Rebuttal from Martin MacInnis, Lauren Skelly and Martin Gibala

We commend Bishop et al. (2019) for their comprehensive summary but respectfully disagree with their conclusion that training volume is more important than exercise intensity for increasing mitochondrial content in human skeletal muscle. Their position, based on the compiled data, centres on: (1) a positive correlation between training volume and citrate synthase (CS) activity and mitochondrial volume density (MitoVD); and (2) no correlation between training intensity and CS activity or MitoVD. The analysis presented by Bishop et al. (2019) is problematic for isolating the importance of exercise intensity per se. Firstly, on average, ∼37% more training sessions were performed for the moderate-intensity continuous training (MICT) as compared to high-intensity interval training (HIIT) and sprint interval training (SIT) (Granata et al. 2018): the HIIT and SIT interventions involved ∼15 sessions (lasting ∼15 min) compared to 21 sessions (lasting ∼70 min) for MICT (Granata et al. 2018). As indicated by time-course data (e.g. Murias et al. 2011), mitochondrial content increases as a training intervention progresses, which complicates comparisons among the intervention types. Secondly, despite the interval protocols being performed at higher intensities, the training volume for MICT (n = 33) was ∼60% and ∼700% greater than for HIIT (n = 9) and SIT (n = 20), respectively (Granata et al. 2018). As Bishop et al. (2019) demonstrated that training volume was positively associated with changes in mitochondrial content, the similar increase in CS activity across training types, despite differences in training volume, is strong evidence that HIIT and SIT were more efficient than MICT for increasing mitochondrial content. Given the disparities across intervention types, the lack of correlation between exercise intensity and the change in mitochondrial content is unsurprising and is not evidence against the importance of exercise intensity. Finally, while we agree that studies comparing different training protocols are important (Daussin et al. 2008; Granata et al. 2016; MacInnis et al. 2017; Montero & Lundby, 2017; Shepherd et al. 2017), we wish to clarify that the training interventions employed by Gillen et al. (2016) elicited similar increases in CS activity despite SIT involving about one-fifth the total volume of MICT (i.e. not equal volumes), demonstrating that high intensities of exercise can ‘compensate’ for low training volumes. Thus, in addition to our initial arguments (MacInnis et al. 2019), we contend that the pooled analysis from Bishop et al. (2019) supports our position that training intensity is more important than training volume for increasing human skeletal muscle mitochondrial content. Readers are invited to give their views on this and the accompanying CrossTalk articles in this issue by submitting a brief (250 word) comment. Comments may be submitted up to 6 weeks after publication of the article, at which point the discussion will close and the CrossTalk authors will be invited to submit a ‘LastWord’. Please email your comment, including a title and a declaration of interest, to [email protected] Comments will be moderated and accepted comments will be published online only as ‘supporting information’ to the original debate articles once discussion has closed. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. The authors declare that they have no competing interests. M.J.M., L.E.S. and M.J.G. contributed equally to the writing and critical revision of this manuscript. All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. The research programmes of M.J.M. (ID: RGPIN-2018-06424) and M.J.G. (ID: RGPIN-2015-04632) are funded through Discovery Grants from the Natural Sciences and Engineering Research Council of Canada (NSERC). L.E.S. was funded by an NSERC Vanier Canada Graduate Scholarship.

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