Periodization is a core scientific concept of training theory and methodology, and is widely acclaimed as being beneficial in exercise prescription, both for performance and health purposes (Issurin, 2008; Naclerio et al., 2013). It consists of the "systematic planning and structuring of training variables throughout designated training timeframes aimed at maximizing performance gains and minimizing the potential for overtraining or decrements in performance" (Harries et al., 2015, p. 1113). As its definition implies, periodization requires training variation (Gamble, 2006), but extends well beyond that. Indeed, it aims at achieving peak performances in certain, pre-specified periods in time (Fleck, 2008; Turner, 2011), while also avoiding overtraining and reducing the risk of injury (Naclerio et al., 2013).
It might be argued that performance or learning environments can rarely be well predicted in advance (Davids, 2012). Learning settings bring about novelty, which by definition cannot be predicted (Ellis, 2005), as it is the result of dynamic self-organizational properties that cannot be established a priori (Davids et al., 2003; Lames, 2003). Furthermore, all predictions have to deal with the sensitivity of systems to the initial conditions (Aicinena, 2013), whereby the slightest differences may result in extremely amplified divergences after a period of time (Cubitt et al., 2015). In this respect, even so-called non-linear periodization is linear in its prediction of outcomes (Denison and Scott-Thomas, 2011), as it uses a sum-of-components approach, which is incompatible with the nature of complex systems (Ellis, 2005).
In light of these concerns, theoretical criticisms have been addressed to research on periodization of training. For example, it has been suggested that research appears to bypass a core principle of training theory and methodology: the divergence between external and internal load (Bailey and Pickard, 2010; Scanlan et al., 2014). Indeed, considerable inter-individual differences in response to training, nutrition, supplementation, and medicine have been well established in sport (Kenney et al., 2012). It is known that some persons are non-responsive to certain types of stimuli, e.g. pulmonary rehabilitation (Stoilkova-Hartmann et al., 2015), cardiac resynchronization therapy (Auricchio and Prinzen, 2011), viral infections (Perng and Chokephaibulkit, 2013), and use of antidepressants (Kudlow et al., 2014), among others. Even within those who are responders, there is a wide range of variation, from low- to high-responders (e.g., Perng and Chokephaibulkit, 2013; Stoilkova-Hartmann et al., 2015).
The existence of non-responsiveness is extensive to training regimes. Following a protocol of live-high, train-low training at 1650 m, Hamlin et al. (2011) found that some athletes markedly changed the sympathetic-to-parasympathetic ratio, while others were non-responsive, i.e., their autonomic nervous systems' activity did not change after the training protocol. Non-responders to altitude training, specifically live high-train low protocols, comprise nearly 50% of the tested population (Paula and Niebauer, 2012). In a study with youth football players, Faude et al. (2013) found that 40% of players subjected to High-Intensity Interval Training did not change their individual anaerobic thresholds. With regard to resistance training, extensive variation in responses have been verified. Subjects range from low-responders to high-responders when changes in muscle size and strength are considered, suggesting we need to further focus research further on inter-individual variation in responses (Ahtiainen et al., 2016; Fisher et al., 2014; Garcia et al., 2016).
Although proponents of periodized programs have underlined the need to respect inter-individual variations in accommodation to a given training stimulus (e.g., Bompa, 1999; Mujika, 2007), research on periodization appears to have ignored such variations (Kiely, 2012; Lames, 2003). Moreover, intra-individual variation in time has escaped analysis in periodization research (Aicinena, 2013; Kiely, 2012; Lames, 2003). Finally, periodization is being equated with variation, although periodized programs are more than random variations; conversely non-periodized programs can be varied (Harries et al., 2015; Kiely, 2012; Turner, 2011). This leads to errors of analysis due to conceptual equating of what are two distinct concepts.
Systematic reviews and meta-analyses have synthesized empirical findings on the application of periodized training programs. Dantas et al. (2010) conducted a systematic review of 103 papers on periodization, having concluded that the models of Classical Periodization, Accumulation-Transformation-Realization, and Structural Bells were superior to models of Prioritized or Block Practices. However, this systematic review included book chapters, technical papers, and non-accredited web sources. In addition, non-periodized programs were equated with constant volume programs, despite the fact that non-periodized programs can be varied. A meta-analysis was published by Dantas et al. (2011), and the results suggested the superiority of the models of Matveev, Verkhoshansky, and Bompa, in comparison to those of Accumulation-Transformation-Realization and Forteza. Unfortunately, this paper suffers from the same problems as the above mentioned systematic review.
In the meta-analysis conducted by Rhea and Alderman (2004), periodized models presented superior results when compared to non-periodized models with respect to strength and power outcomes, but once again non-periodized programs were equated with non-varied programs. A systematic review with meta-analysis (Harries et al., 2015) has shown no differences in the effectiveness of linear versus undulating periodization on strength. The authors stated that their results suggested that variety and novelty in training were the important factors, whereas the specific type of variation might have not been so relevant. Hence, variation alone and not periodized variation might be the key.
Finally, when applying training protocols, it is important to control for nutrition, supplementation and medicine, as these factors may influence outcomes (Kenney et al., 2012). Overall, nutrition strategies and timings, as well as supplementation profoundly impact the outcomes of training programs (e.g., Helms et al., 2014; Perez-Schindler et al., 2014; Pyne et al., 2014). Protein supplementation, for example, enhances hypertrophic gains, being more effective than resistance training alone (Pasiakos et al., 2015; Phillips, 2016). Medicine also interferes with responses to exercise, such as the combined use of statins and exercise training (Deichmann et al., 2015). Even caffeine intake can alter the responses to training stimuli (Kenney et al., 2012). As such, well-designed studies should attempt to at least report on some of these parameters.
We propose, therefore, to conduct a comprehensive review of how empirical research on training periodization with human subjects has been performed. In particular, we aim to answer the following research questions: (1) Are the concepts of periodization and variation actually being used as synonyms? (2) Is research on periodized exercise programs actually testing the direction, timing, and magnitude of adaptations? (3) What timeframes are being considered in such research (e.g., short-, medium-, and/or long-term)? (4) What dimensions of load are being investigated? And (5) Are confounding factors being declared?
The search was conducted in late February 2016 on the following databases: EBSCO + SportDISCUS (specifically selecting Academic Search Complete, CINAHL Plus with Full Text, MedicLatina, MEDLINE with Full Text, PsycINFO, and SPORTDiscus with Full Text), PubMed, Scielo, Scopus and Web of Science. No limitations were imposed concerning date of publication, and in press papers were included. Search and retrieval of papers was conducted by two of the researchers independently and simultaneously. The study was conducted in accordance with the principles of the Helsinki Declaration and was formally approved by the ethics committee of the Faculty of Sports of the University of Porto.
Only original empirical articles published in peer-reviewed journals with a full manuscript available were included. Boolean operators were used for the searches. The titles had to include the terms "periodization" OR "periodized"; orthographic variations such as "periodization" and "periodized" were considered and accepted. The operator AND conjugated these words in the title with "exercise" OR "sport" OR "training" in the title or abstract. As these search engines automatically translate titles written in other languages, articles were included if a full manuscript had been written in one of the following languages: English, French, Italian, Portuguese or Spanish. Only studies with human participants were included. Duplicate papers (i.e., emerging in several databases or more than once in the same database) were counted only once. Overall, 118 papers were retrieved in this stage. Again, two of the authors independently conducted the whole process. Where disagreements were found, a reanalysis of the search and inclusion criteria was implemented.
Twenty-six papers were excluded because they did not actually analyze periodization (e.g., plyometric program vs. weight training program). Observational studies (n = 31) were excluded, as with such designs it is always possible to question: i) if these results have been derived from sport-specific practice and not from the experimental protocol? and ii) if the experimental protocol actually have had a detrimental effect upon what would be the benefits of the sports-specific practice? Case studies (n = 3) were also excluded, as they were observational reports with...