INTRODUCTION
The ability to generate power plays a vital role in sport success (6,28). Power is defined as the capacity of the neuromuscular system to generate maximal force in the shortest period or at high velocity (12). Vertical jump (VJ) is a common task in sports performance that is dependent on lower-body power. Strength and conditioning practitioners in sports with jumping tasks consider VJ as one of the critical parameters in monitoring strength and conditioning training adaptations. In the past decade, complex training has received notable attention as a training scheme to improve power (5,9,17,18,38). Complex training combines heavy resistance and plyometric/ballistic/speed training in one training session in an effort to enhance the force-velocity continuum characteristics of an individual. The heavy resistance training (RT) in complex training addresses the force curve component by increasing muscle force output, whereas the high-velocity training targets the velocity curve by producing force at high speeds or over a short period (5,12,30). The neurophysiological mechanism in complex training has been attributed to postactivation potentiation, which refers to an improvement in muscle kinetics from enhanced muscle phosphorylation through calcium sensitivity and h-reflex activity (24,33,34). One complex training variation is contrast training (CT), which is performed by alternating a set of resistance exercise with a set of plyometrics or speed drill (4,14,30). This variation has been proposed to promote an acute ergogenic effect and enhance subsequent performance in power-oriented tasks (9,14).
Given the increasing interest in CT as a means to enhance performance, it is important to gain consensus on the literature on the topic to determine its efficacy and develop evidence-based recommendations on its use. To date, no systematic review or meta-analysis has been conducted to accomplish this task. Thus, the purpose of this article was to conduct a systematic review and meta-analysis on the effect of CT on VJ performance.
METHODS
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement was used to perform a literature search in Google Scholar, SPORTDiscus, World of Science, SpringerLink, and PubMed from all time points until January 30, 2018. The following search terms and Booleans were used: (complex training) OR (contrast training) OR (combined weight training and plyometrics) OR (combined strength training and plyometrics) OR (combined resistance training and plyometrics) AND (vertical jump or jump performance). In addition, manual searches from relevant journals and reference lists from articles were conducted. To be included in the meta-analysis, studies had to meet the following criteria: (a) randomized trials peer-reviewed in English; (b) complex training intervention comparing any RT or plyometric training (PLYO) or control (CON), wherein complex training involves a set of RT followed by a set of high-velocity exercise/plyometrics; (c) included countermovement jump (CMJ) as a dependent variable; and, (d) training intervention performed at least twice a week with a duration of ≥4 weeks.
A single investigator (J.P.) initially assessed the eligibility of the studies for inclusion. In the first stage of screening, titles and abstracts of identified articles were checked for relevance. Reference lists of included articles were also checked for possible inclusion. In the second stage, full-text articles of potential studies were retrieved and assessed individually. Each study was evaluated for 4 times separated by 7 days to reduce selection bias. Studies that were deemed eligible were coded based on the following variables: author/s, participant information, description of exercise/s and activity, intensity of exercise/s, training frequency, study duration, CMJ test protocol, and study result. A second investigator (H.P.) independently checked the data extraction. After coding of data, both investigators rated the included studies for “risk of bias” using an 8-point scale from Consolidated Standards of Reporting Trials (CONSORT) statement. The instrument uses a binary scale with each item scored as either 0 (absently or inadequately described) or 1 (explicitly described and present). A study with a score of 0–2 is regarded as having a high risk of bias, 3–5 as having a medium risk of bias, and 6–8 considered as having a low risk of bias. A consensus was reached for any disagreement presented in data extraction and CONSORT output.
Meta-analysis was conducted using a free software program (RevMan ver 5.3; The Nordic Cochrane Centre, Copenhagen, Denmark). The meta-analysis aimed to examine CMJ height, considered as continuous variable, between: CT versus RT; CT versus PLYO; and CT versus CON. The standard mean difference (mean CMJ difference from CT and comparison group/pooled SD), also known as effect size (ES), was used to compare CMJ differences between groups (10,39,41). ES was interpreted using the following classification system: 0.2—small effect; 0.5—moderate effect; and 0.8—large effect. An inverse-variance random-effects model was used due to the heterogeneous study methods and subject populations. Statistical heterogeneity was examined using chi-squared and I2-Index tests (23). An I2 value greater than 25% is considered to exhibit low heterogeneity, 50% as moderate heterogeneity, and 75% as high heterogeneity.
A subgroup analysis was also conducted to identify potential moderating variables for any difference in CMJ between CT and comparison group. Variables included age (≥20 years versus <20 years) and level of physical ability (competitive versus noncompetitive). Training load intensity (≥70% repetition maximum [RM] versus <70% RM) and study duration (≥6 weeks versus <6 weeks) were also included in the subgroup analysis.
RESULTS
The literature search uncovered 1,067 potential articles and 2 articles identified from reference lists. Removal of duplicates (n = 345) left 742 articles. After screening of title and abstracts, 83 articles underwent a more detailed evaluation and led to the exclusion of 70 articles. Thirteen studies (1–3,8,12,16,19,21,25,27,29,36,37) were ultimately deemed to meet the eligibility criteria for systematic review. Three studies from the systematic review (3,8,29) failed to qualify for meta-analysis due to missing CMJ data for prestudy, poststudy, or both. This left 10 studies (1,2,12,16,19,21,25,27,36,37) for inclusion in the meta-analysis. Flow diagram of the search process is displayed in Figure 1.
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Figure 1.: Flow diagram of search process.
Risk of bias of studies based on CONSORT is presented in Table 1. Eleven studies (1,3,8,12,16,19,21,25,29,36,37) fall within the category of “medium risk” for bias, whereas 2 studies (2,27) fall within the “high risk” category.
Table 1: CONSORT scores of studies included in the systematic review
The total number of participants involved in the review was 441 (males, n = 408; females, n = 33) with ages ranging from 8 to 30 years. Nine studies (1,2,8,12,16,19,21,27,36) included an athlete population, whereas 3 studies (25,29,37) used an active but nonathletic population. Only one study was conducted in an untrained population (3). CT using pairing (1,3,12,21,29,36,37), triad (2,8,25) and combinations of both (16,19,27) were among the CT strategies identified. A majority of studies (1–3,8,12,16,19,21,25,27,29,37) were conducted 2 days a week; only one study (33) used a frequency of 3 times a week. Interventions lasted from 4 to 12 weeks. For identifying CMJ performance, 9 studies used the hands-on-waist CMJ protocol (1,2,8,16,19,21,25,27,29). In one study, the Vertec was used to measure CMJ using the jump and reach protocol (12) and one study used the chalk method (37). One study also used a modified jump and reach protocol without arm swing (3). Finally, CMJ in one study was measured using an unloaded bar in a Smith machine (36). Characteristics of studies are displayed in Table 2.
Table 2: Characteristics of studies
Table 2-A: Characteristics of studies
Table 2-B: Characteristics of studies
Table 2-C: Characteristics of studies
Table 2-D: Characteristics of studies
Table 2-E: Characteristics of studies
Table 2-F: Characteristics of studies
Table 2-G: Characteristics of studies
In the meta-analysis, 4 studies compared CT with RT (1,12,21,36), one differentiated CT and PLYO (12), and 6 studies compared CT with CON (1,16,19,21,27,36). The CON in one study was reclassified into RT because participants underwent stability and light RT (1).
OUTCOMES
Table 3 displays the CMJ output from CT, RT, PLYO, and CON.
Contrast training versus resistance training
Five of 10 studies compared CT with RT (1,12,21,25,36). These studies were found to have considerable heterogeneity at χ2 = 21.03, df = 4, P = 0.0003, I2 = 81%. Meta-analysis showed an effect favoring CT compared to RT with an ES of 1.30 (0.31–2.30), Z = 2.56, P = 0.01. CMJ from CT showed a 12.7% (95% confidence interval [CI] = 3.11–6.51 cm) increase compared with a 5.90% (95% CI = 0.95–3.72 cm) CMJ enhancement in RT. Figure 2 depicts the forest plot comparing CT versus RT.
Figure 2.: Forest plot comparing contrast training (CT) with resistance training (RT). CI = confidence interval.
Subgroup analysis in CT versus RT (Table 3) revealed a nonsignificant CMJ improvement for age (20 years ≥ versus <20 years; P = 0.08) with CT compared to RT. However, longer participation in CT showed a significantly greater CMJ performance (≥6 weeks versus <6 weeks; <0.00001) than in RT.
Table 3: Subgroup analysis for CT versus RT
Contrast training versus control
Comparison between CT and CON was analyzed in 6 of 10 studies. These studies displayed high heterogeneity at χ2 = 33.32, df = 5, P < 0.00001, I2 = 85%. Meta-analysis showed greater CMJ improvements in CT compared to CON with an ES difference of 1.46 (0.46–2.46), Z = 2.85, P = 0.004. On a percentage basis, CT resulted in an increase in CMJ of 8.6% (95% CI = 1.17–4.61 cm), whereas CON showed a decrement in performance of −0.91% (95% CI = −2.38 to 1.13 cm). Figure 3 depicts the forest plot comparing CT and CON. Subgroup analysis in CT versus CON (Table 4) showed a nonsignificant difference in CMJ between subjects ≥20 years and <20 years, P = 0.44. There was also no significant difference in CMJ between ≥70% RM and <70% RM loading schemes (P = 0.34). No subgroup difference in CT duration (≥6 weeks versus <6 weeks; P = 0.05) was also observed.
Figure 3.: Forest plot comparing contrast training (CT) with control (CON). CI = confidence interval.
Table 4: Subgroup analysis for CT versus CON
DISCUSSION
The purpose of this study was to conduct a systematic review and meta-analysis on the effect of CT, a complex training strategy that involves completing a set of resistance exercise followed by a set of high-velocity exercise, on VJ performance. In this review, CMJ was used as the lower-body power parameter when comparing CT with RT/PLYO/CON. Thirteen studies were included in the systematic review with CT interventions administered from healthy untrained population to elite-level athletes of various age population. CT schemes ranged from the traditional complex pairing method to triad or a combination of both, conducted twice to thrice a week over a 4 to 12-week study period. In the meta-analysis, 10 of 13 studies were analyzed. An analysis comparing CMJ output from CT and PLYO was discarded due to lack of available studies to qualify for meta-analysis (12). When comparing CT versus RT, results showed that CT enhanced CMJ to a markedly greater extent compared with RT (ES = 1.30; i.e., large effect). Similar findings were exhibited with CT in comparison with CON (ES = 1.46, i.e., large effect). Superior increases resulting from CT can be attributed to a postactivation potentiation–induced phosphorylation and excitation of h-reflex (11,15,32,34). This hypothesis is partially supported by Labib (2013), who showed increased CD34/CD45 immune system stem-cell secretions after CT (13,26,35). Moreover, evidence shows CT preserves type IIX muscle fibers, which are responsible for generating faster contraction velocities, power, and rate of tension development than type IIA and type I fibers (7,22,37). Another possible explanation for CT-induced enhancement in CMJ performance may be linked to greater volume in CT than in other interventions, which may in turn have produced a greater stimulation of the neuromuscular system (40). However, it should be noted that higher training volumes may induce higher levels of neuromuscular fatigability than in the other groups (20,39). This, in turn, may negatively influence neuromuscular adaptations when combined with sport training (9,32,40).
In this study, subgroup analysis was conducted to determine possible moderators that may have contributed to the difference in CMJ between CT and RT. These covariates included age, training level, loading intensity for resistance exercise, and duration of intervention. Both subgroups for age improved CMJ; however, no difference in CMJ enhancement was seen between age groups. It should be noted that one study in the age subgroup was not included due to ambiguous age data (12). Analysis for loading intensity was not possible in availability of at least two studies in the <70 RM group. Engaging in CT for shorter duration (<6 weeks) did not show improvement in CMJ compared to RT. However, there was a significant enhancement in CMJ with ≥6 weeks of CT compared to RT. There was a significant difference in CMJ between both CT durations. This may be linked to reduction of fatigue induced by CT at the onset of its intervention (13,24,39). No analysis was conducted as to the level of activity due to the presence of only one study in the noncompetitive group.
Subgroup analysis was also administered in CT versus CON. Analysis showed that athletes in the <20-year-old-group exhibited greater gains in CMJ compared with athletes in the ≥20-year-old group; however, the difference between subgroups was nonsignificant. The CT loading intensity of <70 RM showed significant improvement in CMJ than CON. On the other hand, CT loading intensity of ≥70 RM did not post any significant CMJ enhancement compared to CON. There was no significant CMJ difference between both CT loading intensity schemes. One study subgroup analysis for loading schemes was not included due to combining both loading schemes during the intervention (16). Subgroup analysis for level of physical activity was not possible due to lack of available studies in the non-competitive group. Similarly, subgroup analysis for duration was not administered because of non-availability of at least two studies in <6 week CT intervention.
There are several limitations to this meta-analysis that must be considered when attempting to draw practical inferences. First and foremost, analysis is based on a fairly small number of studies on the topic (10 total), and there was substantial heterogeneity in methodology between these studies. In particular, differences exist in the use of specific exercises (i.e., high pulls, squats, box jumps, etc.) as well as the CMJ testing measure (i.e., hands on waist, Vertec, chalk, etc.). Thus, although the data do indicate a positive benefit to CT, the ability to draw strong inferences is limited. Moreover, the combination of mechanical stimuli within a CT program may impair the ability to determine direct causality (18,40). The review also included CT in comparison with RT/PLYO/CON groups only. Future studies including other complex training and RT (e.g., compound training) schemes comparing CT is warranted. Furthermore, only CMJ was used as the dependent variable; other performance parameters may produce different outcomes from CT and provide valuable information on the parameter-dependent effect of CT (2,12,27,31). Finally, the findings are limited to athletes and healthy nonathletes and cannot necessarily be generalized to other populations.
In conclusion, transference of CT in VJ enhancement was superior in comparison with RT and CON. However, changes in CMJ from the use of CT compared with PLYO are unclear.
PRACTICAL APPLICATION
Current evidence supports CT having greater efficacy in improving VJ performance when compared with RT and CON. For sports that include VJ actions, integration of CT for twice to thrice a week for more than 6 weeks into the comprehensive sports training program can help increase leg power in athletes. A common CT exercise scheme for the lower body involves a multijoint exercise (i.e., squat, leg press) followed by a VJ activity (i.e., CMJ jumps). However, coaches may use other CT variations by adding other plyometric/speed exercise(s) after the VJ task. In addition, a rest interval in between exercises (2–6 minutes) should be used to maximize CT results. Finally, when CT is conducted before sports training, applying one CT exercise performed for 3–5 sets and 10–20 minutes of active recovery after CT would seem to be a sound strategy.
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