or no rFE. Various factors related to neural excitation/inhibition and motor unit recruitment, as well as mechanical factors such as series compliance could all contribute to the variability in reported magnitudes of rFE. Understanding the underlying mechanisms of muscle contractility is crucial, but the implications of an intact human neuromuscular system on modulating rFE cannot be overlooked when investigating this historydependent property of muscle.
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