Resistance training (RT) is a specialized form of conditioning using a range of resistive loads and a variety of training modalities designed to enhance health, fitness, and sports performance (9). Participation in RT results in numerous performance- and health-related benefits in adolescent and adult populations (9,22,33). These benefits include improvements in athletic performance, musculoskeletal health, muscular strength, power and endurance, motor performance including jumping ability, balance and coordination, and cardiovascular and metabolic health (9,11,14,22,33,38,40). The American College of Sports Medicine (ACSM) recommends the use of periodized RT programs based on evidence that such programs are more effective than nonperiodized programs (33). Periodization is the systematic planning and structuring of training variables (intensity, volume, frequency, and rest) throughout designated training timeframes aimed at maximizing performance gains and minimizing the potential for overtraining or decrements in performance (3,5,16,25–27,29,32,35,36).
There is debate regarding the terminology used to describe periodized programs (13,19,31) and the most effective manipulation of key training variables to improve neuromuscular performance for a wide variety of populations is yet to be determined (5,28,32,35,36). Two of the most commonly referred to periodization models in the literature are linear periodization (LP) and undulating periodization (UP). Linear periodization has been described as involving the breakdown of the training year into weekly (microcycle), monthly (block or mesocycle), and multi-monthly (cycle or macrocycle) periods. A key characteristic of LP is an initial high volume and low intensity of training with gradual increases in intensity and decreases in volume within and across training periods (3,5,15–17,20,25–27,33,35–37). Undulating periodization has been described as more frequent, daily, weekly, or biweekly variation of intensity and volume and generally uses repetition maximum zones to prescribe exercise intensity (5,16,17,20,26,28,29,32,33,35–37). Undulating periodization is commonly identified as daily undulating periodization (DUP) or weekly undulating periodization (WUP) depending on whether volume and intensity of RT is manipulated on a daily or weekly basis. It has been proposed that these nonlinear manipulations of volume and intensity, providing more frequent changes in stimuli and periods of recovery, are more conducive to strength gains (3,5,16,26,27,32,35,36).
A number of studies have compared the effects of LP RT programs with UP or nonlinear periodized programs. The aims of this review were to (a) systematically identify and examine all studies directly comparing linear and undulating periodized RT programs and to synthesize the results, (b) quantitatively compare linear and undulating periodized RT programs' effects on muscular strength using meta-analysis, (c) evaluate the risk of bias in previous studies and provide recommendations to improve the quality of future studies, and (d) review the study populations in which the comparisons of these resistance training programs have been investigated.
Experimental Approach to the Problem
The conduct and reporting of this review was guided by the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement (23). A comprehensive search of the MEDLINE, SCOPUS, and SPORTDiscus databases was conducted on April 27, 2012. A librarian assisted in the development of unique search strategies for the different databases. No year restriction was placed on the search. Titles and abstracts of identified articles were checked for relevance in the first stage of screening. In the second stage, full-text articles were retrieved and considered for inclusion. Finally, the reference lists of included articles were screened for additional articles. The search was updated to the July 28, 2014, during the revision process.
Studies were assessed for eligibility based on the following inclusion criteria: (a) participants were from a nonclinical population, (b) study compared the use of a linear RT program (LP) with an undulating periodized RT program (UP) (free weights, bodyweight resistance [including plyometrics], elastic tubing, machine weights, isokinetic devices), (c) study involved a randomized controlled trial (RCT) or quasi-experimental design, (d) study included a quantitative assessment of muscular strength, and (e) study was published in English. Conference abstracts, dissertations, theses, and articles published in non–peer-reviewed journals were not included. No restriction was placed on participant age or training experience.
Studies had to meet the following additional criteria to be included in the meta-analysis: (a) assessed muscular strength by a bench press, squat, or leg press repetition maximum test; and (b) data were reported as means and SDs for the linear and undulating periodized groups at post-test. Separate meta-analyses were conducted for studies that assessed bench press, squat, or leg press. Authors were contacted in attempts to obtain further details when required.
Meta-analyses have been strongly emphasized for their utility to provide a quantitative summary of treatment effects and their use as a tool to bridge the gap between the science and practice of exercise prescription (30). All meta-analyses were performed in RevMan (6). The meta-analyses sought to determine the effect of the periodization approaches on upper- and lower-body muscular strength. Muscular strength was considered a continuous data variable; therefore, the mean difference (MD) with 95% confidence intervals were used to determine effect measures. The inverse-variance random effects model was used for the meta-analysis procedure because of studies being performed with varied populations and methods. The χ2 and the I2-Index tests were used to examine statistical heterogeneity. A previous meta-analysis provided the following guide for the interpretation of heterogeneity based on the I2-Index: 0–40% might not be important, 30–60% may represent moderate heterogeneity, 50–90% may represent substantial heterogeneity, and 75–100% considerable heterogeneity (8,14).
Studies were assessed for “risk of bias” using criteria adapted from the Consolidated Standards of Reporting Trials (CONSORT) statement by 2 authors independently, and in the case of disagreement, further discussion was undertaken to achieve consensus. A “risk of bias” score for each study was completed on an 8-point scale by assigning a value of 0 (absent or inadequately described) or 1 (explicitly described and present) to each methodological item listed in Table 1. Studies that scored 0–2 were regarded as having a high risk of bias, studies that scored 3–5 were classified as having a medium risk of bias, and those that scored 6–8 were classified as having a low risk of bias.
The flow of studies through the review process is reported in Figure 1. Twenty-five full-text articles were assessed; 17 met the inclusion criteria (Table 2), and 17 were included in the meta-analyses. Twelve studies compared the effectiveness of LP and DUP RT programs (7,12,15–17,20,26,27,29,32,35,36). Three studies compared LP and WUP RT programs (1,3,39). One study compared LP, WUP, and DUP programs (5), and 1 study compared an LP program with a program incorporating both WUP and DUP (37).
There were a total of 510 participants in the included studies. Twelve of these studies assessed males only, 3 studies females only and 2 studies assessed both males and females. The average age of participants was 24 years (SD: 5), with a range of 19–39 years. One study did not report participant age. Resistance training experience was reported in all studies. Seven studies were conducted in untrained participants (<1 year RT experience), whereas participants in 10 studies were identified as trained (≥1 year RT experience). No studies were conducted in advanced resistance trained participants (>5 years RT experience). Participants in 10 studies were identified as recreational trainers, in 1 study participants were sedentary, 2 studies were conducted in American football athletes, 1 study was conducted with Judo athletes, and participants in 2 studies were tactical service operators (firefighters or military personnel).
The mean duration of RT programs was 12.6 ± 4.1 weeks. Nine of the studies were of 12-week and 3 studies were of 9-week duration. Mean training frequency was 3.2 ± 0.7 sessions per week. Session duration was not reported in 10 of the 17 studies. Most RT programs (11 studies) used a combination of both multi-joint and single-joint free weight and machine-based exercises. Five studies consisted of mostly multi-joint free weight exercises (3,16,17,27,29). One study used single-joint machine-based exercises only (15).
Maximal strength was assessed in all studies. Sixteen studies assessed upper-body strength by a repetition maximum bench press test. Lower-body strength was assessed using a repetition maximum squat test in 7 studies and a leg press repetition maximum test in 7 studies. Of the included studies, 16 reported statistically significant increases in maximal strength for both LP and UP RT programs (1,3,5,7,12,15,16,20,26,27,29,32,35–37,39). Twelve of these studies found no significant difference in maximal strength gains between LP and UP RT programs (3,5,7,12,15,16,20,26,29,32,36,39). Three studies found a significant difference favoring UP RT programs (27,35,37), whereas 2 studies found a significant difference favoring the LP group (1,17).
After the initial risk of bias assessment, there was 96% agreement between authors and full consensus was achieved after discussion (Table 1). There was a high risk of bias in 4 studies (23.5%) and a medium risk in 13 (76.5%) studies. No studies had a low risk of bias. Thirteen studies reported randomizing participants to groups; however, no study adequately described the randomization process. No studies reported using blinded assessors. No studies reported a power calculation to determine whether their study was adequately powered to detect their hypothesized effects. In addition, effect sizes were reported in only 5 of the included studies (7,20,26,36,37). Eighty percent of participants completed follow-up assessments in 13 studies. Analyses in all studies accounted for potential baseline differences. All but 3 studies equated the volume and intensity between training groups.
Sixteen studies were evaluated in a meta-analysis comparing 1 repetition maximum (1RM) bench press at postintervention (Figure 2). Overall, the studies were found to be moderately heterogeneous (χ2 = 33.41, df = 15 [p = 0.004], I2 = 55%). The meta-analysis showed no clear effect for either LP or UP (MD = 1.71 [−2.05 to 5.47] kg, Z = 0.89 [p < 0.37]). Seven studies were evaluated in a meta-analysis comparing 1RM leg press at postintervention (Figure 2). These studies had significant heterogeneity (χ2 = 16.55, df = 6 [p = 0.01], I2 = 64%). No clear effect was shown for either LP or UP (MD = 25.93 [−2.48 to 54.35] kg, Z = 1.79 [p = 0.07]). Seven studies were evaluated in a meta-analysis comparing 1RM squat at postintervention (Figure 2). These studies were homogenous (χ2 = 7.83, df = 5 [p = 0.17], I2 = 36%). No effect favoring LP or UP was found (MD = −1.67 [−10.88 to 7.54] kg, Z = 0.36 [p = 0.72]).
This review identified 17 studies that directly compared LP and UP programs. Studies were mostly conducted in young adult males with limited RT experience. Most studies found no differences between the 2 periodization models, and this was supported by the findings of the meta-analyses where no difference was identified for both upper- and lower-body strength. There is substantial room to improve the quality of future studies comparing training manipulations to reduce the risk of bias. There is also a lack of studies investigating more athletic or highly resistance trained populations as well as adolescents and over long time frames.
The RT programs evaluated in this review were predominantly short-term interventions and only 4 studies had a duration greater than 12 weeks. Two studies found DUP RT resulted in increased strength in the initial weeks of the intervention with no increase in strength with LP RT until later stages of the intervention (27,35). The short-term nature of these interventions makes it difficult to draw conclusions regarding the long-term effectiveness of LP or UP. Longer-term interventions are needed in order to assess the purported advantage of greater variation in UP being more effective at breaking strength plateaus than LP (5,10,17,20,26,29,35).
The majority of study participants were adult males and none of the studies investigated the effects of LP and UP in adolescents. Additionally, supporting evidence for both forms of periodization is lacking in novice and athletic populations. Generalized training theories underpin the rationale for periodized training programs (32,35,36,40). Selye's General Adaptation Syndrome states that if a stress or bout of exercise is experienced by a system, the system will respond with a temporary decrease in performance followed by restitution returning to or above the initial level of physical fitness (2,40). This enhancement of physical fitness is termed supercompensation (2,40) and is the primary purpose of all training interventions where an improvement in physical fitness is sought. If the applied stress remains at the same magnitude (intensity, volume and frequency) the system will accommodate to this stress and no further improvements in physical fitness will occur (2,40). To avoid this accommodation, training programs must be varied over time (40).
Previous training history and training status will influence adaptations to further training interventions, particularly in respect to muscular strength. Over a 4-week to 2 year period muscular strength increases of 40, 16, 10, and 2% are representative of the expected improvements in untrained, trained, advanced and elite resistance trained individuals, respectively (21,33). Most participants in this review had some prior RT experience and were identified by study authors as trained. When planning training interventions it is important to consider generalized theories of training adaptation and in particular the initial level of physical fitness or physical preparedness of participants. The description of study participants' previous training history was poorly reported in most of the included studies. For example, Rhea et al. (35) indicated that all participants in their 12-week study reported undertaking RT equivalent to a LP approach during the 2 years prior to the study but did not describe the volume or frequency of training. They found a significant difference favoring DUP for strength improvement only in the first 6 weeks of the intervention. However, no significant difference in strength gains between groups was found in the last 6 weeks of the intervention. Prior experience with LP RT creates the potential for UP to provide a more novel stimulus. It is reasonable to suggest that the novelty or variation in stimulus compared to participants' previous training experience is of greater importance for eliciting strength improvements and overcoming accommodation than the specific type of periodization approach employed. There is a need for authors to clearly describe the training experience of their participants with different periodization approaches.
The majority of studies included in this review found significant increases in muscular strength for both periodization approaches, whereas significant differences between approaches were rarely found. One possibility is that studies were underpowered to detect statistically significant differences. Considering the small sample sizes often involved in sports science studies, the reporting of effect sizes may be more practically meaningful in RT interventions (4,18,34). Only 5 of the studies included in this review reported effect sizes (7,20,26,36,37). In a previous review, Rhea (34) reported that the effect sizes in RT studies were much larger than those typically observed in the social/behavioral fields. He recommended scales for assessing the practical significance of effect sizes in RT research based on participant's training status (34). This scale highlights the importance for RT research studies to adequately describe the training history and background of participants. Comprehensive reporting of effect sizes in the scientific literature will enable strength and conditioning professionals to use theoretical knowledge and implement practical evidence-based training programs. Further within group variations in baseline strength and responsiveness may also influence the capacity to detect differences between training approaches.
To the authors knowledge this is the first systematic review and meta-analysis comparing linear and undulating periodized RT programs. A strength of a systematic review is that the criteria for inclusion is determined prior to the search and is designed to minimize reviewer bias in regards to what is included. This objectivity is strengthened by adherence to the PRISMA reporting guidelines and CONSORT statement. A strength of combining a systematic search with a meta-analysis is that it allows data from multiple studies to be combined to determine an outcome. This is particularly advantageous when studies have small sample sizes and risk an inability to identify differences due to lack of statistical power. A number of limitations should be noted. Firstly, there may be bias in the selection of studies as abstracts, theses, or studies published in non peer-reviewed journals were not included. Additionally, there was considerable heterogeneity between studies and no study adequately described the randomization of participants. Therefore, caution should be taken in the interpretation of the meta-analysis results.
The results of this systematic review and meta-analysis reveal that both LP and UP RT programs can increase maximum strength substantially, but no clear evidence favoring either periodization approach was found for the development of upper or lower body strength. The results suggest that novelty or training variety are important for stimulating further strength development. When the work performed is equal, neither periodized approach is necessarily superior and either approach can be used to provide variety and therefore enhance adaptation. Potentially the implementation of short training blocks, of 2–6 weeks duration, using either LP or UP RT within current training regimes may provide an adequate and novel stimulus to promote further strength increases and overcome plateaus. Therefore, strength and conditioning professionals are advised to design periodized training programs taking into account the RT principle of “variety” to prevent stagnation and accommodation to a particular training approach. Careful consideration should be given to the previous training history and current training status of participants.
Further research is needed in adolescent, athletic, and possibly for rehabilitation (24) populations to investigate the effects of different periodized approaches to RT. Furthermore, longer-term studies are also needed to determine and compare the long-term effectiveness of LP and UP RT on strength development. Researchers are advised to adequately report the previous training history of participants, stratify assignment to groups on the basis of prior training experience, or implement standardized pre-intervention training to reduce the influence of training history on intervention effects.
The authors of this article would like to thank the investigators who responded to requests to provide additional information for the meta-analysis. No external funding was used for this project. The authors have no competing interests relating to the content of this manuscript. There were no other contributors to this manuscript.
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