Table 3.

Characteristics of the non-randomised studies included in the systematic review.

Reference	Participants Level	Intervention	Comparison	Outcomes	Compliance Rate	Reliability /SWC	Results	Comments
Malliou et al. 2004 [75]	100 young soccer players (IG = 50, age: 16.7 ± 0.5; CG = 50, age: 16.9 ± 0.7 years) First Greek division	IG: Proprioception training program Duration: 12 weeks (competition period) Frequency: 2/week Session duration: ≈20 min Training components: Balance exercises (in order to maintain balance while they were performing soccer agilities, such as headers) performed on Biodex Stability System device, mini trampoline and balance boards.	CG: Same training than IG but without any special-extra balance training	Total stability, anterior-posterior and medial-lateral index in a balance test	NR	NR	↓ Total index (ESc = 1.13, p < 0.001), A-P Index (ESc = 1.20, p < 0.001), and M-L Index (ESc = 0.69, p < 0.05) in IG	Supervisor not specified.
Gioftsidou et al. 2008 [67]	68 male soccer players (age: 24.1 ± 5.7 years) Professional	IG: Isokinetic training program Duration: 8 weeks Frequency: 3/week Session duration: ≈60 min Training components: 10 sets in a velocity spectrum exercise (5 sets with both flexor and extensor muscle groups + 5 sets with only the weak muscle group based on the initial measurement)	CG: NR	Concentric H/Q ratio at 60 and 180 °/s. Differences between limbs in peak torque for flexors and extensors.	NR	NR NR	↑ H/Q ratio at 60 °/s and 180 °/s at both legs (ESc = 0.51–0.87) in IG. ↓ Difference between limbs in peak torque in knee extensors and knee flexors at 60 and 180 °/s (ESc = 1.18–1.75)	Supervisor not specified. Lack of information regarding CG, and weak in the IG. Disparity between groups (IG = 41, CG = 27 players). p values and results in CG not specified.
Grooms et al. 2013 [54]	41 male soccer players (CG = 30, age: 20.3 ± 1.6; IG = 34, 20.0 ± 2.4 years) NCAA Division III	IG: F-MARC 11+ Duration: ≈12 weeks Frequency: 5–6/week Session duration: ≈20 min Training components: (1) running exercises with dynamic stretching and controlled perturbations; (2) strength, balance, and jump-landing control with progression; and (3) higher-speed running drills with cutting manoeuvres.	CG: Standardised warm-up (1st season)	Relative risk (per 1000 athlete-exposures) of ACL injuries	NR	NA	↔ No ACL injury occurred during either season.	Supervisor: Athletic trainer. Low sample to analyse risk ratio of ACL injuries. 7 dropouts in CG
Sliwowski et al. 2015 [79]	24 junior male soccer players (IG = 14, age: 17.0 ± 0.78; CG = 10, age: 17.1 ± 0.71 years) Polish U17 championship	IG: RT program specific to imbalance Duration: 6 weeks Frequency: 2/week Session duration: ≈90 min Training components: Two parts: (1) a set of 5 reps at 80% of 1RM of 12 upper and lower body exercises, 3–5 min of rest between sets, and (2) 2–3 additional series of 5–7 reps at 80% of 1RM. Depending on the imbalance, the additional exercises were performed for the muscle groups of a given extremity.	CG: Only performed the first part of the RT program	Conventional H/Q ratio for D and ND (54.9% considered deficient) at 60 °/s. Bilateral ratio between knee extensors and flexors at 60 °/s.	NR	NR/ NR	↑ H/Q ratio in D leg (Esc = 0.43, p < 0.05) in IG. ↔ No changes in the bilateral differences in any group.	Supervisor: Qualified strength training instructor. Disbalance between groups. Higher H/Q ratio in CG than in IG at baseline.
Ibis et al. 2018 [72]	42 male soccer players (CG = 14, age: 22 ± 1.35; IG1 = 14, age: 23.21 ± 2.29; IG2 = 14, age: 23 ± 1.51 years) Amateur	IG1: Strength training group Duration: 8 weeks Frequency: 3/week Session duration: ≈60 min (estimated) Training components: 4 series of additional strength training at increasing load (80–95% of 1RM) of 7 lower extremity strength exercises in a pyramidal method. IG2: Individual-specific strength training group Duration: 8 weeks Frequency: 3 weeks Session duration: ≈75 min (estimated) Training components: Same training than IG1 but with 4 additional series of 2 extra exercises depending on the participant deficiencies (those with H/Q rate low or bilateral flexor deficiency performed two extra knee flexor exercises, and those with bilateral extensor deficit performed two extra knee extensor exercises).	CG: Regular training	Conventional H/Q ratio and bilateral differences (BLD) for hamstrings and quadriceps at 60, 180 and 300°/s	NR	NR/ NR	↓ H/Q ratio at 300 °/s in ND (ESc = 0.58, p = 0.026) in CG. ↑ H/Q ratio at 60 °/s in D (ESc = 0.43) in IG1 and in D and ND, 180 °/s in D and ND, 300 °/s in D and ND (ESc = 0.58–1.68) in IG2.	Supervisor not specified. Low sample. 2 training/week CG vs. 5 training/week IG1 and IG2, and also IG2 > IG1 regarding workload.
Arundale et al. 2018 [59]	68 women soccer players (IG = 48, CG = 20 in one testing point; IG = 22, CG = 15 all time points). NCAA Division I and II	IG: FIFA 11+ Duration: 1 competitive season Frequency: 3/week Session duration: ≈20 min Training components: (1) Running exercises that encompass cutting, COD, decelerating and proper landing techniques; (2) strength, plyometric and balance exercises that focus on core strength, eccentric control and proprioception; (3) running exercises.	CG: Standard warm-up	Peak hip flexion, adduction and internal rotation angles and moments, and peak knee flexion and abduction angles and moments in a DVJ from 40 cm.	NR	ICC = 0.63–0.92/ SDC and MID directly measured from the pre-season data.	1st season: ↔ No differences between groups in postseason peak knee abduction when controlling for baseline differences. No significant time x group interactions for valgus collapse value for either limb. Both groups increased peak knee abduction angle and peak hip abduction an external rotation on the ND (IG) and on the D (CG), but no time x group interaction. ↓ Clinically meaningful decrease (>MID) in peak knee flexion angle in CG for D leg (F(1,35) = 7.64, p = 0.05, np2 = 0.18) 2nd season: ↑ ND hip abduction angle (>SDC), ND hip external rotation angle (>SDC), D knee abduction angle (>MID) in 1st season vs. 2nd season, and ND peak hip flexion angle (>SDC) in 2nd season vs. 1st season (time x season). ↓ ND hip flexion angle (>SDC) in 1st vs. 2nd season (time x season). ↔ No significant time x season interaction in valgus collapse in either leg.	Supervisor: Athletic trainer.
Dos’Santos et al. 2019 [66]	26 male soccer players (IG = 13, age: 16.9 ± 0.2; CG = 13, age: 17.8 ± 0.3) Professional club (U17)	IG: COD speed and technique modification Duration: 6 weeks Frequency: 2/week Session duration: 20 min Training components: COD velocity and technique modification	CG: Regular field-based warm-up	CMAS score in a 90° COD with both limbs	IG = 88.5% CG = 90%	ICC = 0.774–0.934 SEM = 0.49–0.69 CV = 11.4–22.2/ SDD = 1.36–1.84	↓ CMAS in right (p = 0.025, g = −0.85, −22.5%), and left (p = 0.018, g = −1.46, −33.9%) legs in IG. ↔ No change in CG.	Supervisor: Certified S&C specialist. Some dropouts of participants in the IG (n = 5).
Krutsch et al. 2020 [55]	1527 male football players (IG = 529, age: 22.7 ± 4.3, CG = 601, age: 21.9 ± 4.1 years) Elite	IG: Newly implemented football-specific training modules Duration: one season Frequency: At least 2/week Session duration: ≈12 min Training components: One main exercise and several alternative with specific variations in movements and techniques (established by the researchers; decided by the coach) of 5 modules: mobilisation, core stability, leg axis stability, jumping, and landing exercises and agility.	CG: Their usual training program	Severe knee and ACL/PCL injury incidence per 1000 AE	NR	NA	↓ Severe knee injury incidence (0.38 vs. 0.69/1000h, p < 0.05) in IG compared to CG. ↔ ACL/PCL injury incidence (0.11 vs. 0.18/1000h, p > 0.05) in IG compared to CG	Supervisor: Coach. High number of dropouts, but do not specify how many of each group. Weeks of intervention not specified.

ACL—anterior cruciate ligament, PCL—posterior cruciate ligament, AE—athlete exposures, CMAS—cutting movement assessment score, D—dominant, ND—nondominant, SDC—smallest detectable change, COD—change of direction, SWC—smallest worthwhile change, ICC—intraclass correlation coefficient, CV—coefficient of variation, IG—intervention group, CG—control group, NR—non-reported, ES—effect size, ESc—effect size calculated through Hedge’s g, NA—non-applicable, ↓—decrease, ↑—increase, ↔—no change.