Table 2.

Study characteristics: experimental design and sample size, exercise prescription, pre-post changes, and statistical outcomes (p-values and effect sizes [Cohen’s d])

Study	Participants’ characteristics	Sport discipline, expertise status	Experimental design	Exercise intervention prescription	Adverse events	Group-specific pre-post changes (deltas Δ)	Inferential statistics	Level of evidencea
Metastable resistance training (e.g., metastable plyometric training) in youth athletes
Büsch et al. [35]	Healthy elite male youth athletes; age: 16–18 years; biological maturity status: post-PHV	Handball; 9.8 years of systematic handball training	RCT; metastable resistance training (n = 9) vs stable plyometric training (n = 10)	Metastable resistance training 10 weeks of training during the in-season with 2 sessions/week; 60–90 min/session; 100–150 jumps/session; plyometric exercises included single and two-legged SJs, CMJs, DJs on BOSU balls, balance pads, and balance beams Stable plyometric training 10 weeks of training during the in-season with 2 sessions/week; 60–90 min/session; 100–150 jumps/session; Plyometric exercises comprised single and two-legged SJs, CMJs, and DJs on stable ground	None	Metastable resistance training SJ: Δ 4.7% CMJ: Δ 8.5% DJ: Δ 5.3% Stable plyometric training SJ: Δ 11.6% CMJ: Δ 3.6% DJ: Δ 11.2%	Main effects of time for SJ and CMJ (all p < 0.004, d = 1.64 for CMJ, d = 2.04 for SJ; no significant group by time interactions for SJ, CMJ, DJ (p > 0.05, d = 0.72–1.16)	2
Granacher et al. [26]	Healthy sub-elite male youth athletes; age: 15 years; biological maturity status: N/A	Soccer; 4.0 years of systematic soccer training	RCT; metastable resistance training (n = 12) vs stable plyometric training (n = 12)	Metastable resistance training 8 weeks of training during the pre-season with 2 sessions/week; 90 min/session; plyometric exercises included CMJs, DJs, hurdle CMJs and DJs on stability trainers, balance pads, and balance beams Stable plyometric training 8 weeks of training during the pre-season with 2 sessions/week; 90 min/session; plyometric exercises comprised CMJs, DJs, hurdle CMJs, and DJs on stable ground	None	Metastable resistance training CMJ: Δ 4.5% DJ: Δ 7.8% CoP: Δ -14.8% Stable plyometric training CMJ: Δ 12.9% DJ: Δ 11.1% CoP: Δ -6.7%	Main effects of time for CMJ, DJ, CoP (all p < 0.016, d = 2.88 for CMJ, d = 1.68 for DJ, d = 1.12 for CoP); no significant group by time interactions for DJ, CoP; for CMJ significant interactions in favour of stable group (p = 0.005, d = 1.32)	2
Negra et al. [36]	Healthy sub-elite male youth athletes; age: 12–13 years; biological maturity status: pre-PHV	Soccer; 4.0 years of systematic soccer training	RCT; metastable resistance training (n = 16) vs stable plyometric training (n = 18)	Metastable resistance training 8 weeks of training during the in-season with 2 sessions/week, 80–90 min/session; 50–120 jumps/session; plyometric exercises included SLJs, CMJs on balance pads and balance beams Stable plyometric training 8 weeks of training during the in-season with 2 sessions/week; 80–90 min/session; 50–120 jumps/session; plyometric exercises comprised SLJs, CMJs on stable ground	None	Metastable resistance training CMJ: Δ 7.0% SLJ: Δ 6.0% SYBT: Δ 12.0% UYBT: Δ 19.0% Stable plyometric training CMJ: Δ 13.0% SLJ: Δ 6.0% SYBT: Δ 9.0% UYBT: Δ 10.0%	ANCOVA showed no significant between-group difference at post for CMJ, SLJ, SYBT, UYBT (p > 0.05, d = 0.08–0.81)	2
Negra et al. [37]	Healthy sub-elite male youth athletes; age: 12–13 years; Biological maturity status: pre-PHV	Soccer; 4.0 years of systematic soccer training	RCT; combined metastable and stable plyometric training (n = 16) vs stable plyometric training (n = 17)	Combined metastable and stable plyometric training 8 weeks of training during the in-season with 2 sessions/week; 80–90 min/session; 50–120 jumps/session; plyometric exercises included CMJs, ankle hops forward on stable ground and balance pads, balance beams, and stability trainers Stable plyometric training 8 weeks of training during the in-season with 2 sessions/week; 80–90 min/session; 50–120 jumps/session; plyometric exercises included CMJs, ankle hops forward on stable ground	None	Combined metastable and stable plyometric training CMJ: Δ 7.1% SLJ: Δ 5.4% SSBT: Δ 34.0% USBT: Δ 84.0% Stable plyometric training CMJ: Δ 8.4% SLJ: Δ 25.30% SSBT: Δ 32.0% USBT: Δ 53.0%	ANCOVA showed no significant between-group difference at post for CMJ, SLJ, SSBT (p > 0.05, d = 0.20–0.41); significant between-group difference at post for USBT (p < 0.01, d = 1.49) in favour of combined metastable and stable plyometric training	2
Combined or sequenced balance and resistance training in youth and youth athletes
Chaouachi et al. [38]	Healthy boys; age: 12–15 years; biological maturity status: circa and post-PHV	Physical education students	RCT; combined balance and stable plyometric training (n = 14) vs stable plyometric training (n = 14) vs active control (n = 14)	Combined balance and stable plyometric training 8 weeks of training with 3 sessions/week in addition to physical education classes with 2 lessons/week; plyometric exercises included CMJs, LJs, DJs, SLLH, and SLS with a focus on minimal ground contact time (40% of exercise time); balance exercises were combined with plyometric exercises with an emphasis on proper landing for 3 s after the performance of plyometrics (60% of exercise time) Stable plyometric training 8 weeks of training with 3 sessions/week in addition to physical education classes with 2 lessons/week; plyometric exercises included CMJs, LJs, DJs, SLLH, and SLS with a focus on minimal ground contact time (100% of exercise time) Active control 8 weeks of regular physical education classes; 2 lessons/week	None	Combined balance and stable plyometric training CMJ: Δ 14.1% SLJ: Δ 10.8% SEBT: Δ 7.7% SSBT: Δ 87.9% Stable plyometric training CMJ: Δ 11.7% SLJ: Δ 9.6% SEBT: Δ 6.3% SSBT: Δ 59.6% Control CMJ: Δ 1.7% SLJ: Δ 0.6% SEBT: Δ 2.3% SSBT: Δ 22.3%	For CMJ, SLJ, SEBT, SSBT within-group magnitude-based inferences showed large (combined balance and plyometric training, d = 0.88–1.62), moderate (stable plyometric training, d = 0.64–0.79) and unclear effects (control, d = 0.05–0.36)	2
Hammami et al. [23]	Healthy elite male youth athletes; age: 12–13 years; biological maturity status: pre-PHV	Soccer; national level elite players	RCT; mesocycle of balance followed by mesocycle of stable plyometric training (n = 12) vs mesocycle of stable plyometric followed by mesocycle of balance training (n = 12)	Mesocycle balance followed by stable plyometric training 4 weeks of balance training followed by 4 weeks of plyometric training during the in-season with 2 sessions/week; balance exercises included kneeling on a Swiss ball, single-legged and two-legged stance on unstable surfaces (e.g., BOSU ball); plyometric exercises comprised CMJs, DJs, LJs, and hurdle jumps; 40–75 jumps/plyometric session Mesocycle stable plyometric followed by balance training 4 weeks of plyometric training followed by 4 weeks of balance training during the in-season with 2 sessions/week; Plyometric exercises included CMJs, DJs, LJs, hurdle jumps; 40–75 jumps/plyometric session; balance exercises comprised kneeling on a Swiss ball, single and two-legged stance on unstable surfaces (e.g., BOSU ball)	None	Mesocycle balance-plyometric training CMJ: Δ 14.3% SLJ: Δ 18.6% SSBT: Δ 169.5% SYBT: Δ 29.5% Mesocycle plyometric-balance training CMJ: Δ 8.6% SLJ: Δ 16.8% SSBT: Δ 130.2% SYBT: Δ 22.0%	Main effects of time for CMJ, SLJ, SSBT, SYBT (all p < 0.01, d = 1.71 for CMJ, d = 3.49 for SLJ, d = 2.38 for SSBT, d = 5.29 for SYBT); significant group by time interactions for THT, SYBT in favor of the balance- plyometric training group (all p < 0.05, d = 2.15 for THT, d = 0.87 for SYBT)	2
Chaouachi et al. [22]	Healthy elite male youth athletes; age: 13–14 years; biological maturity status: circa-PHV	Soccer; national level elite players	RCT; within-session alternated balance and stable plyometric exercises (n = 13) vs within-session blocked balance followed by blocked stable plyometric exercises (n = 13)	Alternated balance followed by stable plyometric exercises 8 weeks of combined balance and plyometric training applied as alternating exercise pairs within training sessions during the in-season with 2 sessions/week; balance exercises included kneeling on a Swiss ball, one-legged stance, leg bridge on the Swiss ball, lunge on BOSU; Plyometric exercises comprised CMJs, DJs, LJs, and lateral hops Blocked balance followed by stable plyometric exercises 8 weeks of combined balance and plyometric training applied as blocked balance exercises followed by blocked plyometric exercises within a single exercise session during the in-season with 2 sessions/week; Balance exercises included kneeling on a Swiss ball, one-legged stance, leg bridge on the Swiss ball, lunge on BOSU; Plyometric exercises comprised CMJs, DJs, LJs, and lateral hops	None	Alternated balance-plyometric exercises CMJ: Δ 20.0% SLJ: Δ 12.0% SSBT: Δ 159.0% SYBT: Δ 12.0% Blocked balance-plyometric exercises CMJ: Δ 25.0% SLJ: Δ 12.0% SSBT: Δ 139.0% SYBT: Δ 6.0%	Main effects of time for CMJ, SLJ, SSBT, SYBT (all p < 0.01, d = 1.40 for CMJ, d = 3.30 for SLJ, d = 1.82 for SSBT, d = 2.44 for SYBT); significant group by time interaction for SYBT in favour of the alternated balance—plyometric exercise group (p = 0.02, d = 0.82)	2
Makhlouf et al. [41]	Healthy elite male youth athletes; age: 10–12 years; biological maturity status: pre-PHV	Soccer; national level elite players	RCT; within session blocked balance followed by stable plyometric exercises (n = 21) vs within-session blocked agility followed by blocked stable plyometric exercises (n = 20) vs active control (n = 16)	Blocked balance followed by stable plyometric exercises 8 weeks of combined balance and plyometric training applied as blocked balance exercises followed by blocked plyometric exercises within a single exercise session during the in-season with 2 sessions/week; Balance exercises included kneeling on a Swiss ball, one-legged stance, leg bridge on the Swiss ball, lunge on BOSU; Plyometric exercises comprised CMJs, DJs, LJs, and lateral hops Blocked agility followed by stable plyometric exercises 8 weeks of combined agility and plyometric training applied as blocked agility exercises followed by blocked plyometric exercises within a single exercise session during the in-season with 2 sessions/week; agility exercises included different ladder drills, with CoD tasks in response to an external stimuli; plyometric exercises comprised CMJs, DJs, LJs, and lateral hops Active control 8 weeks of regular soccer training with comparable training volumes as intervention groups; 2 sessions/week	None	Blocked balance-plyometric exercises CMJ: Δ 18.4% THT: Δ 9.2% SSBT: Δ 145.0% SYBT: Δ 14.7% Blocked agility-plyometric exercises CMJ: Δ 13.4% THT: Δ 8.4% SSBT: Δ 210.5% SYBT: Δ 10.9%	Main effects of time for CMJ, THT, SSBT, SYBT (all p < 0.001, d = 3.13 for CMJ, d = 3.34 for THT, d = 1.96 for SSBT, d = 2.56 for SYBT); significant group by time interaction for CMJ and SYBT in favour of the blocked balance—plyometric exercise group (p < 0.002, d = 1.03–1.32); significant group by time interaction for SSBT in favour of the blocked agility—plyometric exercise group (p < 0.001, d = 1.13)	2
Muehlbauer et al. [28]	Healthy sub-elite male youth athletes; age: 13 years; biological maturity status: circa-PHV	Soccer; 2nd division players	RCT; mesocycle of balance followed by mesocycle of stable plyometric training (n = 8) vs microcycle of balance followed by microcycle of stable plyometric training (n = 9)	Mesocycle of balance followed by mesocycle of stable plyometric training 6 weeks of combined balance and plyometric training applied as a 3-week mesocycle of balance training followed by a 3-week mesocycle of plyometric training during the off-season with 2 sessions/week; balance exercises included the one-legged stance on unstable surfaces, backward beam walk; plyometric exercises comprised ankle jumps, SJs, skater jumps Microcycle of balance followed by microcycle of stable plyometric training 6 weeks of combined balance and plyometric training applied in alternated sequence with a microcycle of balance training followed by a microcycle of plyometric training during the off-season with 2 sessions/week; Balance exercises included the one-legged stance on unstable surfaces, backward beam walk; plyometric exercises comprised ankle jumps, SJs, skater jumps	Two players reported competition-related injuries	Mesocycle balance-plyometric training SJ: Δ 9.0% CMJ: Δ 10.2% DJ: Δ 22.7% SYBT: Δ 5.0% Microcycle balance-plyometric training SJ: Δ 1.2% CMJ: Δ 7.3% DJ: Δ 9.3% SYBT: Δ 4.9%	Main effects of time for SJ, CMJ, DJ, SYBT (all p < 0.03, d = 1.36 for SJ, d = 2.21 for CMJ, d = 1.96 for DJ, d = 1.38 for SYBT); no significant group by time interactions for SJ, CMJ, DJ, SYBT (all p > 0.05, d = 0.99 for SJ, d = 0.32 for CMJ, d = 0.74 for DJ, d = 0.06 for SYBT)	2
Bouteraa et al. [39]	Healthy sub-elite female youth athletes; age: 16 years; biological maturity status: post-PHV	Basketball; regional level sub-elite players	RCT; combined balance and stable plyometric training (n = 16) vs regular basketball training (n = 10)	Combined balance and stable plyometric training 8 weeks of combined balance and stable plyometric training with 3 balance (i.e., kneeling on a Swiss ball, one-legged stance, chest pass balance exercise) followed by 3 plyometric exercises (i.e., vertical jump and reach, double leg zig zag jump, DJs) during the in-season with 2 sessions/week in addition to regular training Regular basketball training 8 weeks of standard basketball training with similar training volume compared with the experimental group	None	Combined balance-plyometric training SJ: Δ 10.3% CMJ: Δ 7.3% DJ: Δ 15.2% SSBT: Δ 127.2% SYBT: Δ 9.7% Basketball training SJ: Δ − 1.8% CMJ: Δ − 3.5% DJ: Δ − 0.7% SSBT: Δ 11.0% SYBT: Δ 1.7%	Main effects of time for DJ, SSBT, SYBT (all p < 0.05, d = 0.096 for DJ, d = 0.96 for SSBT, d = 0.123 for SYBT); significant group by time interactions for DJ (p = 0.016, d = 0.115) and SSBT (p = 0.01, d = 0.77) in favor of the combined balance and plyometric training	2
Chaabene et al. [40]	Healthy elite female youth athletes; age: 17 years; biological maturity status: post-PHV	Handball; national level elite players; 8.0 years of systematic handball training	RCT; balance and complex training (n = 11) vs complex training (n = 12)	Combined balance and complex training 8 weeks of combined balance and complex training during the in-season with 2 sessions/week; Balance exercises included single leg balance stance on unstable surfaces directly followed by complex training exercises in the form of back half squats at 80% 1-RM directly followed by 3–4 sets with 6–10 reps of CMJs Complex training 8 weeks of complex training during the in-season with 2 sessions/week; complex training included back half squats at 80% 1-RM directly followed by 3–4 sets with 6–10 reps of CMJs	None	Combined balance and complex training CMJ: Δ 3.2% SLJ: Δ 6.2% SYBT: Δ 1.0% Complex training CMJ: Δ 7.1% SLJ: Δ 6.2% SYBT: Δ − 0.5%	Main effects of time for CMJ, SLJ (all p < 0.002, d = 1.50 for CMJ, d = 1.70 for SLJ); significant group by time interaction for SYBT (p = 0.007, d = 1.30) in favor of the combined balance and complex training; no significant group by time interactions for CMJ, SLJ (p > 0.05, d = 0.60 for CMJ, d = 0.40 for SLJ)	2

1-RM one-repetition maximum, ANCOVA analysis of covariance, CMJ countermovement jump, CoD change of direction, CoP center of pressure displacements during balancing, DJ drop jump, LJ line jump, N/A not applicable, OLDJ one leg distance jump, PEDro Physiotherapy Evidence Database, PHV peak height velocity, RCT randomized controlled trial, reps repetitions, SEBT star excursion balance test, SJ squat jump, SLCJ single-leg cone jumps, SLJ standing long jump, SLLH single-legged line hop, SLS single-legged squats, SSBT stork balance test on stable ground, SYBT Y balance test on stable ground, THT triple hop test, USBT stork balance test on unstable ground, UYBT Y balance test on unstable ground

^aLevel of evidence for individual studies was rated according to the Strength of Recommendation Taxonomy (SORT) [34]. Level 1 is indicative of good-quality evidence, which is present with high-quality RCTs (i.e., allocation concealed, blinding, intention-to-treat analysis, adequate statistical power, adequate follow-up). See PEDro scores in Table 3. Level 2 means limited-quality evidence which is present with lower quality clinical trials, cohort studies, or case–control studies. Level 3 stands for other evidence (i.e., consensus guidelines, usual practice, opinion)