High-Load Strength Training Reduces Injury Incidence and Injury Burden and Improves Physical Fitness in Young Highly Trained Soccer Players

Abstract

Background:

There is little available information on the reduction of injury incidence and injury burden after strength training programs. This study aimed to analyze the effects of a 12-week high-load strength training program on injury incidence, injury burden, and fitness in young, highly trained, soccer players.

Hypothesis:

It was hypothesized that well-targeted high-load training focused on the gluteal and hamstring musculature could aid in injury prevention and increase physical fitness.

Study Design:

A randomized controlled trial design was applied, which followed the CONSORT Statement.

Level of Evidence:

Level 2.

Methods:

Twenty players were assigned randomly to the experimental group (EG, n = 10 players), who performed a high-load strength training program, or to the control group (CG, n = 10 players), who performed only their usual soccer training. Injury incidence (injuries per 1000 hours exposure) and injury burden (days of absence per 1000 hours exposure) were recorded during the intervention, as well as the physical fitness attributes before and after the training program.

Results:

A significant (P < 0.05) lower injury incidence was observed in the EG (CG, 11.34 vs EG, 1.31 injuries per 1000 hours of exposure) and a significant (P < 0.001) lower injury burden in the EG (CG, 304.66 versus EG, 19.72 days of absence per 1000 h of exposure). The analysis of covariance model revealed significant between-group differences favoring the EG, showing significantly greater improvements in jumping, change of direction ability, sprinting, and imbalance strength tests (P < 0.001; effect size, 3.02 to −7.23).

Conclusion:

This study demonstrated the beneficial effects of a 12-week high-load strength training program on injury incidence, injury burden and physical fitness, in highly trained soccer players.

Clinical Relevance:

This study provides positive information for implementing this type of strength training in the daily training sessions of young soccer players for both performance enhancement and injury prevention.

Soccer is a highly demanding team sport that requires the execution of repetitive high-intensity neuromuscular actions (eg, jumps, accelerations, changes of direction and sprints) to obtain adequate on-field performance.^14,36,56 Developing these actions at early ages can play a part in determining the chance of players proceeding to higher achievement levels. ³² This raises the question of the most effective training strategies to improve key physical determinants of soccer. However, it is also necessary to consider that the relationships between training/match demands and the risk of injury. ²² Major concerns resulting from injuries include acute or chronic alterations in health and the inability to compete during the absence and rehabilitation period.^19,33 For this reason, injuries have become a relevant topic of investigation.^3,26 Regarding nonelite soccer players, some authors have noted injury rates ranging from 6.2 to 12.4 injuries per 1000 hours of exposure, with severe injuries (ie, >28 days of absence) accounting for approximately 12% to 18% of injuries.^4,42 Moreover, other authors showed an injury burden of 15.6 days of absence per 1000 hours of exposure for semiprofessional soccer players. ⁴² Thus, it should be a priority to establish training strategies to reduce the injury incidence and injury burden in soccer players.

The implementation of a strength training program, using intensities of 70% to 80% of maximal effort, is a strategy employed to enhance performance and reduce the risk of injury, as investigated in several studies examining its efficacy in soccer players.^7,41 Similarly, a recent study has shown that strength training with near to maximal loads (ie, >85% 1 repetition maximum [RM]) has beneficial effects on increasing physical fitness attributes and decreasing the incidence and burden of injury. ¹⁸ Considering that most noncontact injuries in soccer are due to a lack of strength in the posterior chain of the lower body (eg, gluteus medius and hip abductors), which hinders proper load production and absorption during sports actions by these muscle groups, ²¹ it seems relevant to implement strength training programs focused on this musculature.

High-load strength training improves coordination between motor units and the muscle that they innervate ⁵¹ and increases muscle fascicle length.^8,9 This improved coordination and muscle activation causes type IIa fibers (intermediate twitch fibers) to act like myosin heavy chains.^10,45 In addition, these fibers are known to have a lower stretching capacity, to fatigue more easily, and to have a higher viscosity due to their morphological characteristics. ⁸ Therefore, due to all these improvements, there could be less muscle fatigue and a consequent reduction in the number of musculotendinous injuries. ⁵⁵ Previous research has shown promising results in sports performance after the application of strength training programs with split-based exercises, improving both eccentric strength in the hamstrings, sprints, and jump performance. ³⁰ Other authors have observed an increase in vertical jumping and sprinting abilities after a squat strength training program over 8 weeks in male soccer players aged <19 years. ¹⁶ However, it seems necessary to analyze in greater depth the benefits of applying a specific high-load strength training program in isolation on the injury profile of soccer players as well as on injury incidence and burden.

Most previous studies have examined the improvements in physical performance, but there is limited knowledge about the benefits of high-load strength training in reducing both injury incidence and injury burden.^5,24,44 This study aimed to analyze the effects of a 12-week high-load strength training program on injury incidence, injury burden, and physical fitness attributes in highly trained soccer players. The hypothesis was there would be an improvement in physical performance in highly trained soccer players after the application of various high-load strength-oriented training programs.^5,44 Also, considering the relevance of resistance training programs to reduce injuries, ⁴⁶ another hypothesis was that this type of training could aid in reducing injury incidence and burden.

Methods

Subjects

Twenty highly trained young male soccer players participated voluntarily in this study. An a priori power calculation was not performed because this study enrolled a sample derived from 1 team. ⁶ This allowed the researchers to evaluate the impact of a training program versus a control limiting the confounding factors such as different coaches, different training regimes, player levels, etc. The players belonged to the same team and competed in the “Division de Honor” under-18 Spanish National League. Participants were assigned randomly to the control group (CG, n = 10; age, 17.7 ± 0.5 years; height, 177.6 ± 7.8 cm; body mass, 68.6 ± 8.4 kg; body mass index (BMI), 21.7 ± 1.3 kg m⁻²) and to the experimental group (EG, n = 10; age, 17.5 ± 0.7 years; stature, 174.9 ± 4.3 cm; body mass, 66.7 ± 4.3 kg; BMI, 21.8 ± 1.0 kg m⁻²). Both groups were composed of 4 defenders, 4 midfielders, and 2 attackers. Participants who sustained injuries at the commencement of the investigation were omitted from the study (n = 4). The EG performed high-load strength training in addition to their regular soccer training routine, whereas the CG players performed only their regular soccer training with the team. During the intervention period, the players completed a recovery/compensatory session of 90-minute duration and 3 soccer-acquisition training sessions, each lasting approximately 90 minutes, resulting in a total training time of 270 minutes per week and 1 official match at the weekend. These acquisition training sessions (ie, microcycle) aimed, in this order, to recover (ie, starters) from match demands or to compensate (ie, nonstarters) the match stimuli, a tension day (ie, priority of small spaces, using positional and small-side games), an endurance day (ie, focused in large spaces, using positional and large-side games), and a speed day (ie, finding speed stimuli, using tactical drills, strategy and speed training situations). The players were accustomed to resistance training; however, until the implementation of this study, load orientation was not a relevant factor in the prescription of resistance training. All the players met the inclusion criteria ensuring a minimum of 80% of the strength training sessions (including both soccer and strength sessions) throughout the 12-week period and to be free of injuries the month preceding the investigation (Figure 1). Goalkeepers were not selected for the study due to their special role. Participants were provided detailed information regarding the advantages, procedures, and potential risks of taking part in the study. They also signed an informed consent form before the start of the investigation. The study was conducted according to the Declaration of Helsinki (2013), and the protocol received approval from the ethics committee of Valladolid East Health Area (Code PI 22-2793 NO HCUV).

Figure 1.

CONSORT diagram of participants’ recruitment, allocation follow-up, and analysis. CG, control group; EG, experimental group.

Design

To examine the effect of 12 weeks of a high-load strength training program on lower limb injuries (eg, incidence and burden) and physical fitness attributes, a randomized-controlled trial design was applied, which followed the CONSORT 2010 Statement. ⁵⁰ During the in-season period (ie, from February to April) the soccer players in the EG completed a 12-week strength training program alongside their regular soccer training regimen. The participants performed some physical testing sessions before (Pre) and after (Post) the training intervention on 3 separate days in the same week. On the initial day, the 505 change of direction ability test (505-CODA test) and linear sprints over distances of 10, 20, and 40 m were executed. After a 48-hour rest period (second day), the countermovement jump (CMJ), squat jump (SJ), and repeated sprint ability (RSA) test were carried out in that order. Finally, on the third day, after another 48-hour rest, isometric strength exercises were performed in the following order: hamstring dominant (ISOHAMSd) and nondominant (ISOHAMSnd), quadriceps dominant (ISOQUAd) and nondominant (ISOHAMSnd), abductor dominant (ISOABDd) and nondominant (ISOABDnd), and adductor dominant (ISOADDd) and nondominant (ISOADDnd) limbs. For jumps, the 505-CODA test and speed assessments, a specific warm-up comprised running and joint mobility exercises lasting 10 minutes. In contrast, isometric strength tests involved a dedicated 15-minute warm-up, incorporating the 5 exercises developed for the test. This warm-up was executed at an intensity ranging from 40% to 60% of 1RM, involving 2 sets of each exercise (comprising 10-15 repetitions) with a rest interval of 30 to 60 seconds between sets. The performance laboratory was used to measure jump testing and isometric strength. The laboratory was maintained at a temperature of 18°C with a relative humidity of 60% to 70%. The assessment of the 505-CODA test, sprints, and RSA took place on an artificial grass field where the team usually trained. During these tests, players wore their own soccer boots. All testing occurred in the afternoon, specifically between 5:00 and 7:00 pm. To ensure consistency, players were instructed to have their last meal 3 hours before the start of the tests and to abstain from consuming caffeinated beverages or engaging in intense physical exercise. Throughout the protocols, a strength and conditioning specialist supervised the tests and provided verbal encouragement. ⁴⁷ The intervention was focused on high-load strength training. The strength sessions occurred on 2 nonconsecutive days weekly (Tuesdays and Thursdays at 6:00 pm), lasting approximately 45 to 50 minutes. These sessions were scheduled after the conclusion of the regular soccer training sessions.^34,37

Procedures

High-Load Strength Training

A total of 24 sessions were completed for the high-load strength training by the EG, ensuring a minimum of 2 rest days between each session before proceeding to the post-tests. Before each strength session, a tailored warm-up routine was undertaken, involving the execution of the 5 exercises used in the intervention period. The selected exercises included hip thrust with a barbell, Bulgarian split squat on a Multipower machine, clamshell with elastic band (TheraBand), split with external hip rotation, and Bulgarian split squat on Bosu. All exercises used free weights (barbell and dumbbells), except the exercise performed with an elastic band. The exercises were incorporated into circuit training sessions with intensities and repetitions shown in Table 1.

Table 1.

Types of exercises, volume, intensity, and recovery time for the 12-week intervention strength training period

Exercise	Temporal Sequence	Series	Repetitions	Rest Between Sets
Hip thrust	W1-2, 70% 1RM W3-4, 75% 1RM W5-7, 80% 1RM W8-12, 85% 1RM	3	W1-2, 10 Rep. W3-4, 10 Rep. W5-7, 8 Rep. W8-12, 6 Rep.	2 minutes
Bulgarian split squat multipower	W1-2, 70% 1RM W3-4, 75% 1RM W5-7, 80% 1RM W8-12, 85%1RM	3	W1-2, 10 Rep. W3-4, 10 Rep. W5-7, 8 Rep. W8-12, 6 Rep.	2 minutes
Clamshell	Blue elastic band Blue elastic band Black elastic band Black elastic band	3	W1-2, 10 Rep. W3-4, 10 Rep. W5-7, 8 Rep. W8-12, 8 Rep.	2 minutes
Split external hip rotation	W1-2, 70% 1RM W3-4, 75% 1RM W5-7, 80% 1RM W8-12, 85% 1RM	3	W1-2, 10 Rep. W3-4, 10 Rep. W5-7, 8 Rep. W8-12, 6 Rep.	2 minutes
Bulgarian split squat Bosu	W1-2, 70% 1RM W3-4, 75% 1RM W5-7, 80% 1RM W8-12, 85% 1RM	3	W1-2, 10 Rep. W3-4, 10 Rep. W5-7, 8 Rep. W8-12, 6 Rep.	2 minutes

1RM, 1 repetition maximum; Rep., repetitions; W, week.

A submaximal and modified protocol was employed to determine the 1RM in free-weight exercises. ⁴⁰ Initially, players selected a weight for 8 to 10 repetitions, allowing 3 minutes for recovery. Subsequently, they increased the load to perform 1 set of 5 to 6 repetitions, with a 5-minute rest period. Finally, the load was increased once more for 1 set of 3 to 4 repetitions. An estimated calculation of 1RM was then derived using the formula proposed by O’Connor et al ⁴⁰ (1RM = weight lifted in kilograms × [1 + 0.025 × n repetitions]), and corresponding training percentages were computed for each player. To calculate the intensity in the Clamshell exercise (performed with an elastic band), the “TheraBand Manual” was used as a reference and the intensity was increased by modifying the resistance of the bands from blue to black.^2,38

Measures

Injuries

Throughout the 12-week intervention, records were maintained for the number, mechanism, type, body region, muscle structure, time, and duration of injuries, as well as lower extremity load. The Union of European Football Associations (UEFA) model criteria guided data collection. ²⁵ Lower extremity injuries were diagnosed and documented by the medical staff. Treatment and recovery follow-up were overseen by the medical team. An injury was defined as “an injury occurring during a scheduled training session or match that caused absence from the next training session or match.” ²⁵ Subsequently, incidence rate, ²⁸ injury incidence (injuries per 1000 hours exposure), and injury burden (days of absence per 1000 h exposure) were calculated, with the latter defined as “the number of days lost per 1000 h of exposure.” ³ Exposure was determined based on the time (in hours) spent in training and matches, and the incidence rate referred to the number of injuries sustained during both practice settings per 1000 hours of exposure. ⁴⁹ A player was deemed fully recovered postinjury when cleared by the medical staff for full participation in team training and matches. ⁴⁶

Vertical Jump Performance

The soccer players executed a bilateral CMJ, CMJ with the dominant (CMJd) and nondominant (CMJnd) leg, and a bilateral SJ. ¹¹ Dominant leg determination criteria were based on each player's soccer ability (ie, kicking leg). ²⁹ The CMJ involved a flexion-extension of the hips and knees at the highest velocity, reaching a maximum knee flexion angle of 90°, and keeping hands on the hips. The SJ was performed with hands on the hips, a straight trunk, and executing a maximum vertical jump from the 90° knee flexion position, without any rebound or countermovement. For all jumps, 2 attempts were made, and the best one was selected for further analysis, with a 1-minute rest between attempts. Jump height (cm) was measured using a contact platform (Optojump Next, Microgate), calculated as h = gt²/8 (where h is height in cm, g is acceleration due to gravity [9.81 m s⁻²], and t is flight time in seconds of the jump). ²³ The intraclass correlation coefficient (ICC) values for CMJ, CMJd, CMJnd, and SJ were 0.986, 0.975, 0.935, and 0.906, respectively. The coefficients of variation (CVs) were in the range of 2.82% to 5.57% for all jumping measures.

CODA Test

To assess CODA, the players carried out the 505-CODA test. The test involved an acceleration run from the starting line to the first marker at 10 m, followed by a 5-m sprint to the second marker (positioned 15 m from the starting line). Subsequently, a 180° change of direction was executed, followed by a 5-m sprint back past the first marker (placed 10 m from the starting line). ⁵² Timing was recorded using a photocell (Polifemo, Microgate) positioned over the start/finish line. Each player performed 2 attempts to turn with each leg (dominant, 505-CODAd; nondominant, 505-CODAnd), and the best result was selected. A 2-minute rest interval was observed between attempts. The ICC values for CODA tests were 0.858 to 0.878, and the CVs were 2.46% to 2.61%.

Linear Test Sprints

The soccer players performed a 40 m linear sprint, with splits at 10 (SPR10), 20 (SPR20), and 40 m (SPR40) distances. ³⁹ The sprint commenced 0.5 m before the starting point. Three attempts were made, and the fastest time was selected. A 4-minute rest period was implemented between sprints, and players received verbal encouragement to achieve optimal performance. Four photoelectric cells (Polifemo, Microgate) measured the sprint times. The ICC values for SPR10, SPR20, and SPR40 were 0.930, 0.954, and 0.968, respectively. The CVs were in the range of 0.64% to 1.60% for all sprint tests.

Repeated Sprint Ability

Each player completed 5 sprints of 30 m at maximum speed with 25 s of recovery between each sprint. ⁵⁴ The starting point for the sprints was 0.5 m before the start. Sprint times were measured using 2 photoelectric cells (Polifemo, Microgate) positioned at 0 m and 30 m distances, and the total time for the 5 sprints (RSAtotal) was calculated.

Isometric Strength

The soccer players engaged in isometric strength contractions lasting 5 seconds for the quadriceps, hamstrings, hip abductors, and hip adductors. A previously validated dynamometer (Carp Spirit Water Queen Digital Scale 50, BIODEX System Pro 4, System 4 advance Version 4.2), ⁴⁸ was used to measure isometric strength. For ISOHAMSd and ISOHAMSnd, players sat on the stretcher with the knee fixed at 90° flexion, performing knee flexion against the dynamometric tape. ¹⁸ ISOQUAd and ISOQUAnd involved players seated on the stretcher with the knee completely fixed at 90° flexion, performing knee extension against the dynamometric tape. ISOABDd, ISOABDnd, ISOADDd, and ISOADDnd required players to be supine with the knee fully extended and the hip neutral, performing hip abduction and adduction against the dynamometric tape while keeping the knee fully extended. Two attempts were made with each leg, and the best result was chosen for analysis. A 1-minute rest occurred between attempts, with verbal encouragement given to maximize force. The ICC values were in the range of 0.994 to 0.997, and the CVs were 0.87% to 1.74% for all isometric strength tests.

Statistical Analysis

Data are reported as means and standard deviations. Normality of data distribution and homogeneity of variances were assessed using the Shapiro-Wilk test and Levene test, respectively. Metrics related to lower extremity injuries, including incidence and burden, are presented as the number per 1000 hours of exposure and the number of absence days per 1000 hours of exposure, each accompanied by 95% CI. Rate ratios (RRs) with 95% CI and Z-tests were computed to assess between-group differences (ie, EG and CG) in lower extremity injury incidence and burden. ³¹ Parametric tests were used for all the variables analyzed and independent t tests were employed to assess initial differences at the preintervention stage. An analysis of covariance (ANCOVA) was conducted to identify potential training effects, with baseline values treated as covariates. ⁶ Within-group pre- to postdifferences were evaluated using paired-samples t tests. Cohen’s d effect size (ES) was calculated to assess practical significance, ¹⁵ with results interpreted as small (0.00 ≤ d ≤ 0.49), moderate (0.50 ≤ d ≤ 0.79), and large (d ≥ 0.80). Data analysis was conducted using the Statistical Package for the Social Sciences (SPSS Inc, Version 27.0 for IOS). The significance level for all analyses was set at P < 0.05.

Results

Table 2 shows the characteristics of injuries in total for the CG and EG. Eight injuries were suffered during the intervention period of the study, with 7 belonging to the CG and 1 to the EG. A total of 7 muscle-tendon injuries were reported in CG, of which 6 were muscle injuries (2 hamstring muscle injuries, 2 hip adductor injuries, 1 hip flexor injury, and 1 quadriceps injury), and 1 was a ligamentous injury to the medial collateral ligament. As such, injured players suffered 188 absence days. The maximum number of absence days was 58, and the minimum was 18. In contrast, the EG experienced only 1 musculoskeletal injury, leading to 15 absence days. In the EG, only 1 player suffered an injury throughout the 12-week intervention period. Examining the distribution of injuries over time, 5 injuries occurred in weeks 1 to 6 (4 in the CG and 1 in the EG), while 3 injuries were reported in weeks 7 to 12 (all in the CG, with none in the EG).

Table 2.

Number, type, mechanism, region, structure, time of injury, and absence time values of injuries of CG and EG players

	Total	CG	EG
Injuries, n	8 (100%)	7 (87.5%)	1 (12.5%)
Type of injury
Muscular	6 (75%)	6 (75%)	0
Ligament	2 (25%)	1 (12.5%)	1 (12.5%)
Mechanism of injury
Direct	1	0	1 (12.5%)
Indirect	4 (50%)	4 (50%)	0
Overuse	3 (37.5%)	3 (37.5%)	0
Body region
Pubis	3 (37.5%)	3 (37.5%)
Thigh	3 (37.5%)	3 (37.5%)	0
Knee	1 (12.5%)	1 (12.5%)	0
Ankle	1 (12.5%)	0	1 (16.7%)
Musculoskeletal structure
Hamstrings	2 (25%)	2 (50%)	0
Quadriceps	1 (12.5)	1 (12.5%)
Knee ligament	1 (12.5%)	1 (12.5%)	0
Adductors	3 (37.5%)	3 (37.5%)	0
Ankle ligament	1 (12.5%)	0	1 (12.5%)
Time and epoch
Training session	3 (37.5%)	3 (37.5%)	0
Competition session	5 (62.5%)	4 (50%)	1 (12.5%)
Weeks 1-6	5 (62.5%)	4 (50%)	1 (12.5%)
Weeks 7-12	3 (37.5%)	3 (37.5%)	0
Time of absence (days)
Total	203 (100%)	188 (92.6%)	15 (7.4%)

Percentage values correspond to the percentage with respect to the total number of injuries. CG, control group; EG, experimental group.

Regarding injury profile, significantly (P < 0.05) higher injury incidence (CG, 11.34 vs EG, 1.31 injuries per 1000 hours of exposure; RR, 8.63; 95% CI, 1.06-70.12) and significantly (P < 0.001) higher injury burden (CG, 304.66 vs EG, 19.72 days of absence per 1000 hours of exposure; RR, 15.45; 95% CI, 9.13-26.14) were observed in the CG compared with the EG.

Table 3 presents alterations in physical fitness attributes after the 12-week intervention period. There were no initial differences between groups in any variable. The ANCOVA model revealed significant between-group differences (P < 0.001-0.02) favoring the EG in most physical fitness attributes and imbalance variables. The EG demonstrated improvements from pre- to post-training in most physical fitness attributes (P < 0.001-0.002; ES, 1.57 to −7.23, large).

Table 3.

Changes in physical fitness attributes after the 12-week period intervention in CG and EG players

	CG					EG					Between-Group Differences
Variables	Pre	Post	%Diff	P	ES	Pre	Post	%Diff	P	ES	P	F
CMJ, cm	37.00 ± 3.34	36.98 ± 3.39	−0.05	0.85	0.06	38.84 ± 2.28	41.33 ± 2.14	6.02	<0.001***	−7.23	<0.001***	248.19
CMJd, cm	20.17 ± 3.02	20.15 ± 3.02	−0.10	0.83	0.07	22.78 ± 1.31	24.12 ± 1.03	5.56	<0.001***	−3.69	<0.001***	75.84
CMJnd, cm	21.81 ± 2.24	21.62 ± 2.19	−0.88	0.25	0.39	22.30 ± 2.15	23.44 ± 1.90	4.86	<0.001***	−3.64	<0.001***	63.73
SJ, cm	29.09 ± 3.57	28.92 ± 3.49	−0.59	0.11	0.56	31.64 ± 3.62	33.47 ± 3.45	5.47	<0.001***	−4.34	<0.001***	159.66
505-CODAd, s	2.29 ± 0.08	2.28 ± 0.07	−0.44	0.26	0.38	2.19 ± 0.07	2.14 ± 0.06	−2.34	<0.001***	2.06	<0.001***	35.05
505-CODAnd, s	2.32 ± 0.07	2.31 ± 0.06	−0.43	0.33	0.33	2.25 ± 0.08	2.17 ± 0.04	−3.69	<0.001***	2.02	<0.001***	37.85
SPR10, s	1.72 ± 0.06	1.72 ± 0.06	0.00	0.15	0.49	1.69 ± 0.05	1.62 ± 0.06	−4.32	<0.001***	2.37	<0.001***	28.07
SPR20, s	2.95 ± 0.11	2.95 ± 0.11	0.00	0.59	0.18	2.88 ± 0.09	2.81 ± 0.08	−2.49	<0.001***	2.16	0.002**	40.18
SPR40, s	5.29 ± 0.17	5.29 ± 0.17	0.00	0.41	−0.28	5.16 ± 0.13	5.06 ± 0.14	−1.98	<0.001***	3.02	<0.001***	51.69
RSAtotal, s	21.46 ± 0.87	21.48 ± 0.85	0.09	0.22	−0.42	21.09 ± 0.47	20.58 ± 0.33	−2.48	<0.001***	1.57	<0.001***	31.68
ISOQUAd, kg	40.82 ± 4.28	40.88 ± 4.11	0.15	0.68	−0.14	39.34 ± 3.27	42.52 ± 3.30	7.48	<0.001***	−5.46	<0.001***	169.08
ISOQUAnd, kg	38.32 ± 4.10	38.64 ± 4.26	0.83	0.01*	−1.01	38.03 ± 4.84	42.17 ± 3.54	9.82	<0.001***	−1.86	<0.001***	35.89
ISOHAMSd, kg	22.09 ± 2.16	22.19 ± 2.21	0.45	0.40	−0.28	22.20 ± 2.29	25.78 ± 2.48	13.89	<0.001***	−6.40	<0.001***	267.26
ISOHAMSnd, kg	21.86 ± 2.22	21.90 ± 2.30	0.18	0.78	−0.09	21.78 ± 2.54	25.72 ± 2.26	15.32	<0.001***	−5.61	<0.001***	230.54
ISOABDd, kg	30.59 ± 4.49	30.78 ± 4.39	0.62	0.04*	−0.76	28.15 ± 4.22	30.85 ± 3.76	8.75	<0.001***	−3.24	<0.001***	83.33
ISOABDnd, kg	29.22 ± 3.92	29.48 ± 4.03	0.88	0.009**	−1.04	27.57 ± 4.38	30.60 ± 3.98	9.90	<0.001***	−2.11	<0.001***	32.31
ISOADDd, kg	24.76 ± 2.94	24.76 ± 2.96	0.00	>0.99	0.00	26.13 ± 2.99	29.41 ± 2.67	11.15	<0.001***	−7.04	<0.001***	359.12
ISOADDnd, kg	23.94 ± 2.73	24.21 ± 2.90	1.12	0.060	−0.68	25.43 ± 1.56	29.43 ± 1.77	13.59	<0.001***	−3.22	<0.001***	69.02

CG, control group; CMJ, counter movement jump; CMJd, dominant leg counter movement jump; CMJnd, nondominant leg counter movement jump; 505-CODAd, dominant leg change of direction ability; 505-CODAnd, nondominant leg change of direction ability; %Diff, difference in percentage between groups; EG, experimental group; ES, effect size; ISOABDd, isometric strength in abductors muscles in dominant leg; ISOABDnd, isometric strength in abductors muscles in nondominant leg; ISOADDd, isometric strength in adductors muscles in dominant leg; ISOADDnd, isometric strength in adductors muscles in nondominant leg; ISOHAMSd, isometric strength in hamstrings muscles in dominant leg; ISOHAMSnd, isometric strength in hamstrings muscles in nondominant leg; ISOQUAd, dominant leg isometric strength in quadriceps muscles; ISOQUAnd, nondominant leg isometric strength in quadriceps muscles; RSAtotal, repeated sprint ability total; SJ, squat jump; SPR10, linear sprint in 10m; SPR20, linear sprint in 20m; SPR40, linear sprint in 40m.

^*P < 0.05, **P < 0.01, *** P < 0.001.

Discussion

This study analyzed the effects of a 12-week high-load strength training program on injury incidence, injury burden, and physical fitness attributes in highly trained soccer players. Given the limited scientific evidence on high-load strength training and injury prevention, this study examined both the performance improvements resulting from this type of training and its potential impact on reducing injury incidence and burden throughout the season. It found that players in the CG experienced higher injury incidence (~10 injuries per 1000 hours of exposure) and burden (~285 days of absence per 1000 hours of exposure) compared with those in the EG. In addition, the EG showed greater improvements (~2%-15%) in all the assessed physical fitness attributes. Therefore, the proposed high-load strength training program can be suggested as an effective way to reduce injuries (ie, incidence and burden) as well as to improve the physical fitness level of the participants.

Previous studies have revealed that the application of strength training programs could lead to the reduction of the number of injuries^46,53 and the improvement of physical fitness. ⁴⁹ However, few investigations have analyzed the effect of strength training programs with high-load orientation on risk of injury and physical fitness levels. ⁵ In our study, the EG players suffered a significantly lower injury incidence compared with those in the CG (ie, EG, 1.31 vs CG, 11.34 injuries per 1000 hours of exposure). In addition, the EG players had a significantly lower injury burden in comparison with their counterparts in the CG (ie, EG, 19.72 vs CG, 304.66 days of absence per 1000 hours of exposure). Although not measured directly, we speculate that these results might be attributed to the benefits of strength training programs with a high-load orientation, which enhances coordination between motor units and the muscle they innervate, ⁸ as well as increases in muscle fascicle length, due mainly to eccentric work.^35,57 Regarding the period of injuries, the majority occurred during weeks 1 to 6. The EG suffered only 1 injury during weeks 1 to 6 (compared with 4 injuries in the CG) and none during weeks 7 to 12 (compared with 3 injuries in the CG). Thus, it seems that the program was also effective during weeks 1 to 6. This aspect is particularly relevant as it highlights that the intervention program had positive effects in reducing injuries, even during the initial weeks.

It is worth noting that the 5 strength exercises performed in this study targeted primarily the activation of the gluteus medius, gluteus maximus, and hamstrings.^1,17 Recent studies have highlighted the significance of lumbo-pelvic muscle function in preventing and treating hamstring injuries, demonstrating the correlation between gluteus medius activation and strength during running and the occurrence of hamstring injuries. ²¹ It is crucial to emphasize the role of the gluteus medius and maximus in specific soccer tasks such as jumps and changes of direction. Some authors have revealed the electromyographic activation of over 80% in these 2 muscles during various types of jumps, including the crossover jump, hurdle jump, and split jump ²⁷ . In addition, research has shown that, besides enhancing jumping performance, there is also a notable improvement in reducing the knee valgus angle by up to 60%. ²⁰ These improvements could potentially aid in preventing major knee injuries such as patellofemoral pain syndrome or anterior cruciate ligament injuries. ²⁰ Moreover, it is believed that a significant proportion of hamstring injuries occur during the eccentric phase of the movement. ³⁰ Exercises such as splits have been shown to enhance both eccentric strength in the hamstrings and power in sprints and jumps. ³⁰

By enhancing the physical fitness of soccer players, improvements can be seen in actions such as sprints, jumps, and changes of direction, thereby boosting overall performance. The players in the EG demonstrated improvements in all the physical fitness tests conducted. These findings are consistent with previous studies after the application of various high-load strength-training programs.^5,24 In addition, our findings are consistent with those of Barjaste and Mirzaei, ⁵ who demonstrated that high-load strength training (ie, loads between 70% and 85% of 1RM) led to improved maximal strength (1RM) and vertical jump, in semi-professional soccer players. Furthermore, Griffiths et al ²⁴ observed significant improvements in strength, sprint speed, and jump height in young soccer players after high-load training. This could be attributed to the development of tendons with greater stiffness and thickness, leading to increased power and maximum strength. ⁴³ Moreover, strength training targets the physical aspects that determine the speed of CODA, such as muscle strength and power.^13,58 It is worth noting that different CODA actions are either force or velocity-oriented, depending on approach speed and the angle of change. Modest CODA angles (<90°) are more speed-oriented, whereas angles >90° are more strength-oriented. ¹² Thus, high-load strength training may be more beneficial for improving strength-oriented CODA (>90°), as observed in this study. Moreover, significant differences (P < 0.001) in the percentage change in fitness tests between the EG and CG were observed in this study, with the EG players experiencing fewer and less severe injuries and greater improvements in physical fitness tests. Therefore, future studies focusing on high-load strength training and its effects on physical abilities and injury reduction would be valuable to corroborate and complement these findings.

This study has some limitations that warrant consideration. The main limitation is the difference in overall training load between the EG and CG, since the EG included additional strength training without reducing the regular soccer training. This may suggest that the fact of having a greater stimulus than the CG is enough to have some specific physical adaptations. Secondly, the generalizability of the results may be limited to young, highly trained players; however, most soccer players are not professionals, and, therefore, we believe that this study has great ecological validity. Third, the study was conducted with a single team, and, therefore, the unique characteristics of that team may have influenced the outcomes; however, this is also an advantage because the technical and tactical elements of the training were controlled (and so no other confounding factors were added to the study). Finally, this study has not provided information on whether the EG were protected against injury in the long term because only injuries sustained during the intervention period were registered. To address these limitations, future studies could consider (1) implementing this training program across a larger number of teams of different competitive levels to assess its effectiveness; (2) extending the study period to quantify injury incidence and injury load over the entire season, providing a more comprehensive understanding of the long-term effects of the training program; (3) exploring the effects of high-load training at different periods during the season to determine optimal timing for implementation; (4) equating training load (eg, soccer exposure) between the CG and EG, thus eliminating training time as a confounding factor (the groups should have similar training and competition load); and (5) recording injuries over a longer period to examine the long-term effect of preventive programs.

Conclusion

The implementation of a 12-week high-load strength training program in addition to usual regular soccer training showed reductions in injury incidence and injury burden in the young soccer players participating in this study. In addition, this type of training demonstrated improvements in all physical fitness attributes, such as jumping ability, CODA, sprinting, RSA, and isometric strength. Overall, it may be positive to implement this type of strength training in the daily training sessions of young, highly trained soccer players for both performance enhancement and injury prevention.