Proximal core strengthening training for anterior cruciate ligament injury prevention with biomechanical improvements in knee/hip kinematics and strength gains: a systematic review and meta-analysis of low-certainty evidence

Abstract

Objectives

This study evaluated the effects of proximal core training on biomechanical risk factors and strength parameters in individuals at high risk of anterior cruciate ligament (ACL) injury (specifically: those exhibiting pathological movement patterns, neuromuscular deficits or biomechanical risk factors) and compared direct versus indirect interventions. We hypothesised that targeted training enhances dynamic knee stabilisation and hip control during high-risk manoeuvres, with direct approaches providing superior biomechanical benefits through neuromuscular control optimisation.

Design

Systematic review and meta-analysis using the Grading of Recommendation, Assessment, Development and Evaluation (GRADE) approach.

Data sources

We searched (PubMed, Web of Science, EBSCO Academic Search Premier (ASP)+Business Source Premier (BSP)) for relevant literature published between its inception and the date of retrieval (22 April 2024).

Eligibility criteria for selecting studies

This study included studies comparing the effects of proximal core intensification training and lower extremity training, evaluated their influences on biomechanical risk factors and strength parameters in three types of high-risk ACL populations, and compared the direct and indirect intervention effects. The three types of people include: (1) athletes with pathological exercise patterns, (2) those with neuromuscular defects after ACL injury/reconstruction and (3) those without injury but with biomechanical risk factors.

Data extraction and synthesis

Two independent reviewers used standardised methods to search, screen and code included studies. Risk of bias was assessed using the Cochrane Collaboration and Evidence Project tools. A meta-analysis was conducted using random effects models. Findings were summarised in GRADE evidence profiles and synthesised qualitatively.

Results

24 studies with a total of 749 participants were included. Meta-analyses demonstrated that proximal core strengthening training may increase lower extremity muscle strength (quadriceps peak torque: standardised mean differences (SMD)=0.65, 95% CI (0.29 to 1.01), I²=0%; hamstring peak torque: SMD=0.53, 95% CI (0.14 to 0.92), I²=0%; both p<0.05, low-quality evidence). For movement kinematics, task-dependent improvements were observed: peak knee flexion angle likely increased during jumping (SMD=0.58, 95% CI (0.20 to 0.96), I²=32%) and single-leg squatting (SMD=0.60, 95% CI (0.07 to 1.26), I²=0%) (both p≤0.05, low-quality evidence) but may have little meaningful effect during walking (low-quality evidence). Peak hip flexion may substantially increase during jumping (SMD=0.83, 95% CI (0.19 to 1.47), I²=0%, p<0.05, low-quality evidence), while core endurance may improve (plank time: SMD=0.63, 95% CI (0.12 to 1.13), I²=0%, p<0.05, low-quality evidence).

Conclusions

Proximal core training has the potential to improve core endurance, knee kinematics and lower limb strength in individuals at high risk of ACL injury, but evidence remains limited to late postoperative and exercise studies. Standardised protocols and harm rate validation are needed to confirm the preventive effectiveness of multidimensional protocols.

PROSPERO registration number

CRD42024532199.

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• This systematic review and meta-analysis conducted a comprehensive search of clinical trials on proximal core strengthening training, with an explicit focus on its effectiveness in anterior cruciate ligament (ACL) injury prevention.

• We applied the Grading of Recommendation, Assessment, Development and Evaluation framework to rigorously assess the strength and quality of evidence for biomechanical and neuromuscular outcomes.

• Limitations include low methodological rigour in included studies, small sample sizes and not all utilising randomised controlled designs.

• While differentiating direct versus indirect core training interventions enhanced mechanistic interpretation, high heterogeneity in training protocols (frequency, intensity and duration) limited comparability.

• Critical gaps persist as studies lacked validation through ACL injury rate tracking, restricting clinical relevance assessments.

Introduction

Proximal core strengthening training is a training programme that focuses on core stability and hip strength training to enhance knee stability and function, reduce the risk of anterior cruciate ligament (ACL) injury and promote early recovery of knee function by strengthening proximal core stability, hip strength and proprioception.^{1 2} Proximal core strengthening exercises have demonstrated effective improvements in balance stability and hip muscle strength^{3 4} and have been associated with reduced risk of ACL injury and knee injuries.^{5 6} Current proximal core strengthening training can be classified into direct proximal core strengthening training and indirect proximal core strengthening training. Direct proximal core strengthening training is defined as programmes that directly target core and hip strength and stability, such as neuromuscular control training⁷ that focuses on balance training,⁸ core training,⁹ hip abduction training.¹⁰ Indirect proximal core strengthening training can be defined as a neuromuscular training programme with different training combinations in the plane of instability, which can be classified as perturbation training¹¹ and whole body vibration therapy (WBVT).¹² Both direct and indirect proximal preoperative core strengthening exercises can be useful in improving core control, knee function and preventing the risk of ACL injury in patients after ACL reconstruction (ACLR).¹³ It has been demonstrated that proximal core strengthening training is more effective than knee function training for patellofemoral pain syndrome,¹⁴ but proximal core strengthening training is often neglected or only used as a small part of ACL injury prevention and rehabilitation training programmes.¹⁵ There is less evidence for proximal core strengthening training for the prevention of sports injuries and reinjuries to the ACL, and this uncertainty makes the application of proximal core strengthening training in the prevention and rehabilitation of ACL injury difficult, which may lead to incomplete preventive training and increased rates of reinjury to the ACL in patients.

Patients with ACL injury or reconstruction exhibit significant lower extremity muscle weakness and aberrant kinematics due to arthrogenic muscle inhibition.^{16 17} Decreased strength and coordination of the quadriceps and hamstrings, stiff landing kinematics of the knee and knee valgus can lead to increased loading on the ACL and lead to ACL injury.^{18 19} Given that quadriceps-hamstring strength underpins athletic performance ²⁰and is directly compromised by ACL injury,²¹ prophylactic neuromuscular interventions are essential. This meta-analysis primarily aimed to evaluate the efficacy of proximal core strengthening training—as a preventive neuromuscular strategy—in modifying lower extremity kinematics and muscle strength among high-risk populations. Specifically, we quantified between-group differences (training vs control) in biomechanical risk factors (kinematics) and isokinetic strength parameters to inform ACL injury prevention protocols.

Method

The strategy and methods for this systematic evaluation follow the Preferred Reporting Items for Systematic Reviews and Meta-Analyses reporting guidelines²² and were registered on 22 April 2024 at PROSPERO (Registration number: CRD42024532199).

Search strategy

In this review, we will use Exercise Therapy, Plyometric Exercise, Endurance Training, Resistance Training, Anterior Cruciate Ligament, Anterior Cruciate Ligament Reconstruction, Anterior Cruciate Ligament Injury, Anterior Cruciate Ligament Injuries as Mesh subject terms in PubMed, Web of Science and EBSCO Academic Search Premier (ASP)+Business Source Premier (BSP)’s databases to research and we searched from the creation of the databases to the retrieval date (22 April 2024) for all studies. The final search strategy was ((Exercise Therapy) OR (Plyometric Exercise) OR (Endurance Training) OR (Resistance Training) OR (core training) OR (trunk) OR (Core-Muscle Training) OR (Suspension training) OR (perturbation training)) AND ((Anterior cruciate ligament) OR (Anterior Cruciate Ligament Reconstruction) OR (Anterior Cruciate Ligament Injury) OR (Anterior Cruciate Ligament Injuries) OR (Lower limb muscle strength)). PubMed was used as the primary search library for all screening. Only clinical trials were screened in Web of Science. In EBSCO ASP+BSP, since it was not possible to screen trials directly, studies with full text in all EBSCO ASP+BSP databases were selected for screening. We only included clinical trials with control groups, including non-randomised controlled trials (NRCTs) and randomised controlled trials (RCTs), cohort trials and excluded trials without control groups as well as reviews and academic reports. Duplicates in studies were removed and only studies published in English were screened. If missed by the initial search, other supplementary materials were sought by cross-referencing. A complete search strategy can be found in online supplemental appendix 2.

Study selection

PICOS tools for constructing and organising study inclusion criteria:

• Population (P): (1) Sport-exposed individuals: Athletes or physically active adults participating in pivoting/cutting sports with biomechanical risk factors. (2) Post-ACL injury cohort: Patients post-ACL injury or reconstruction with persistent neuromuscular deficits. (3) Non-injured high-risk group: Healthy individuals demonstrating pathological movement patterns despite no prior ACL injury.

• Intervention (I): Proximal core strengthening training was defined as integrated neuromuscular interventions primarily targeting trunk/hip musculature through: (1) Direct core training: Combined core-lower extremity protocols emphasising simultaneous activation during functional tasks. (2) Indirect core training: Isolated core activation preceding lower extremity exercises (eg, perturbation training and WBVT).

• Comparison (C): Athletic population and patients with ACL injury or ACLR who have no relevant interventions or who have a routine training programme based on lower limb mobility training, agility training or receiving an isolated quadriceps exercise programme. Excluded from the study population were those aged >50 years. Studies that received any type of invasive rehabilitation method (eg, surgery or injections) were excluded. Patients with established osteoarthritis of the knee and patellofemoral pain syndrome were excluded. Studies that focused solely on physical therapy treatment and blood flow restriction training were excluded. In addition, studies that included orthotics, tapes or any form of support were also excluded.

• Primary outcome (O): (a) lower limb kinematics and (b) isokinetic muscle strength measurement. Secondary outcome: core endurance.

• Study design (S): Systematic reviews were included only for clinical trials. Only RCTs were included in the meta-analysis. Two evaluators (JX and JC) ultimately screened eligible studies for agreement on study inclusion. If there were any discrepancies, they were reviewed by an independent third party.

Data collection and analysis

Data extraction included study characteristics (authors, year, population demographics), intervention details (programme content, session frequency) and outcome metrics (baseline/postintervention means±SD). Independent extraction and cross-verification by two authors (JXand CF) ensured accuracy. The primary outcome measures assessed included (1) kinematics: peak knee flexion angle (PKFA), peak hip flexion angle (PHFA) and knee flexion excursion, (2) isokinetic muscle strength: quadriceps/hamstring torque, concentric/eccentric hip work. The secondary outcome was core endurance: core endurance (prone plank duration). Interventions were grouped by type: direct proximal core strengthening training (D), indirect proximal core strengthening training (ID) and control group (CG). For multiphase studies, postintervention data were prioritised. Non-blank control groups, bilateral jump data where available, and affected or dominant limbs were selected following predefined criteria. These data have been retrieved and published.

Meta-analysis (Review Manager V.5.3) calculated standardised mean differences (SMD) with random-effects models. Effect magnitudes were classified: <0.20 (trivial), 0.20–0.49 (small), 0.50–0.79 (moderate), ≥0.80 (large). Heterogeneity was assessed via Q-test/I² (thresholds: p<0.05, I²≥50%).²³ Subgroup (exercise type) and sensitivity analyses enhanced robustness. Missing data were obtained through author contact or digitised via GetDataW2.26 (graph-to-value conversion error <1%).

Risk of bias assessment

Risk of bias was assessed using the Cochrane Collaboration’s recommended standardised tool for randomised study reviews through Review Manager V.5.3. The tool contains five domains: selection, performance, detection, attrition and reporting bias. Bias analysis related to NRCTs will be conducted using ROBINS-I. The heterogeneity of the results is analysed by the vulnerability map (online supplemental figures 1 and 2). Two authors (JX and CF) independently assessed the risk of bias of included studies, and any disagreements were resolved by discussion until consensus was reached or a third author was consulted.

Grading of Recommendation, Assessment, Development and Evaluation evidence

We used the Grading of Recommendation, Assessment, Development and Evaluation (GRADE) system to assess the level of certainty of Meta-analysis results through the GRADEpro tool (www.gradepro.org) by two authors (JX and CF) who ensured accuracy.²⁴ If the results of the two people are inconsistent, JC will determine the final result. This approach considers five key areas that may affect the credibility of the evidence and the strength of the recommendation: study limitations, consistency of results, indirectness of evidence, precision and reporting bias. Evidence was categorised into one of four quality levels based on the GRADE assessment: high, moderate, low or very low.²⁵

Results

Search results

We performed a comprehensive search of databases and only clinical trials were screened in Web of Science. In EBSCO ASP+BSP, since it was not possible to screen the studies for study type, only studies with full text were selected for screening. Studies were screened again after reading the abstracts and full text to select the studies that best met the criteria for full text screening, resulting in 24 studies.26,49 These studies were eventually included in the descriptive review, and 16 of them were included in the meta-analysis.26,41 Eight studies were not included in the comprehensive meta-analysis due to the lack of common indicators that could be combined, but the effect sizes of the individual literatures were calculated. For details, please refer to online supplemental table 3.42,49 The specific flow chart is shown in figure 1.

Figure 1. Screening flow chart. ASP, Academic Search Premier; BSP, Business Source Premier.

Risk of bias assessment results

All studies were rated at high risk of bias. The main source of risk of bias was blinding of subjects and experimenters. Only one study out of all the experiments achieved a double-blind trial. All of the studies included in the meta-analysis achieved random allocation. The results for risk of bias can be found specifically in figure 2. The results of the funnel plot analysis can be found in online supplemental appendix 1.

Figure 2. Bias analysis. ROBINS tool: D1: Bias due to confounding; D2: Bias in selection of participants into the study; D3: Bias in classification of interventions; D4: Bias due to deviations from the intended intervention; D5: Bias due to missing data; D6: Bias in measurement of outcomes; D7: Bias in selection of the reported result. NRCT, non-randomised controlled trial; RCT, randomised controlled trial.

Results of GRADE

The GRADE quality of evidence results can be seen in online supplemental table 1. All 12 results were low-quality evidence.

Results of systematic review

Participants and exercise programme characteristics

A total of 749 people were included in the studies, including 18 RCTs,26,3133 2 non-RCTs,^{44 45} 2 randomised repeated-measurement trials,^{32 36} 1 quasi-experimental design⁴⁶ and 1 random inter-block design.⁴¹ 13 RCTS,26,3133 2 randomised repeated measures trials^{32 36} and 1 randomised block group design⁴¹ were included in the meta-analysis. The included studies included nine studies in athletes,2627 30 33,36 44 48 one in an athlete with quadriceps strength deficits,⁴⁶ one in an athletic population with dynamic knee valgus and internal rotation,³² one in an athletic population with dynamic knee valgus in an athletic population,⁴⁵ two studies in a regular athletic population,^{31 41} eight studies in patients after ACLR,^{28 29 37 39 40 42 43 47} two studies in patients with ACL injury who did not undergo surgery and^{38 49} two studies that belonged to the same cohort.^{42 43} The subjects in the study were predominantly young adults with exercise experience. The mean age of the subjects was 20 years old, and the average age range was between 14 and 31 years old. The included subjects were 262 males and 354 females, with a male to female ratio of 0.7:1. Sex data were unavailable for 133 participants across four studies.^{32 37 42 43}

15 studies implemented direct proximal core training26,3642 (including a paired cohort^{42 43}), while 9 employed indirect approaches: perturbation training (n=538,4046 49) and whole-body vibration (n=4^{37 41 47 48}). Among 24 controlled trials, 18 used conventional training comparators,26,2932 with 3 exceptions: one multiarm study incorporated intensive training,³⁶ another included a non-blank comparator⁴¹ and three used blank controls44,46 (excluded from meta-analysis). One study omitted control protocol details.³¹ Interventions predominantly featured multijoint, stability-focused regimens targeting hip/core musculature, with progressive training phases (duration: 10–80 min/session; frequency: 10–32 sessions; programme length: 1–6 months). Online supplemental table 2 details baseline characteristics, study designs, interventions and outcome metrics.

Extracted data from experimental/control groups included pretraining/post-training means±SD, minimum clinically important difference (MCID) and within-group/between-group effects. Hedges’ g SMDs were calculated to quantify intervention effects. Comprehensive statistical outputs are detailed in online supplemental table 1.

Effect of training programmes on lower limb kinematics

A total of 12 studies were available for extracting and analysing PKFA, knee flexion excursion and PHFA in proximal core strengthening training compared with conventional training for lower limb kinematics.2730,32 34 36 38 The knee flexion angle was reported in F. Saki’s study as PKFA was analysed.²⁹

Six studies (N=159) analysed changes in PKFA during jump landings between direct proximal core strengthening training and control groups,^{27 29 30 32 34 36} with training effects ranging from −0.87 to 1.26. Three studies demonstrated a significant increase in PKFA before and after training,^{27 29 30} one study demonstrated a significant difference between groups and MCID,²⁹ and one study demonstrated a trend inconsistent with the other studies, which may be related to its small sample size and was excluded from the meta-analysis.³⁶

One study analysed the change in PKFA during tuck jumping between interval proximal end core strengthening training and a control group, with a training effect of 4.61, demonstrating a significant pretraining and post-training effect.⁴⁶

Three (N=74) studies analysed the change in PKFA during single-leg deep squatting between direct proximal core strengthening training and control groups,^{32 34 44} with effects ranging from 0.09 to 0.61, and one study demonstrating significant pretraining and post-training improvement.³⁴

Two studies (N=71) describing changes in PKFA during walking between indirect proximal core strengthening training and control groups, both of which did not demonstrate significant between and within group changes.^{39 40}

One study analysed changes in PKFA during side-step tangent exercise between direct proximal core strengthening training and the control group, and no significant changes between or within groups were observed.³¹

Two studies (N=54) analysed the effect of indirect proximal core strengthening training on knee flexion excursion,^{38 40} both at time points midway through the standing phase, with between-group effects ranging from −0.10 to 0.45, with no significant changes in between-group and within-group effects observed. Detailed outcomes are catalogued in online supplemental table 1.

Four studies (N=134) analysed the changes in PHFA between direct proximal core strengthening training and control group in jump landing and side-step cutting manoeuvre, with training effects ranging from −0.37 to 0.94.^{27 31 34 36} Two studies demonstrated significant enhancement before and after training.^{27 36} One study showed a trend that was inconsistent with the other studies, which may be related to its small sample size and was excluded from the meta-analysis.³⁶ Detailed outcomes are catalogued in online supplemental table 1.

Effect of training programmes on isokinetic muscle strength

13 studies from 12 cohorts (N=427) were extracted and analysed for indicators related to lower limb isokinetic and isometric muscle strength.2628 31 33 34 37 41,45 47 48

Eight studies (N=240) correlated changes in isometric peak quadriceps torque for proximal core strengthening training and controls,^{26 28 33 34 37 41 44 47} with between-group effects ranging from 0.02 to 2.01. Five studies demonstrated significant pretraining and post-training gains and showed significant between-group changes.^{28 33 37 44 47}

One (N=48) of these studies described changes in isokinetic muscle strength of the quadriceps in direct proximal core strengthening training and control groups, did not show statistical differences and was not included in the meta-analysis because the measured speed was not described.³¹

One (N=40) study described changes in isometric quadriceps ratios between indirect proximal end core strengthening training and a control group which did not show significant differences.⁴⁸

Eight studies (N=240) described changes in peak hamstring torque after proximal core strengthening training and control,^{26 28 31 33 34 37 41 44 47} with between-group effects ranging from 0.45 to 5.75, with seven studies demonstrating significant pretraining and post-training gains^{26 28 33 34 41 44 47} and five studies demonstrating significant between-group changes.^{26 28 33 34 47}

One (N=48) of these studies described changes in hamstring isokinetic muscle strength between direct proximal core strengthening training and the control group, did not show statistical differences and was not included in the meta-analysis because the speed of measurement was not described.³¹

Two studies (N=65) from a cohort analysing changes in knee flexion total work at 6 months and 2 years after direct proximal core strengthening training and a control group did not demonstrate significant between-group and within-group changes at 6 months and 2 years after training.^{42 43}

One study (N=34) demonstrated significant between-group and within-group changes in total concentric and eccentric work of the hip before and after direct proximal core strengthening training and control.⁴⁵ Detailed outcomes are catalogued in online supplemental table 1.

Effect of training programmes on core endurance

Two studies (N=69) of direct proximal core strengthening training were described analysing prone plank support time, with between-group effects ranging from 0.59 to 0.70, both demonstrating significant pretraining and post-training enhancement.^{31 35} Detailed information can be found in online supplemental table 3.

Meta-analysis

Meta-analysis of random effects was performed on sixteen studies comparing eleven direct proximal core strengthening training groups (N=161) and five indirect proximal core strengthening exercises (N=73) with control groups (N=211).

Peak knee flexion angle

For PKFA during jumping, direct proximal core strengthening training may improve outcomes compared with controls (5 studies; SMD=0.58, 95% CI (0.20 to 0.96), I²=32%, p<0.05; low-quality evidence).^{27 29 30 32 34} A formal leave-one-out sensitivity analysis revealed initial heterogeneity (I²=63%, p=0.03; SMD=0.39 (0.03 to 0.74)) was predominantly driven by Brown et al’s study.³⁶ Exclusion reduced heterogeneity to I²=32% while maintaining consistent effect direction and significance across all other iterations. This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), with detailed methodology in online supplemental table 1.

Direct proximal core strengthening training may increase PKFA during single-leg squatting compared with control groups (2 studies; SMD=0.60, 95% CI (0.01 to 1.20), I²=0%, p=0.05; low-quality evidence).^{32 34} This evidence was downgraded due to lack of double-blinding in included studies and a total sample size below 500 participants. Detailed methodology and quality assessments are provided in online supplemental table 1.

Indirect proximal core strengthening training may have little to no meaningful effect on PKFA during walking compared with control groups (2 studies; SMD=−0.18, 95% CI (−0.65 to 0.29), I²=0%, p>0.05; low-quality evidence).^{39 40} This evidence was downgraded due to lack of double-blinding and imprecision (total sample size <500 participants). Detailed methodology is available in online supplemental table 1.

Proximal core strengthening training may slightly increase performance across different sports (7 studies; SMD=0.35, 95% CI (0.08 to 0.61), I²=39%, p<0.05; low-quality evidence).^{27 29 30 32 34 39 40} Subgroup analyses confirmed statistically significant variations (p<0.05, figure 3A). This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants).

Figure 3. Forest plot demonstrating the results of the meta-analysis of the effects of proximal core strengthening training (with 95% CIs) on (A) Peak knee flexion angle (overall effect: p<0.01), (B) Knee flexion excursion during walking (overall effect: p=0.74) and (C) Peak hip flexion angle during jumping (overall effect: p=0.01). CG, control group; EG, experimental group; PKFA, peak knee flexion angle.

Knee flexion excursion

Indirect proximal core strengthening training may have little to no meaningful effect on knee flexion excursion angle during walking compared with control groups (2 studies; SMD=0.09, 95% CI (−0.45 to 0.63), I²=0%, p>0.05; low-quality evidence)^{38 40} (figure 3B). This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), as detailed in online supplemental table 1.

Peak hip flexion angle

For PHFA during jumping, direct proximal core strengthening training may substantially increase outcomes compared with controls (3 studies^{27 34 36}; SMD=0.83, 95% CI (0.19 to 1.47), I²=0%, p<0.05; low-quality evidence).^{27 34 36} A formal leave-one-out sensitivity analysis revealed initial heterogeneity (I²=56%, p=0.29; SMD:0.43 (−0.37 to 1.22)) was predominantly driven by Brown et al’s study.³⁶ Exclusion of this study resolved heterogeneity (I²=0%) while maintaining consistent effect direction. This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), with full details in online supplemental table 1 (figure 3C).

Isokinetic peak quadriceps torque

Six studies compared proximal core strengthening training with controls for peak quadriceps torque at 60°/s.^{26 28 33 34 37 41} Separate meta-analyses were performed due to unit discrepancies. Leave-one-out sensitivity analysis revealed high heterogeneity in torque outcomes (I²=67%, p<0.05; SMD=1.31 (0.47, 2.14)), predominantly driven by Sabet et al’s study.³³ Exclusion reduced heterogeneity to I²=0% while maintaining consistent effect direction and significance across iterations.

Direct proximal core strengthening training may increase peak hamstring torque (Nm/kg) at 60°/s compared with controls (3 studies; SMD=0.52, 95% CI (0.05 to 0.99), I²=0%, p<0.05; low-quality evidence).^{26 28 34} This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants). Detailed methodology is available in online supplemental table 1.

Proximal core strengthening training may substantially increase peak quadriceps torque (Nm) at 60°/s compared with controls (3 studies; SMD=0.85, 95% CI (0.28 to 1.42), I²=0%, p<0.05; low-quality evidence).^{33 37 41} Two studies were on indirect proximal core strengthening training,^{37 41} and one study was on direct proximal core strengthening training.³³ This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants). Detailed methodology is available in online supplemental table 1.

Proximal core strengthening training may increase quadriceps strength compared with controls (7 studies; SMD=0.65, 95% CI (0.29 to 1.01), I²=0%, p<0.05; low-quality evidence). Subgroup differences were not statistically significant (p>0.05). This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), with full outcomes catalogued in online supplemental table 1 (figure 4A).

Figure 4. Forest plot demonstrating the results of the meta-analysis of the effects of proximal core strengthening training (with 95% CIs) on (A) isokinetic muscle force, quadriceps 60°/s peak torque (overall effect: p<0.01), (B) isokinetic muscle force, hamstring muscle 60°/s peak torque (overall effect: p<0.01) and (C) plank plate support time (s) (overall effect: p=0.01), CG, control group; EG, experimental group.

Peak isokinetic hamstring torque

In six papers on peak hamstring torque of 60°/s in proximal core strengthening training versus controls, meta-analyses were performed separately due to different units.^{26 28 33 34 37 41}

Direct proximal core strengthening training may increase peak hamstring torque (Nm/kg) at 60°/s compared with controls (three studies).^{26 28 34} Initial high heterogeneity (I²=90%, p=0.05; SMD=2.05 (-0.01, 4.12)) was primarily driven by Dello Iacono et al’s study; exclusion reduced heterogeneity to I²=0% while maintaining effect directionality.²⁶ The pooled effect likely represents a moderate increase (SMD=0.60, 95% CI(0.05 to 1.15), I²=0%, p<0.05; low-quality evidence). This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), with detailed methodology in online supplemental table 1.

For peak hamstring torque (Nm) at 60°/s, proximal core strengthening training may have little to no meaningful effect compared with controls (three studies).^{33 37 41} Two were on indirect proximal core strengthening training^{37 41} and one on direct proximal core strengthening training.³³ Initial substantial heterogeneity (I²=95%, p>0.05; SMD=1.98 (-0.60, 4.55)) was predominantly driven by Sabet et al’s study. Postexclusion heterogeneity dropped to I²=0% while maintaining effect consistency.³³ The pooled effect showed no statistical significance (SMD=0.45, 95% CI(−0.09 to 1.00), I²=0%, p>0.05; low-quality evidence). This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), with full methodology in online supplemental table 1.

Proximal core strengthening exercises may increase hamstring strength compared with controls (4 studies; SMD=0.53, 95% CI (0.14 to 0.92), I²=0%, p<0.05; low-quality evidence). Subgroup differences were not statistically significant (p>0.05). This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), with full outcomes catalogued in online supplemental table 1 (figure 4B).

Core endurance

Direct proximal core strengthening training may improve plank support time compared with controls (2 studies; SMD=0.63, 95% CI (0.12 to 1.13), I²=0%, p=0.10; low-quality evidence).^{31 35} This evidence was downgraded due to risk of bias (incomplete blinding) and imprecision (total sample size <500 participants), with full outcomes catalogued in online supplemental table 1 (figure 4C).

Discussion

This systematic review indicates that proximal core strengthening training may confer biomechanical advantages for individuals at elevated risk of ACL injuries, particularly through enhanced dynamic knee stabilisation during high-intensity movements such as jump landings and cutting manoeuvres. Our analysis demonstrates its potential utility as an adjunctive therapeutic approach in two critical domains: (1) postsurgical rehabilitation, where it addresses compensatory neuromuscular strategies contributing to asymmetric movement patterns and (2) athletic performance optimisation, where it promotes sport-specific kinetic chain efficiency. These benefits appear to be mediated through improved proximal-to-distal force transmission and neuromuscular coordination during task-specific activities.

Lower limb kinematics

Regarding the comparison groups (‘conventional training/no training’ referenced throughout this section): Conventional training in the included studies primarily consisted of standard rehabilitation protocols for the respective populations (eg, post-ACLR rehabilitation focusing on range of motion, basic strengthening and proprioception) or traditional lower limb resistance training programmes (eg, squats, lunges, leg presses). No training refers to control groups receiving no specific intervention beyond usual activities.

Regarding lower limb kinematics, low-quality evidence suggests that direct proximal core strengthening training yields a moderate improvement in PKFA during jumping and single-leg squatting compared with conventional training or non-training interventions. For PHFA during jumping, the initial pooled analysis showed non-significant improvement (SMD=0.43 (−0.37 to 1.22)). However, leave-one-out sensitivity analysis revealed this result was substantially influenced by Brown et al’s study: on its exclusion, the effect became statistically significant with a large improvement (SMD=0.83 (0.19 to 1.47)) and resolved heterogeneity (I² decreased from 56% to 0%).³⁶ This indicates the overall PHFA effect is highly sensitive to individual study characteristics, requiring cautious interpretation of its clinical relevance. One of the risk factors for ACL injury is asymmetry in lower limb kinematics,⁵⁰ and patients after ACLR demonstrate the use of interlimb compensatory strategies during jumping⁵¹ and single-leg deep squatting,⁵² which creates an asymmetry in the lower limb, which can have a direct impact on the patient’s subjective knee function. The included studies on jumping and during the single-leg squatting were direct proximal core strengthening exercises and included populations of healthy athletes and patients after ACLR. Increased symmetry of both lower limbs in the jump test following proximal core strengthening training improves landing kinematics by enhancing symmetry during lower limb kinematic movements.⁵³ Enhanced bilateral symmetry during jumping following such training may improve landing kinematics, though current evidence shows inconsistent knee flexion angle changes in ACLR limbs during landing, with observable reductions in hip flexion on the affected side during bilateral landings.⁵⁴

Biomechanically, the knee and hip joints are the primary impact relieving joints during the landing of body jumps, and stiffer landings with smaller knee and hip flexion angles are not conducive to reducing the impact of the landing,⁵⁵ and a decrease in the knee flexion angle during landing increases the knee abduction torque.⁵⁶ Proximal core strengthening may mitigate ACL injury risk by improving dynamic trunk control during deceleration and potentially reducing abduction torque—though the sensitivity of PHFA results warrants verification of hip-specific kinematic responses in future studies.⁵⁷

Conversely, low-quality evidence indicates indirect proximal core strengthening training demonstrates no significant effect on PKFA (SMD=−0.18 (−0.65 to 0.29)) or knee flexion excursion (SMD=0.09 (−0.45 to 0.63)) during walking in ACL-injured or ACLR populations. All included during the walk were indirect proximal core strengthening exercises, and the population included were patients with ACL injury and ACLR. Inconsistent with the biomechanical properties during high-risk sports such as jump landings is the fact that patients with ACL injury and ACL reconstructions show functional fixedness of the knee during walking out of a protective response⁵⁸ and maintain knee stability by compensating for ankle joint movement.⁵⁹ Given that proximal core training shows limited advantage for improving ankle kinematics, the absence of significant effects on knee parameters during walking is biomechanically plausible.²⁷

Lower limb muscle strength

Regarding quadriceps strength and hamstring strength, low-quality evidence suggests proximal core strengthening training may increase quadriceps torque. Leave-one-out sensitivity analysis confirmed this heterogeneity was primarily attributable to some studies.^{26 33} Its exclusion resolved heterogeneity (I²=0%) while maintaining consistent direction and significance of effects across all iterations, reinforcing the robustness of the positive finding. The included studies on jumping and during single-leg squatting involved direct and indirect proximal core strengthening exercises. The included population involved healthy individuals who exercise regularly, athletes and patients after ACLR. In the meta-analysed papers, we chose all peak torque at 60°/s because 60°/s isokinetic muscle strength measurement is currently the most commonly used test speed, which effectively responds to lower limb muscle strength. There is a biomechanical link between the core and the lower limb muscles, and although the lower limb muscles are not directly trained, proximal core strengthening training can enhance the activation of the quadriceps and hamstrings through biomechanical transmission and improve quadriceps and hamstrings strength.⁶⁰ Hamstring strength affects the loads placed on the ACL during exercise by influencing knee flexion, whereas quadriceps strength both affects knee extension and has a positive tendency to correlate with overall kinematic performance, which can have a direct impact on functional knee stability.⁶¹ Reduced peak knee extension torque and reduced knee flexion angle at landing after ACLR are directly related to muscle strength performance.⁶² Improvements in knee muscle strength and knee muscle neural control are also beneficial in reducing the risk of ACL injury. Routine training focuses on strength on the side of the injured limb and fails to focus on the fact that strength loss can also exist on the side of the uninjured limb. People with asymmetrical quadriceps strength will exhibit less knee extension torque and knee-to-support torque ratios, which affects the overall movement process.⁶³ If only the muscle strength of the affected side is improved during the rehabilitation process without focusing on the overall biomechanical performance, it is also not conducive to the recovery of the patient’s motor performance. Proximal core strengthening training focuses more on the whole body, improves neuromuscular control and improves asymmetry of strength between limbs, which can effectively restore the knee’s ability to perform in sports, thus reducing the risk of ACL injury.^{26 33 38 40}

After ACL injury, knee energy reception strategy changes and chronic osteoarthritis may occur.^{64 65} The knee is the main energy cushion during landing and the hip is one of the main energy absorbers for men.⁵⁷ In the studies included in this paper, no significant effect of proximal core control training on total flexion work was found,^{42 43} but a rise in hip concentric and eccentric work was found after proximal core control training.⁴⁵ The ability of the hip to cushion effectively during exercise may also reduce ACL injury. As proximal core strengthening does not involve full weight-bearing walking of the lower limbs, proximal core strengthening is uniquely valuable as an early postoperative intervention in terms of timing of intervention. Conventional resistance training can only be performed safely 6 weeks after the patient’s ACL reconstruction.⁶⁶

The potential biomechanical benefits of proximal core strengthening training—particularly in enhancing dynamic knee stability during high-risk movements—support its strategic integration into targeted clinical and athletic populations. For post-ACLR patients, early-phase implementation (2–6 weeks postsurgery) may disrupt compensatory neuromuscular patterns, while athletes could leverage task-specific protocols to optimise movement mechanism preseason. However, clinicians must contextualise these interventions within multidimensional prevention frameworks, prioritising individualised progression (eg, static-to-dynamic drills) and objective monitoring (eg, wearable sensors for real-time kinematic feedback).⁶⁷

This meta-analysis has several limitations. First, while significant heterogeneity was initially observed in key outcomes (eg, quadriceps torque, I² = 61%), subsequent leave-one-out sensitivity analysis successfully reduced this heterogeneity to negligible levels (I²≈0%), substantially strengthening the robustness of these pooled estimates. Second, the overall low study quality—evidenced by pervasive risks of bias from inadequate blinding, limited randomisation and universally small sample sizes (<500)—undermines confidence in the pooled estimates. Third, while funnel plots were examined (online supplemental appendix 1), potential publication bias cannot be ruled out given the restricted number of included trials. Finally, the certainty of evidence for all outcomes was graded as low certainty per GRADE criteria, reflecting persistent methodological limitations and the resolution of initial inconsistency through sensitivity analysis.

Importantly, the stability of results varied: statistical significance remained consistent across most outcomes, except for PHFA. The exclusion of Brown et al’s data during sensitivity analysis revealed a statistically significant effect that was masked in the primary analysis, suggesting this particular finding is sensitive to individual study contributions and warrants cautious interpretation.³⁶ Collectively, these factors limit definitive conclusions regarding efficacy in early postoperative patients and injury prevention. Future multicentre trials should implement: (1) standardised dose-controlled protocols; (2) longitudinal tracking of biomechanical adaptations (eg, knee flexion symmetry) and (3) monitoring of injury epidemiology across competitive cycles. Until such higher-quality evidence emerges, proximal core training should be viewed as a biomechanically informed adjunct—not a standalone solution—within ACL injury mitigation frameworks.

Conclusions

Proximal core strengthening exercises not only improve core endurance but also demonstrate potential to optimise knee kinematics and enhance lower limb muscle strength in high-risk ACL populations—specifically defined in this review as: (1) individuals exhibiting biomechanical risk factors predisposing to primary ACL injury, (2) post-ACLR patients with persistent neuromuscular deficits (including graft rupture risk) and (3) athletes with pathological movement patterns. While these adaptations may benefit postoperative recovery and secondary prevention, current evidence remains constrained by methodological limitations (eg, heterogeneous interventions, unvalidated injury/graft failure rates). We recommend integrating this approach within multidimensional prevention strategies, with future research prioritising standardised protocols and prospective tracking of ACL injury and graft rupture outcomes.

Data availability statement

No data are available.