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Potential Confounding Effects of Intensity on Training Response.

We read with great interest the important study by Zinner et al. (1) that suggested a greater risk of nonresponding when recreational runners complete both polarized (POL) and high-intensity (HIGH) training mesocycles, when compared to a low intensity (LOW)mesocycle. Although this study presents several methodological strengths and valuable information,we feel compelled to point out some potential challenges in the interpretation of the results. First, from a coaching perspective, we would argue that the loads completed in all the training programswere very demanding, based on the training background of the novice runners who served as participants. This is not a trivial issue given the high number of evaluations conducted, including eight maximal efforts over 8 wk, which, when added to the training workload (volume–intensity), may well have induced suboptimal adaptations or even overreaching in some cases. For this group of runners, the training plus testing may have been an excessive load increase. In our experience as both researchers and coaches, a lower total workload, independent of intensity distribution, might be expected to induce a better performance outcome in the three experimental groups in this study (1). For instance, a previous study (2) with active nonrunners showed significant improvements in running performance after 4 wk of interval versus continuous training regimes equated by total load, but with an importantly lower training volume when compared to the current study (1). Furthermore, this is especially relevant in the POL and HIGH groups because only these groups were exposed to the highly demanding 4 4 min sessions (3). Therefore, although the minimum dose for optimal adaptations in novice runners remains to be clarified in further studies, we would like to know the authors’ opinion about these issues, considering also that habitual (nontraining) physical activity has been previously demonstrated to influence aerobic adaptations of recreational runners (4). Second, the methods selected for training prescription and monitoring are not without some limitations. For instance, as the training impulse load was increased by 50% during the 3-wk mesocycle, based on the 4-wk wash-in mesocycle, the POL group seems likely to have been exposed to a greater training load than the other groups as they exhibited a significantly greater weekly training impulse during the wash-in mesocycle. Further, the intensity distribution of POL (see Table 1) during the 3-wk mesocycle is not a good reproduction of the typical