Variable Versus Constant Resistance Squat Training for Lower-Limb Strength and Power: A Systematic Review and Meta-Analysis

Abstract

The superiority of Variable Resistance Training (VRT) over traditional Constant Resistance Training (CRT) for enhancing lower-limb performance is debated, with previous meta-analyses limited by aggregating disparate exercises. This systematic review and meta-analysis, the first to focus exclusively on the squat, compared the acute and long-term effects of VRT versus CRT on maximal strength and explosive power. Following PRISMA guidelines, 20 studies were analyzed (literature search up to 15 June 2025), with Hedges’ g used for effect size (ES) calculation. Results demonstrated VRT’s superiority for both acute (ES = 0.34) and long-term adaptations. Acutely, effects peaked with an 8–12 min recovery (ES = 0.43). Long-term, VRT produced greater gains in maximal strength (ES = 0.31) and explosive power (ES = 0.17). Subgroup analyses on maximal strength revealed that elastic bands were highly effective (ES = 0.67), particularly in trained individuals (ES = 0.35), males (ES = 0.41), within cycles < 8 weeks (ES = 0.44), and at frequencies of ≤2 sessions/week (ES = 0.45). For explosive power, chains were most effective (ES = 0.37), significantly improving jumping performance but not sprinting. In conclusion, VRT is a more effective modality for squat training; optimal programs should utilize elastic bands for strength and chains for power, with strength-focused blocks being short-term (<8 weeks) and lower-frequency (≤2 sessions/week) for trained individuals.

1. Introduction

Optimizing resistance training protocols is critical for enhancing athletic strength, a foundational component of physical performance [1,2]. While traditional constant resistance training (CRT), characterized by a fixed load, is a staple method [3,4], advanced modalities such as variable resistance training (VRT) have emerged to improve training efficiency [5,6]. VRT utilizes equipment like elastic bands or chains to dynamically alter the load throughout a movement’s range of motion. This method is designed to accommodate the body’s natural strength curve by applying progressively greater resistance through stronger phases of a lift, thereby inducing distinct neuromuscular adaptations compared to CRT [7].

The theoretical advantages of VRT are multifaceted. During the concentric phase, the ascending resistance profile helps athletes accelerate through and overcome the biomechanical “sticking region,” which may facilitate greater force production, higher movement velocities, and an enhanced rate of force development (RFD) [8,9,10,11]. Furthermore, the accentuated eccentric loading is purported to increase neuromuscular activation and improve the efficiency of the stretch–shortening cycle (SSC), a key determinant of explosive power and performance in dynamic tasks like changing direction [9,12,13,14].

Despite these proposed benefits, the superiority of VRT over CRT remains equivocal, and the existing meta-analytic literature possesses notable limitations. Previous reviews have been constrained by a high risk of bias [15], have focused narrowly on long-term strength outcomes while neglecting explosive power [16], or have aggregated data from disparate upper- and lower-body exercises [17]. Combining distinct movements such as the bench press, deadlift, and squat masks the unique biomechanical demands and specific neuromuscular adaptations associated with each lift. Critically, no systematic review has focused exclusively on the squat—a cornerstone of lower-limb development—to comprehensively investigate the acute and long-term effects of VRT versus CRT on both maximal strength and explosive power.

Therefore, the purpose of this study was to conduct a systematic review and meta-analysis of squat-based training to resolve existing controversies and provide clear, evidence-based guidance for the scientific design of lower-limb VRT programs.

2. Methods

This systematic review and meta-analysis was conducted and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [18].

2.1. Search Strategy and Information Sources

A systematic literature search was performed across three electronic databases (PubMed, Web of Science, and SPORTDiscus) for all relevant articles published from database inception through 15 June 2025. To ensure a comprehensive retrieval of literature, no initial restrictions were placed on publication date or language. The search strategy utilized a combination of Boolean operators (“AND”, “OR”) with the following keywords: (“variable resistance” OR “elastic resistance” OR “elastic band” OR “chains”) AND (“strength” OR “power” OR “jump” OR “force” OR “performance”) AND (“squat”).

To supplement the database search and minimize the risk of missing relevant studies, we employed three additional search techniques: (1) manually screening the reference lists of all included articles and pertinent reviews (i.e., backward citation tracking); (2) reviewing articles that cited the included studies (i.e., forward citation tracking); and (3) using the “similar articles” function within each database.

The literature search was restricted to English-language articles, as non-English full-text articles from which reliable translation of methods and results could not be obtained were excluded.

2.2. Eligibility Criteria and Study Selection

Study eligibility was determined using the Population, Intervention, Comparison, Outcome, and Study Design (PICOS) framework: (1) Population: Healthy individuals of any sex, age, or training background, with no musculoskeletal injuries reported in the six months preceding the study. (2) Intervention: A supervised squat training program that incorporated VRT using equipment such as elastic bands or chains. (3) Comparison: A parallel squat training program using traditional CRT with free weights. (4) Outcomes: At least one quantitative measure of lower-limb maximal strength (e.g., one-repetition maximum [1RM]) or explosive power (e.g., jump height, peak power, rate of force development). (5) Study Design: Between-group controlled trials, including both randomized and non-randomized designs.

Studies were excluded if they (1) did not involve a squat exercise with VRT; (2) lacked a CRT comparison group; (3) were review articles, case studies, conference abstracts, or opinion pieces; (4) had insufficient data for effect size calculation; or (5) were non-English full-text articles from which reliable translation of the methods and results could not be obtained.

The selection process was performed independently by two reviewers (Z.Y., J.W.). After importing all records into EndNote 20 (Clarivate Analytics, Philadelphia, PA, USA) and removing duplicates, reviewers screened titles and abstracts against the eligibility criteria. The full texts of potentially relevant articles were then retrieved for final assessment. Inter-rater reliability for full-text screening was almost perfect (Cohen’s kappa, k = 0.90) [19]. Any disagreements were resolved by consensus or, if necessary, adjudicated by a third reviewer (S.L.).

2.3. Data Extraction

A standardized data extraction spreadsheet was developed in Microsoft Excel 2019. Two reviewers (Z.Y., J.W.) independently extracted the following data from each included study: (1) study characteristics (first author, publication year); (2) participant demographics (sample size, sex, age, training status); (3) intervention details (VRT and CRT protocols, including load, intensity, volume, frequency, duration, and VRT equipment type); and (4) outcome data (mean, standard deviation [SD], and sample size for pre- and post-intervention measurements). Data presented solely in graphical format were extracted using WebPlotDigitizer (Version 4.1) [20]. All discrepancies in extracted data were resolved by consensus between the two primary reviewers. The consistency of extracted continuous data was confirmed to be high (mean Pearson correlation, r = 0.8) [21].

2.4. Risk of Bias and Methodological Quality Assessment

The risk of bias was assessed using tools appropriate for each study’s design. For long-term interventions designed as parallel-group randomized controlled trials (RCTs), the Cochrane Risk of Bias 2 (RoB2) tool was used [22]. For acute interventions employing a randomized crossover design, the Cochrane Risk of Bias 1 (RoB1) tool was selected, as its domains are well-suited to assess potential carry-over effects and other biases specific to this design [23].

In addition, the overall methodological quality of all included studies was evaluated using the 11-point Physiotherapy Evidence Database (PEDro) scale [24]. Studies were categorized as high quality (score ≥ 6), moderate quality (4–5), or low quality (≥3). All assessments were performed by two independent reviewers, with disagreements resolved through discussion.

2.5. Statistical Analysis

All statistical procedures were performed using Stata/MP 17.0 (Stata Corp, College Station, TX, USA) and Review Manager (RevMan, Version 5.4, The Cochrane Collaboration).

2.5.1. Effect Size Calculation

The standardized mean difference (SMD), corrected for small sample bias using Hedges’ g, was calculated to determine the magnitude of the effect. Change-from-baseline scores (mean change and SD of the change) were calculated for both the VRT (experimental) and CRT (control) groups [25]. The SD of the change was calculated using the formula:

$$\mathrm{SD} \mathrm{change} = \surd {{\mathrm{SD} }_{\mathrm{pre}}}^2 + {{\mathrm{SD} }_{\mathrm{post}}}^2 - (2 \times \mathrm{R} \times {\mathrm{SD} }_{\mathrm{pre}} \times {\mathrm{SD} }_{\mathrm{post}})$$

where a conservative pre-post correlation coefficient (R) of 0.5 was assumed when not reported in the primary studies [26].

The effect size was calculated as the difference in mean change scores between the groups, divided by the pooled standard deviation (SD_pooled), and then corrected for small sample bias to yield Hedges’ g [27].

2.5.2. Meta-Analysis and Heterogeneity

Effect sizes were pooled using a random-effects model if significant heterogeneity was present (I² > 50%); otherwise, a fixed-effect model was used. The magnitude of Hedges’ g was interpreted as trivial (<0.2), small (0.2–0.5), moderate (0.5–0.8), or large (≥0.8) [28].

2.5.3. Subgroup and Publication Bias Analyses

To identify sources of heterogeneity and inform practical recommendations, a priori subgroup analyses were conducted based on two potential moderators: (1) VRT equipment type (elastic bands vs. chains) and (2) intervention duration (<8 weeks vs. ≥8 weeks). Publication bias was assessed visually with contour-enhanced funnel plots [29] and statistically with Egger’s regression test [30], which were performed only for outcomes with at least 10 included studies. A p-value > 0.05 for Egger’s test was considered to indicate no significant publication bias.

3. Results

3.1. Literature Search Results

The initial database search yielded 637 articles. After the removal of 360 duplicates, the titles and abstracts of the remaining 277 articles were screened, from which 234 were excluded. The full texts of the remaining 43 articles were assessed for eligibility, leading to the exclusion of an additional 22 articles. One eligible study was subsequently removed due to insufficient data for effect size calculation. Consequently, a total of 20 studies were included in the final meta-analysis. Of these, seven examined acute interventions and thirteen examined long-term interventions. The complete study selection process is illustrated in the PRISMA flow diagram (Figure 1).

3.2. Characteristics of Included Studies

The seven studies reporting on the acute effects of VRT included 95 participants (85% male), with ages ranging from 20.5 to 26 years [31,32,33,34,35,36,37]. The training status of participants varied, including competitive athletes, resistance-trained individuals, and recreationally active individuals. All studies specified the variable resistance load (range: 14% to 40% of 1RM), with six using elastic bands and one using chains. Outcome measures included lower-limb power (five studies) and maximal strength (two studies). Further details are provided in

Table 1. Basic characteristics of acute intervention studies.

Author/Year	Sample Size	Age (years)	Sex	Training Background	Variable Resistance Equipment	Training Type	Sets × Reps	CRT Intensity/1RM	VRT Intensity/1RM	Outcome Measures
Shi/2023 [33]	13	20.5 ± 0.9	Male	University Basketball Players	Elastic Bands	Squat	1 × 3	85%	85% + 20%/30%/40%	CMJ
Marin/2021 [31]	9	21.4 ± 2.1	Male	Elite Baseball Players	Elastic Bands	Squat	3 × 5	85%	55% + 30%	CMJ
Krčmár/2021 [35]	14	21.9 ± 2.3	Female	Athletes	Elastic Bands	Squat	3 × 4	85%	60% + 20% 50% + 30%	CMJ
Nickerson/2019 [34]	12	22 ± 3	Male	Strength-Trained Individuals	Elastic Bands	Squat	1 × 3	85%	71% + 14%	CMJ
Mina/2019 [32]	15	21.7 ± 1.1	Male	Five years high-intensity resistance training experience	Elastic Bands	Squat	1 × 3	85%	50% + 35%	CMJ PP
Mina/2016 [36]	16	26 ± 7.8	Male	Recreationally Trained	Chains	Squat	2 × 3	85%	70% + 30%	1RM Squat
Mina/2014 [37]	16	26 ± 7.8	Male	Recreationally Trained	Elastic Bands	Squat	2 × 3	85%	70% + 30%	1RM Squat

CMJ = Countermovement Jump, PP = peak power, 1RM = One-Repetition Maximum.

.

The thirteen studies examining long-term effects included 331 participants (67% male), with ages ranging from 15 to 44 years [38,39,40,41,42,43,44,45,46,47,48,49,50]. Sex distribution varied, with eight studies including only males, four including only females, and one including both. Intervention durations ranged from 3 to 24 weeks, with frequencies of one to three sessions per week. The variable resistance load ranged from 10% to 44% of 1RM. Eight studies utilized elastic bands, four used chains, and one used a combination. All thirteen studies assessed maximal strength, and seven of these also measured explosive power. Further details are presented in

Table 2. Basic characteristics of long-term intervention studies.

Author/Year	Sample Size	Age (Years)	Sex	Training Background	Variable Resistance Equipment	Training Type	Frequency/Week	Training Period (Weeks)	CRT Intensity/1RM	VRT Intensity/1RM	Outcome Measures
Pan/2025 [38]	30	22	Male	University Students	Elastic Bands	Squat	2	6	80%	64% + 20%52% + 35%	1RM Squat, CMJ, SJ, 20 m Sprint
Liu/2024 [39]	24	25	Female	Elite Boxers	Chains	Squat	1	6	85%	65–70% + 15–20%	1RM Squat, CMJ
Parten/2023 [50]	19	20	Female	Strength-Trained	Elastic Bands	Squat	2	7	80–85%	60–65% + 20%	1RM Squat
Ferland/2022 [47]	17	17–42	Male	Weightlifters	Chains + Elastic Bands	Squat	2	9	80%	80% + 25%	1RM Squat
Shi/2022 [42]	21	21	Male	University Basketball Players	Elastic Bands	Squat	2	8	80–90%	Not Reported + 17%	1RM Squat, CMJ, SJ, SBJ, 20 m Sprint
Jiang/2022 [46]	44	15.48 ± 0.81	Male	Basketball Players	Chains	Squat	2	6	85%	10% VR + 90% CR 20% VR + 80% CR 30% VR + 70% CR	1RM Squat, 30 m Sprint
Sawrer/2021 [43]	40	18–25	Male	Rugby Players	Elastic Bands	Squat	3	3	50–93%	Not Reported + 20%	1RM Squat, VJ
Arazi/2020 [41]	24	24	Female	Untrained	Chains	Squat	3	8	65–80%	50–65% + 15%	1RM Squat
Joy/2016 [45]	14	21	Male	NCAA D II Basketball Players	Elastic Bands	Squat	1	5	40–95%	25–80% + 30%	1RM Squat, CMJ
Andersen/2015 [44]	30	20–44	Female	Healthy Females	Elastic Bands	Squat	2	10	75–85%	Not Reported + 32–44%	1RM Squat
Ataee/2014 [49]	16	20	Male	Strength-Trained	Chains	Squat	3	4	85%	80% + 20%	1RM Squat
Shoepe/2011 [40]	20	19–21	Male/Female	University Students	Elastic Bands	Squat	3	24	67–95%	Not Reported + 20–35%	1RM Squat
Rhea/2009 [48]	32	21	Male	University Students	Elastic Bands	Squat	2	12	75–85%	Not Reported	1RM Squat

CMJ = Countermovement Jump, PP = Peak Power, 1RM = One-Repetition Maximum, SJ = Squat Jump, SBJ = Standing Broad Jump, VJ = Vertical Jump.

.

3.3. Meta-Analysis of Intervention Effects

3.3.1. Acute Effects on Lower-Limb Strength Performance

The meta-analysis of seven acute intervention studies (31 comparisons) revealed that a single bout of VRT squatting produced a small, significant improvement in lower-limb strength and power performance compared to CRT (g = 0.34, 95% CI [0.20, 0.47], p < 0.001) (Figure 2). No significant heterogeneity was detected (I² = 0.0%, p = 0.76), and Egger’s test indicated no publication bias (p = 0.489).

Subgroup analysis based on recovery duration showed the magnitude of this effect was time-dependent. The most significant enhancement occurred with an 8–12 min recovery period (g = 0.43, p < 0.001). A significant improvement was also observed within the 0–3 min window (g = 0.27, p = 0.045). In contrast, the effect during the 4–7 min recovery period was not statistically significant (p = 0.086) (

Table 3. Subgroup analysis of the effect of acute intervention on lower limb explosive power.

Subgroup	Moderator	No. of Comparisons (k)	Hedges’ g	95% CI	p-Value	Heterogeneity
						I2	p-Value
Recovery Time	0–3 min	10	0.27	[0.01, 0.53]	0.045	0	0.89
	4–7 min	10	0.22	[−0.03, 0.47]	0.086	0	0.86
	8–12 min	11	0.43	[0.20, 0.67]	<0.001	0	0.93

).

3.3.2. Long-Term Effects on Lower-Limb Maximal Strength

The analysis of thirteen long-term studies (16 comparisons) demonstrated that VRT was significantly more effective than CRT for increasing lower-limb maximal strength (g = 0.31, 95% CI [0.12, 0.51], p = 0.003) (Figure 3). No heterogeneity was found (I² = 0.0%, p = 0.60), and there was no evidence of publication bias (Egger’s test, p = 0.072).

Subgroup analyses identified several key moderating factors (

Table 4. Subgroup analysis of the effect of long-term intervention on lower limb maximal strength.

Subgroup	Moderator	No. of Comparisons (k)	Hedges’ g	95%CI	p-Value	Heterogeneity
						I2	p-Value
Intervention Type	Elastic Bands	9	0.67	[0.31, 1.03]	<0.001	4.1	0.39
	Chains	6	0.15	[−0.11, 0.40]	0.266	0	0.64
	Chains + Elastic Bands	1	N/A	N/A	N/A	N/A	N/A
Intervention Duration	<8 week	10	0.44	[0.18, 0.70]	0.001	0.4	0.43
Training Status Sex Training Frequency	≥8 week Trained Untrained Male Female Mixed ≤2/week >2/week	6 14 2 11 4 1 8 8	0.09 0.35 0.05 0.41 0.13 N/A 0.45 0.17	[−0.23, 0.42] [0.14, 0.56] [−0.05, 0.62] [0.17, 0.64] [−0.26, 0.51] N/A [0.18, 0.72] [−0.12, 0.45]	0.572 0.001 0.862 0.001 0.510 N/A 0.001 0.247	0 0 0 2.16 0 N/A 8.38 0	0.69 0.52 0.88 0.42 0.84 N/A 0.37 0.84

). Regarding equipment, interventions using elastic bands produced a moderate and significant effect (g = 0.67, p < 0.001), whereas chains did not yield a significant improvement (p = 0.266). The benefits of VRT were significant in programs lasting less than 8 weeks (g = 0.44, p = 0.001), but not in those lasting ≥ 8 weeks (p = 0.572). Furthermore, VRT was significantly more effective in trained individuals (g = 0.35, p = 0.001) compared to untrained individuals (p = 0.862). While a significant effect was found in males (g = 0.41), the analysis for female participants (p = 0.510) was not significant. This finding should be interpreted with caution, as it likely reflects a lack of statistical power due to the small number of studies including women (k = 4), rather than evidence of ineffectiveness. Finally, a training frequency of ≤2 sessions per week resulted in significant strength gains (g = 0.45, p = 0.001), while a higher frequency did not (p = 0.247).

3.3.3. Long-Term Effects on Lower-Limb Explosive Power

The meta-analysis of seven long-term studies (25 comparisons) indicated that VRT produced a small but statistically significant improvement in lower-limb explosive power (g = 0.17, 95% CI [0.02, 0.32], p = 0.02), with no significant heterogeneity (I² = 0.0%, p = 0.50) (Figure 4). Due to the small number of studies (<10), a formal test for publication bias was not performed.

Subgroup analysis revealed several moderators (

Table 5. Subgroup analysis of the effect of long-term intervention on lower limb explosive power.

Subgroup	Moderator	No. of Comparisons (k)	Hedges’ g	95% CI	p-Value	Heterogeneity
						I2	p-Value
Intervention Type	Elastic Bands	17	0.10	[−0.08, 0.28]	0.26	0	0.92
	Chains	8	0.37	[0.06, 0.67]	0.019	32.8	0.17
Intervention Duration	<8 week	17	0.18	[−0.00, 0.37]	0.054	0	0.54
Outcome Type	≥8 week Vertical Jump Horizontal Jump Sprint	8 13 6 6	0.14 0.20 0.38 −0.09	[−0.14, 0.42] [0.00, 0.40] [0.06, 0.70] [−0.40, 0.22]	0.324 0.05 0.02 0.568	0 0 53.42 0	0.48 0.783 0.057 1

). Chains were a significantly more effective stimulus for enhancing explosive power (g = 0.37, p = 0.019) compared to elastic bands, which showed no significant effect (p = 0.26). In terms of intervention duration, programs lasting less than 8 weeks approached statistical significance (p = 0.054), while longer programs did not show a significant effect (p = 0.324). When examining outcome measures, VRT showed a significant improvement for horizontal jump (g = 0.38, p = 0.02) and a nearly significant effect for vertical jump performance (g = 0.20, p = 0.05), but had no significant effect on sprint performance (p = 0.568).

3.4. Methodological Quality and Risk of Bias

The overall methodological quality of the 20 included studies was moderate-to-high, with a mean PEDro score of 5.3 (Appendix A.1). Ten studies were rated as high quality (score ≥ 6) and ten as moderate quality (scores 4–5); no studies were rated as low quality.

The risk of bias assessment using the Cochrane RoB tools identified specific concerns (Appendix A.2). For long-term studies (RoB2), several were rated as having a high risk of bias in domains related to deviations from the intervention (Ferland [47] and Rhea [48], outcome measurement (Jiang [46] and Pan [38]), and selective reporting (Ataee [49] and Shi [42]). For acute crossover studies (RoB1), the primary concern was a high or unclear risk of bias related to the blinding of participants, personnel, and outcome assessors in most studies.

Finally, the assessment for publication bias revealed no significant concerns. Funnel plots for both the acute effects and long-term maximal strength analyses appeared symmetrical, and Egger’s tests were not significant (p = 0.489 and p = 0.072, respectively) (Appendix A.3).

4. Discussion

This systematic review and meta-analysis is the first to focus exclusively on the squat to evaluate the effects of VRT versus CRT. Our findings provide clear evidence that VRT is a superior training modality for enhancing both acute and long-term lower-limb maximal strength and explosive power. Specifically, we found that (1) an acute bout of VRT effectively induces post-activation performance enhancement (PAPE), with an optimal recovery window of 8–12 min; (2) for long-term strength development, elastic bands are most effective; (3) for long-term power development, chains are more advantageous; and (4) the benefits of VRT are most pronounced within training cycles shorter than eight weeks.

4.1. Acute Performance Enhancement

Our analysis confirms that a single bout of VRT squatting significantly enhances subsequent strength and power performance (g = 0.34), primarily through the PAPE mechanism. PAPE describes the phenomenon where a muscle’s performance is acutely enhanced following a conditioning contraction [51,52].Common methods for inducing PAPE include maximal isometric contractions, ballistic contractions, and complex training [53,54,55], and its underlying mechanisms are closely related to the unique biomechanical characteristics of VRT [56,57]. VRT appears to be a particularly effective PAPE stimulus due to its unique “accommodating resistance” profile. The accentuated eccentric load and subsequent concentric acceleration challenge the neuromuscular system by increasing motor unit recruitment and firing frequency, while also enhancing the stretch reflex and efficiency of the SSC [9,58]. Unlike CRT of an equivalent intensity, VRT may provide this potent neural stimulus with less metabolic fatigue, allowing the performance-enhancing effects to dominate [13].

Our findings indicate that the time course of PAPE following VRT is critical. The most significant performance enhancement was observed with an 8–12 min recovery period (g = 0.43), which aligns with the theoretical model where performance peaks after fatigue has subsided but potentiation remains elevated [59]. A smaller, yet significant, effect was also present in the immediate 0–3 min window. This dual-window pattern is consistent with Tillin and Bishop, who noted that PAPE effects depend on the balance between fatigue and potentiation—with early windows (≤3 min) reflecting residual potentiation before fatigue accumulates, and later windows (8–12 min) representing optimal recovery from acute fatigue while preserving potentiation [59]. These results suggest that VRT can be strategically integrated into warm-up protocols, with the specific timing tailored to the athlete’s subsequent activity. However, inconsistencies in the literature regarding optimal VRT intensity and individual responses [32,33,34] highlight the need for further research to delineate a more precise, individualized approach to inducing PAPE.

4.2. Long-Term Adaptations to Variable Resistance

For long-term strength development, this meta-analysis identified a significant advantage for VRT over CRT (g = 0.31). Subgroup analysis revealed two key moderators: equipment type and training duration.

Notably, elastic bands proved superior to chains for increasing maximal strength (g = 0.67). This finding contrasts with a previous, more generalized meta-analysis [17], a discrepancy we attribute to our exclusive focus on the squat. Different exercises possess unique biomechanical and neuromuscular profiles, and the non-linear tension provided by elastic bands may be uniquely suited to the squat’s ascending strength curve [60,61]. The bands demand maximal force output at the top of the movement, potentially leading to greater neural adaptations.

The advantage of VRT was most pronounced in training periods of less than eight weeks (g = 0.44), after which the effect diminished and was no longer statistically significant. This aligns with the established timeline of strength adaptations, where initial gains (first ~8 weeks) are primarily driven by neural factors—such as improved motor unit recruitment and synchronization—while subsequent gains rely more on morphological changes like muscle hypertrophy [62]. VRT, as a potent neural stimulus, likely accelerates these early-phase adaptations. This is further supported by hormonal studies showing that long-term VRT and CRT produce similar anabolic responses, suggesting VRT’s early advantage is not primarily hypertrophic in nature [41].

Subgroup analyses show that when training frequency is ≤2 sessions per week, VRT is significantly more effective than Constant Resistance Training (CRT) in improving lower-limb maximal strength, with a moderate effect size (g = 0.45, p = 0.001). However, this advantage disappears when the frequency increases to >2 sessions per week (p = 0.247). A possible explanation is that VRT’s dynamic load characteristics create a more potent neural stimulus, leading to greater post-training fatigue [63]. A lower frequency of ≤2 sessions per week allows for an adequate recovery window, enabling the nervous system to fully recover and adapt, which is more beneficial for long-term maximal strength gains.

Gender-specific subgroup analyses reveal that VRT’s effects vary. Long-term VRT significantly improves lower-limb maximal strength in males (g = 0.41, p = 0.001), but a similar significant effect was not observed in females (p = 0.51). This could be attributed to physiological differences in hormone levels, muscle mass, and neural control strategies between sexes. However, this conclusion is limited by the sample size, as only four studies on females were included. The subgroup’s low statistical power may have prevented the detection of a smaller, yet real, effect. This reflects a broader gender imbalance in VRT research and highlights the need for more high-quality trials focusing on women.

For individuals without prior training experience, this study did not find sufficient evidence that VRT is superior to CRT for improving squat strength. This is because only two comparison pairs were eligible for this subgroup, leading to insufficient statistical power and unreliable results. Therefore, this finding should be interpreted as a lack of evidence rather than evidence of ineffectiveness. This limitation underscores the need for more high-quality controlled trials on untrained individuals to determine the utility of VRT in the initial phases of strength training.

Our results indicate that VRT produces a small but significant improvement in lower-limb explosive power (g = 0.17). The critical finding from our subgroup analysis was the superiority of chains over elastic bands for enhancing power (g = 0.37). The mechanical properties of chains provide a strong rationale for this outcome.

Explosive power is heavily dependent on the RFD. During traditional free-weight squats, athletes must decelerate the barbell in the final portion of the concentric phase to maintain control. In contrast, the linearly increasing load of chains forces the athlete to continuously accelerate throughout the entire concentric range of motion to complete the lift. This directly trains the capacity for rapid and sustained force application. Furthermore, the inherent instability of swinging chains may impose a greater challenge to neuromuscular control, enhancing intermuscular coordination and neural drive over time [64]. While direct, long-term comparative studies are lacking, our analysis suggests that the continuous acceleration demanded by chains is a more specific and effective stimulus for developing explosive power than the high-end tension provided by bands.

Subgroup analyses for outcome metrics showed that long-term VRT was effective in improving jumping performance. Its improvement in vertical jumping was statistically significant (g = 0.20, p = 0.05), while its effect on horizontal jumping was more definitive with a moderate effect size (g = 0.38, p = 0.02). This positive training transfer is due to the high degree of biomechanical and neuromuscular isomorphism between the VRT squat and jumping movements, and the unique ‘adaptive resistance’ of the VRT, which effectively trains full acceleration and RFD, which is the key to improving jumping performance [9]. It is worth noting that despite the significant improvement in horizontal jumping with VRT, this subgroup analysis also suggested moderate heterogeneity in the results (I² = 53.42%). This inconsistency may stem from methodological differences between the included studies, such as differences in subject characteristics (e.g., Jiang [46] studied adolescent athletes, whereas Pan [38] and Shi [42] studied adults), as well as differences in intervention equipment (e.g., Jiang used chains, whereas the other studies used elasticated bands).

In stark contrast, the improvement in sprint performance by VRT was minimal and not statistically significant (g = −0.09, p = 0.568). The underlying reason for this result, which profoundly reveals the limitations of training migration, is that sprinting is a high-frequency, cyclical, and complex skill, and its motor program is fundamentally different from the non-cyclical, bilaterally synchronized power pattern of the deep squat. Specifically, sprinting requires extremely short touchdown times, rapid single-leg alternation, and strong horizontal propulsion, and these idiosyncratic neurocoordination and biomechanical learning demands [65] cannot be effectively reinforced by VRT squat training. Therefore, although VRT enhances lower limb base explosive power, this gain cannot be effectively migrated to the highly specialized technical movement of sprinting.

5. Limitations

The findings of this review should be interpreted in light of several limitations. First, the included studies exhibited considerable heterogeneity in participant characteristics, particularly in training status and sex. The predominance of male participants limits the generalizability of our conclusions to female athletes and underscores a significant gap in the literature. Second, methodological weaknesses were present in several primary studies. A high or unclear risk of bias, especially concerning allocation concealment and the blinding of participants and assessors, may have influenced the observed effect sizes. Finally, the diversity of metrics used to assess “explosive power” (e.g., various jump types, sprints), each with different sensitivities to force–velocity characteristics and the stretch–shortening cycle, may have introduced variance into the analysis and could mask more subtle, task-specific adaptations.

6. Conclusions and Future Directions

For practical application, this review provides specific guidance. To optimize maximal strength, training programs should utilize elastic bands. The greatest benefits are seen in trained individuals during short-term (<8 weeks) training cycles at a frequency of no more than twice per week. To enhance explosive power, chains are the superior implement, and this training effectively improves jumping ability but not sprinting. Furthermore, VRT can be used as an acute potentiation strategy in warm-ups, with an 8–12 min recovery window being optimal for subsequent performance.

While this review indicates that VRT is most effective when implemented ≤ 2 sessions/week for strength development, it is safe to integrate VRT into weekly training programs with appropriate progression and technical proficiency. For individuals aiming for 3+ weekly squat sessions, the third session could adopt CRT to balance stimulus and recovery, reducing neuromuscular fatigue while maintaining training volume. This hybrid approach ensures consistent overload without compromising adaptation or increasing injury risk.

Practically, VRT is not universally applicable to all resistance exercises. Its efficacy is most pronounced in multi-joint, strength-based movements (e.g., squat, deadlift) that align with the body’s natural strength curve. In typical training sessions, VRT should prioritize these foundational exercises (e.g., placing VRT squat as the primary movement) rather than single-joint exercises, to maximize neuromuscular adaptations and training efficiency.

To build upon these findings, future research should proceed along several key avenues. There is a critical need for studies investigating the differential responses to VRT in underrepresented populations, particularly female athletes of varying ages and training levels. Mechanistic studies should aim to deconstruct the complex interactions between equipment type, load magnitude, and intervention duration; high-quality randomized controlled trials directly comparing the long-term effects of bands versus chains on power are especially warranted. From an applied perspective, research should focus on optimizing VRT protocols for PAPE in warm-up settings and extend this line of inquiry to other foundational exercises, such as the deadlift and bench press.

Overall, while VRT shows significant promise, its effects are moderated by numerous factors. Continued multi-dimensional research is essential to establish a comprehensive, evidence-based foundation for its application in scientific training design.