Currently, strength training (ST) is widely recommended to promote healthy aging. This reflects efforts made over the past three decades through which the role of ST as a sarcopenia countermeasure, preventing physical disability and other poor outcomes, has become evident. However, in recent years, considerable attention has been directed to the phenomenon of ST response heterogeneity. Different adaptive patterns among individuals submitted to the same intervention have increasingly led scholars to conventionally label non-responders or low responders those who do not respond appropriately, either for lack of meaningful improvements or even for worsening parameters. In the latter case, such individuals are specifically classified generally as adverse responders since they present responses in opposite direction to a threshold theoretically or empirically determined (Bouchard et al., 2012; Hecksteden et al., 2018). Although there is still no consensus on the definition of non-responders or low responders to ST, showing that their study is in its infancy, an interesting review by Atkinson et al. (2019) suggests that the categorization of a given response should be rationalized by the researcher after analyzing which threshold is clinically or practically relevant, changing the notion that minimal detectable change is imperative to determine who respond or not to an intervention. In this scenario, the phenomenon of non- or low-responsiveness indirectly favored the development of a more restrictive and less important perspective regarding the role of ST. However, reaffirming the ST relevance in aging, Churchward-Venne et al. (2015) have claimed that there are no older non-responders. In that study, it has been shown that all older adults submitted to ST improve at least one analyzed parameter (e.g., functional capacity and muscle strength), defended its widespread application. As suggested by Pickering and Kiely (2019), there are likely no individuals totally unresponsive to training. Nevertheless, the reason why some individuals show less expressive results than others in apparently homogeneous samples remains unclear, especially concerning muscle hypertrophy. It can be noticed in the study conduct by Churchward-Venne et al. (2015) that more than 35 and 30% of the older individuals had a maximum lean mass increase of 0.5 kg after 12 and 24 weeks of ST, respectively. In addition, about 20% had a decrease in lean mass, regardless of protocol duration. Despite the inherent physiological changes of aging, Ahtiainen et al. (2016) demonstrate that the variability of muscle mass response to ST is not affected by age. Although such changes may indeed lead older individuals to a decreased skeletal muscle tissue sensitivity to anabolic stimuli (Yang et al., 2012), it prohibits putting on account of aging the occurrence of inexpressive morphological adaptations in response to training exclusively. Regarding this muscle mass response heterogeneity, while some scholars (Atkinson et al., 2019; Dankel and Loenneke, 2020) recommend cautiousness when claiming its existence, emphasizing that individual differences need to be attested in studies that consider the random error, verified from a matched control group, many others widely recognize it (Hubal et al., 2005; Davidsen et al., 2011; Sparks, 2017; Stec et al., 2017; Camera, 2018; Rantila et al., 2021). Although more studies need to be conducted to demonstrate the true variability of the response, the body of evidence indicates that such heterogeneity should not be ignored. In this context, it is inevitable to admit that more efforts should be made to clarify the reasons behind the low-responsiveness of some individuals, which, in turn, might assist in planning more effective ST programs.
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Mar 7, 2022
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