Effective Prevention and Rehabilitation Strategies to Mitigate Non-Contact Anterior Cruciate Ligament Injuries: A Narrative Review

Abstract

Non-contact anterior cruciate ligament injuries (NC-ACLs) represent a significant concern in sports medicine, particularly among athletes and physically active individuals. These injuries not only result in immediate functional impairment but also predispose individuals to long-term issues such as recurrent instability and early-onset osteoarthritis. This narrative review examines the biomechanical, neuromuscular, and environmental factors that contribute to the high incidence of NC-ACLs and evaluates the effectiveness of current prevention and rehabilitation strategies. The review identifies key risk factors, including improper landing mechanics, deficits in neuromuscular control, and muscle imbalances, which are pivotal in the etiology of NC-ACLs. Prevention programs that incorporate plyometric exercises, strength training, and neuromuscular education have shown efficacy in reducing injury rates. Rehabilitation protocols that emphasize a gradual return to sport, focusing on pain management, restoration of range of motion, and progressive strengthening, are critical for successful recovery and prevention of re-injury. The evidence suggests that an integrated approach, combining prevention and rehabilitation strategies tailored to the individual, is essential for minimizing NC-ACL risk and improving outcomes in affected populations.

1. Introduction

Anterior cruciate ligament (ACL) tears are among the most common sports injuries worldwide, with an estimated incidence of 0.3–0.8 cases per 1000 individuals [1,2,3,4]. These injuries impose a significant socioeconomic burden due to the resulting disability, loss of productivity, and the onset of post-traumatic knee osteoarthritis (OA) if left untreated. It is estimated that society could save USD 1.1 billion annually (USD 5500 per patient) if the risk of ACL tears were halved [5]. ACL injuries are more prevalent among young and active individuals, particularly female athletes who participate in sports such as soccer and basketball, which often involve jumping, pivoting, and changes in direction [6,7]. Furthermore, the increased participation in sports and recreational activities by the general population may expose more individuals to the risk of an ACL tear. For these reasons, understanding the primary risk factors that predispose individuals to ACL injuries is crucial.

Based on the injury mechanism, ACL tears can occur as either “contact” injuries, caused by direct trauma to the knee, or “non-contact” ACL injuries (NC-ACLs), which are more common and typically occur during landing from a jump, cutting maneuvers, or sudden deceleration [8]. One frequently described mechanism involves landing with an extended hip and knee, a valgus knee position, internal tibial rotation, and a pronated foot [6,9]. Numerous risk factors have been associated with NC-ACLs, which can be categorized as intrinsic or extrinsic and modifiable or nonmodifiable [10]. Nonmodifiable risk factors include genetics and family history, age [11], female sex, narrow intercondylar notch size [12,13], increased posterior tibial slope [14,15], and generalized ligamentous laxity [16]. Previous studies have shown that females tend to have a narrower intercondylar notch and greater knee and general joint laxity compared to males [17], partly explaining the higher predisposition to ACL injuries among the former.

Modifiable risk factors for ACL injury can be divided into extrinsic factors, such as the type and level of sport, footwear, playing surface, and weather conditions, and intrinsic factors, such as body mass index (BMI), neuromuscular and biomechanical deficits, and hormonal status [18]. Controlled laboratory studies have determined that males and females exhibit different movement and muscle activation patterns; however, the relationship between these differences and the risk of ACL injury remains unclear [19]. Females tend to land from jumps and perform cutting and pivoting maneuvers with less knee and hip flexion, greater knee valgus, increased internal hip rotation coupled with increased external tibial rotation, and higher quadriceps muscle activation [20,21]. These movement patterns are hypothesized to elevate the strain on the ACL, potentially contributing to the significant disparity in ACL injury rates between males and females, which may be attributed to neuromuscular differences and resultant mechanics [22].

Understanding the mechanisms underlying NC-ACLs and the main associated risk factors is crucial for developing targeted prevention programs. Factors such as improper landing mechanics, inadequate neuromuscular control, and muscle imbalances are significant contributors to non-contact ACL injury (NC-ACL) risk [23]. Consequently, prevention programs often focus on enhancing proprioception, improving neuromuscular coordination, and correcting biomechanical deficits [24]. Rehabilitation following an ACL injury is equally important, with the primary goal of restoring function and preventing re-injury.

This narrative review aims to synthesize the current knowledge on the biomechanical and neuromuscular factors contributing to NC-ACLs and to evaluate the efficacy of various prevention programs designed to address these risk factors. Additionally, various rehabilitation protocols are discussed, emphasizing the importance of a criterion-based approach that addresses pain management, range of motion (ROM) recovery, muscle strengthening, and neuromuscular re-education.

2. Prevention of NC-ACLs

Effective NC-ACL prevention programs require a comprehensive understanding of the biomechanical and neuromuscular factors that predispose athletes to injury. A significant proportion of NC-ACLs occur during landing after a jump or abrupt deceleration, placing the ACL under stress when the knee is extended or minimally flexed (5–20°) and subjected to valgus stress [25]. Movements involving excessive internal hip rotation with external tibial rotation further drive the knee into valgus, increasing the risk of ACL injury [26]. These biomechanical features, combined with inadequate neuromuscular adaptations, amplify shearing forces on the knee and increase NC-ACL susceptibility [17,25,26,27,28]. Consequently, optimal ACL injury prevention programs should address these high-risk landing mechanics and enhance neuromuscular control to mitigate ACL stress [29]. Since contact ACL injuries are often unavoidable and stem from different causes compared to NC-ACLs, ACL rupture prevention strategies predominantly target NC-ACLs [30,31,32,33,34,35,36,37,38,39]. These strategies focus on mitigating NC-ACL-related risk factors and enhancing physical attributes that may predispose individuals to such injuries. Consequently, athletes at risk for NC-ACLs should thoroughly adhere to prevention programs to reduce the risk of injury, with the main aim of reducing the effect or even eliminating the risk factors associated. Improved proprioception and neuromuscular training, increased trunk/core control, plyometric training, proportional strengthening of hamstring and quadriceps muscles, stretching, and awareness of high-risk positions during sports activities are essential elements to prevent NC-ACL [30,31,32,33,34,35,36,37,38,39]. Previous studies in this field have introduced various programs aimed at decreasing the impact of modifiable risk factors for NC-ACLs and/or reducing the rate of NC-ACLs, which are reviewed below.

2.1. Targeting NC-ACL Modifiable Risk Factors

The correct alignment of the hip and knee joints during sports activities is crucial to avoid injury, with proper posture upon landing being a modifiable risk factor for NC-ACLs. The gluteal muscles act as essential regulators of pelvic stability, ensuring proper hip and knee alignment [40]. A combination of weak hip muscles and excessive hip flexion can significantly limit the gluteal muscles’ ability to stabilize the hip joint during ground contact [41]. Herman et al. [42] described a strength training program focused on major leg muscles to improve landing in female athletes. In this strength-training program [42], the exercises focused on isolated muscle groups rather than compound movements. The program targeted major leg muscles, such as the quadriceps, hamstrings, gluteus medius, and gluteus maximus, primarily through exercises using resistance bands. The exercises were generally structured with a three-limb support base to minimize balance effects, suggesting a focus on double-leg stability. Despite increased muscle strength in the training group, no significant changes in knee and hip kinetics and kinematics during a stop-jump test were found. Based on these findings, the authors concluded that strength training alone might not be enough to reduce the risk of NC-ACLs in these athletes. This could indicate that strength exercises need to be combined with other types of training, such as neuromuscular exercises, to achieve a holistic preventive effect. Further studies might benefit from investigating which additional elements, like proprioceptive or plyometric exercises, could enhance the efficacy of strength training alone.

Anterior pelvic tilt can further accentuate the dynamic valgus of the knee [41]. Tightness of hip flexors and erector spinae muscles, along with weakness of the abdominal and gluteal muscles, are thought to contribute to increased anterior pelvic tilt, thereby increasing the risk of NC-ACLs [40]. Zazulak et al. [43] recommended that athletes with decreased core muscle control should perform core stability and proprioceptive exercises, as well as training to enhance landing techniques, to reduce the risk of NC-ACLs. This finding suggests that interventions aiming to strengthen core stability might be particularly beneficial for female athletes, who are more prone to NC-ACL injuries. Moreover, the trunk musculature may also affect hamstring function, thereby potentially influencing the likelihood of NC-ACLs [44].

The hamstrings play a critical role in reducing anterior, varus–valgus, and internal-external displacement moments applied to the knee [45]. Hamstring strengthening training is thus indicated to reduce the risk of NC-ACLs [40]. A low hamstring to quadriceps (H:Q) isokinetic strength ratio (i.e., the quadriceps muscle outperforms the hamstrings) increases the risk of excessive anterior tibial translation during dynamic activities like landing or changing direction [46]. This poses greater shearing forces on the ACL, and if the hamstrings are not able to counterbalance such displacement forces, ACL injury may occur [40]. Holcomb et al. [47] investigated the role of a specific resistance training program on the H:Q strength ratio in female soccer players. The authors found that a 6-week hamstring-focused training program increased the H:Q strength ratio, potentially reducing the risk of NC-ACLs in this cohort. Likewise, other studies have also shown that improving the H:Q strength ratio may reduce the risk of NC-ACLs in female athletes [48,49].

Muscle fatigue is an additional parameter to consider among risk factors for NC-ACLs [40]. Nyland et al. [50] showed that hamstring fatigue reduces the ability to control knee stability during movements. Targeted activities such as lower extremity closed kinetic chain exercises, mini-squats, single leg-hops, and sideways shuffles in a low squat position improve muscle resistance and may prevent fatigue. Moreover, Nyland et al. [50] emphasize the role of muscle endurance in maintaining knee stability, particularly as fatigue can compromise protective mechanisms. This study advocates for endurance training to be an essential component of the mid-stage rehabilitation process, as improved endurance can enable athletes to maintain proper mechanics even under fatigue, thus reducing the likelihood of NC-ACL re-injury.

One of the most discussed NC-ACL risk factors is the knee abduction moment, especially during landings [25,40]. The ideal landing technique involves first ground contact by the forefoot, with the hip and knee flexed [29]. NC-ACL prevention programs often include modifications of landing techniques [32,34,35,38,51]. A study by Myer et al. [52] classified a population of female athletes into high-risk and low-risk categories for NC-ACLs based on knee kinematics. After a landing training period, the results showed a 13% decrease in knee abduction moment for high-risk athletes.

While muscles play an essential role in dynamic joint stability, additional key factors should be taken into account. To reduce the risk of ligament injuries, the elastic components of the musculotendinous unit, along with the sensory and neural systems, need to be actively engaged [46]. Training programs focused solely on strengthening exercises might neglect other crucial aspects of dynamic knee joint stability [42]. Herman et al. [42] showed increased isometric contraction strength after an adequate NC-ACL prevention program, although without achieving any improvements in the biomechanical factors associated with the risk of injury, and raised some concerns about the difficulties of following cohorts over long periods without subject attrition. Incorporating proprioception and plyometric exercises into training programs is essential to enhance joint kinematics and kinetics during sports activities [46].

2.2. Strategies to Prevent the Occurrence of NC-ACLs

The effectiveness of NC-ACL prevention programs has been a topic of intense debate in recent years, with studies producing conflicting results. One of the main challenges in determining the efficacy of these programs is the difficulty of conducting adequately powered studies, given the relatively low incidence of ACL injuries in the general population. Additionally, the implementation timing of the preventive programs among athletes can vary from preseason to the start of the season, as well as the frequency and duration of the program. Intrinsic factors such as the athlete’s skill level, age group, and type of sport played can further influence the results [46]. Thus, understanding these variables is crucial for designing effective prevention strategies. Neuromuscular training programs have been extensively studied for their helpful reduction in the incidence of NC-ACLs by addressing key risk factors associated with injury mechanics. For instance, LaBella et al. (2011) [53] found that a structured neuromuscular warm-up in high school athletes led to a significant reduction in lower extremity injuries, specifically in NC-ACL injuries. Myer et al. (2007) [52] demonstrated that high-risk athletes showed reduced knee abduction moments, indicating improved dynamic stability following targeted neuromuscular training; Hewett et al. (1999) [34] also reported a notable reduction in knee injuries in female athletes participating in a neuromuscular program focusing on plyometrics and dynamic knee control.

Zazulak et al. (2007) [43] highlighted that core stability, enhanced through neuromuscular training, plays a crucial role in maintaining knee alignment and reducing valgus collapse during high-risk maneuvers, which are strongly linked to ACL injury risk. In a similar vein, Chappell et al. (2008) [30] observed that neuromuscular programs could modify kinetic and kinematic profiles in female athletes, leading to safer movement patterns during jumping tasks. Despite these challenges, critical analysis of data from multiple studies offers valuable insights into defining effective NC-ACL prevention approaches. In particular, neuromuscular training programs have been extensively studied for their potential to reduce NC-ACL rates [25,26,30,34,37,43,49,52,53].

Rengstrom et al. [29] identified Henning as the first author to investigate the higher ACL injury rates observed in female athletes in 1990. They hypothesized that ACL tears primarily resulted from knee position and muscle activation patterns during dynamic movements. The quadriceps muscle, especially in the last degrees of knee extension, applies a significant force that pulls the tibia forward, placing strain on the ACL. Based on this theory, Henning’s intervention program aimed to modify athletes’ technique to promote more knee and hip flexion during cutting, landing, and deceleration maneuvers. This program resulted in an 89% reduction in ACL injuries and laid the groundwork for numerous subsequent ACL injury prevention programs.

Building on this background, Hewett et al. [34] conducted a specific neuromuscular training program to prevent knee injuries in soccer players. The six-week training included exercises for flexibility, strength, and jumping power. The trained soccer players presented no NC-ACLs, compared to six in the untrained group, indicating that the neuromuscular prevention program effectively mitigated the risk of NC-ACLs in these athletes. The success of Hewett et al.’s neuromuscular program suggests that training interventions should not only target strength but also neuromuscular control. This could indicate a shift towards incorporating these multifaceted programs in young athletes’ preseason training to condition their neuromuscular responses before high-intensity sports seasons. Moreover, the findings encourage further exploration into how long-term training can contribute to sustained injury reduction. Similarly, a study by Gilchrist et al. [32] determined the effectiveness of a specific warm-up routine to prevent NC-ACLs in young female soccer players. This randomized controlled trial had the intervention group follow a special 12-week warm-up program called the Prevent injury and Enhance Performance (PEP) Program during the season. The intervention group experienced a 41% reduction in NC-ACLs compared to the control group. For players who had previously undergone ACL reconstruction and completed rehabilitation, the program showed a 100% reduction in re-injury from NC-ACLs and an 80% reduction from contact injuries. Caraffa et al. [54] demonstrated a statistically significant lower risk of ACL injuries in the intervention group that followed a specific proprioceptive training program. However, the study did not report the exact number of NC-ACLs, leaving the complete understanding of the proprioceptive training’s effectiveness in preventing NC-ACLs unclear. Myklebust et al. [55] investigated a neuromuscular training program performed over three years to prevent ACL injuries. Elite players who participated in the program had significantly fewer ACL injuries than those who did not, with a notable reduction in NC-ACLs. However, Heidt et al. [33] found that although the trained group had a lower percentage of ACL injuries (2.4% vs. 3.1% in the control group), the difference was not statistically significant, compromising the perceived effectiveness of the preventive program.

The prospective study by Irmischer et al. [56] evaluated the effectiveness of a Knee Ligament Injury Prevention (KLIP) program over two years among athletes in soccer, volleyball, and basketball. The KLIP program included 15 min of strengthening and plyometric exercises. Although there was no significant difference in ACL injury rates between the intervention and control groups, there were no NC-ACLs among soccer and volleyball players in the intervention group. Intriguingly, all NC-ACLs in the intervention group (with the KIP program) affected basketball players, thus elucidating the need for training programs that should be tailored to the specific sports activity. The lack of statistically significant difference might be attributed to the relatively short duration of the program (9 weeks). Furthermore, since the experimental program was conducted after the classic training sessions, neuromuscular fatigue might have compromised the correct execution of the prevention program [56]. In their study implementing the KLIP program, Pfeiffer et al. [38] reported six NC-ACLs, equally distributed among the treatment and control groups, thus resulting in a non-significant reduction in NC-ACLs [38]. Mandelbaum et al. [35] conducted a study over two years on young female soccer players using a specific 20 min warm-up program that included exercises in athletic gesture education, stretching, muscle strengthening, plyometrics, and agility. The program resulted in an 88% reduction in ACL injuries in the first year and a 74% reduction in the second year compared to the control group, highlighting its effectiveness. However, being a non-randomized prospective study with voluntary enrollment, the study may have suffered from a “Hawthorne effect” and selection or motivational bias, as the teams that chose to participate were likely more interested in injury prevention. Conversely, in a randomized study including 1837 players aged between 15 and 17 years, Olsen et al. [51] demonstrated a significant NC-ACL reduction in the experimental group after 8 months of training.

Research by LaBella et al. [53] and Kiani et al. [57] focused on the Knee Injury Prevention Program (KIPP), which included a warm-up with jumping drills, balance exercises, and core strengthening. Both studies found a significant decrease in NC-ACLs in the trained groups, demonstrating that specific exercise programs can significantly reduce the incidence of these injuries.

Pasanen et al. [58] evaluated a neuromuscular training program for preventing non-contact lower limb injuries but found no preventive effect on NC-ACLs, with both intervention and control groups reporting three injuries each. Ettlinger et al. [59] conducted a prospective and non-randomized trial study involving 4000 skiers, implementing an effective approach to prevent ACL injuries by modifying dangerous lower limb positions during sports activity. The trained group experienced a 62% reduction in ACL injuries by the end of the season, demonstrating the potential effectiveness of this prevention strategy in skiing.

Ultimately, all these studies highlight various ACL injury prevention programs, with a recurring theme being that neuromuscular training programs can significantly reduce the incidence of NC-ACLs in athletes [25,26,30,34,37,43,49,52,53]. The main studies investigating risk factors and prevention strategies for NC-ACLs are summarized in

Table 1. Main studies reporting risk factors and prevention strategies for NC-ACLs.

Study	Study Design	Sample Size (M/F)	Risk Factors	Prevention Strategies	Training Duration	Conclusions
Herman et al., 2008 [42]	Controlled laboratory study	66 (0/66)	Hip muscle weakness and excessive hip flexion	Resistance bands and exercise balls to train quadriceps, hamstrings, gluteus medius, and gluteus maximus muscles	6 weeks	Strength training alone did not improve lower limb biomechanics
Zazulak et al., 2007 [43]	PS	277 (136/141)	Decreased core muscle control	Core stability and proprioceptive exercises, landing technique training	3 years	Greater trunk displacement, particularly in the lateral direction, was associated with an increased risk of knee injuries in female athletes
Holcomb et al., 2007 [47]	PS	12 (0/12)	Altered isokinetic hamstrings to quadriceps ratio	Hamstring strength-focused training program	7 weeks	Eccentric hamstring to concentric quadriceps ratio exceeded 1 after training
Hewett et al., 2005 [25]	PS	205 (0/205)	Increased dynamic knee valgus and high abduction loads	Neuromuscular training program	6 weeks	Correction of landing biomechanics, neuromuscular control, and knee valgus loading decreased the risk of ACL injury
Myer et al., 2007 [52]	PS	53 (0/53)	Muscle co-activation imbalance, increased activation time, decreased strength	Plyometric, core strengthening, balance and resistance training	6 weeks	A comprehensive training program demonstrated multiple benefits in adolescent female athletes
Hewett et al., 1999 [34]	RCT	1263 (434/829)	Not following a neuromuscular training program before the start of the sports season	Neuromuscular, flexibility, strength, and jumping power exercises	6 weeks	Neuromuscular training can reduce the risk of knee injuries in female athletes
Gilchrist et al., 2008 [32]	RCT	1435 (0/1435)	Incorrect neuromuscular and biomechanical control	PEP warm-up program	1 season	On-the-field exercises may prevent NC-ACLs in young female soccer players
Caraffa et al., 1996 [54]	PS	600 (600/0)	Alteration of lower extremity proprioception and dynamic balance	Progressive proprioception training and neuromuscular facilitation program	3 consecutive seasons	Significant reduction in the incidence of ACL injury without distinguishing between NC-ACL and for contact ACL injuries
Myklebust et al., 2003 [55]	PS	2647 (0/2647)	Neuromuscular imbalances, improper landing techniques, and hormonal fluctuations	Neuromuscular training, proprioceptive exercises, and education on proper landing techniques	5 weeks per 2 consecutive seasons	Neuromuscular training could significantly reduce the incidence of knee injuries in female athletes, particularly those who were untrained
Heidt et al., 2000 [33]	PS	300 (0/300)	Alteration of lower extremity dynamic balance	Cardiovascular, plyometric, strength, flexibility exercises and agility drills	7 weeks	The trained group experienced a significantly lower incidence of injury than the untrained group
Irmischer et al., 2004 [56]	PS	28 (0/28)	High impact forces during landing	KLIP program	9 weeks	Plyometric exercise program significantly reduced impact force in female athletes, potentially reducing the risk of knee injuries
Pfeiffer et al., 2006 [38]	PS	1439 (0/1439)	Incorrect jump-landing and running-deceleration mechanics	KLIP program	12 weeks	The employed training program did not decrease NC-ACL rates in high-school female athletes
Mandelbaum et al., 2005 [35]	PS	5703 (0/5703)	Muscle co-activation imbalance with increased activation time; decreased muscular strength and landing mechanics	Warm-up program with athletic gesture education, stretching, strengthening, plyometrics, and agility	12 weeks	A neuromuscular training program may decrease the rate of ACL injuries in female soccer players
Olsen et al., 2005 [51]	RCT	1837 (246/1591)	Lower strength with decreased joint stability	Strength exercises, proprioception exercises, and plyometrics	8 months	Strength training can be an effective strategy for knee injury prevention
LaBella et al., 2011 [53]	RCT	1492 (0/1492)	Lack of neuromuscular control	Neuromuscular warm-up program for the entire sports season with exercises targeting strength, balance, agility, plyometrics, and flexibility	1 season	Neuromuscular warm-up program significantly reduced the rate of non-contact lower extremity injuries in females
Kiani et al., 2010 [57]	PS	1506 (0/1506)	Female sex and young age	HarmoKnee preventive program	1 season	HarmoKnee preventive program reduced the incidence of acute knee injuries by 77% and non-contact knee injuries by 90% in young female soccer players
Pasanen et al., 2008 [58]	RCT	457 (0/457)	Poor neuromuscular control, inadequate balance, and lack of strength	Training program to enhance players’ motor skills and body control	26 weeks	Neuromuscular training significantly prevented NC-ACLs in female floorball players
Ettlinger et al., 2005 [59]	PS	4700 (NS)	Lack of understanding of how to react when a skier is in a position that could lead to an ACL injury	“ACL Awareness Training” program designed to educate skiers on the mechanisms of ACL injuries and how to avoid them	1 season	A 62% reduction in ACL tears was found in the trained group compared to the control group

Abbreviations: ACL = anterior cruciate ligament; KLIP = Knee Ligament Injury Prevention; NC-ACLs = non-contact ACL injury; PEP = Prevent injury and Enhance Performance; PS = prospective study; RCT = randomized controlled study.

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3. Rehabilitation Following NC-ACLs

There is no universally accepted best method for rehabilitation after sustaining an NC-ACL, but adopting a criterion-based approach through multiple stages or phases is considered the best practice [60]. After surgery, we usually recognize three stages of the rehabilitation protocol: early, mid, and late stages [61,62,63].

3.1. Early Stage

The early stage begins immediately following surgery and should start as soon as possible. The primary objective is to control pain and swelling, which can be managed with medication, cryotherapy, compression, and elevation [64,65,66,67]. Similarly, restoring the ROM is a primary goal, achieved through active and passive mobilization exercises. Special attention must be given to recovering full extension, as an extension deficit is associated with altered knee biomechanics, gait disturbances, anterior knee pain, quadriceps inhibition, and an increased risk of cyclops lesion and arthrofibrosis [68,69,70,71]. Unlike the extension, the recovery of flexion and weight-bearing can be less aggressive and may vary depending on postoperative protocols related to concomitant procedures, such as meniscal repairs or cartilage restoration [72]. Gradual and balanced load recovery is also important, although load recommendations may vary depending on the specific procedures performed. For instance, in cases of radial, longitudinal, and meniscal root sutures, immediate weightbearing can be detrimental to hoop stress [73,74,75]. Likewise, for medial and lateral compartment ligament reconstructions, loading may place stress on the valgus or varus, necessitating careful management [76,77]. Hydrotherapy should be initiated as soon as possible, as it can improve ROM, reduce swelling, and stimulate muscle activation without overloading the joint [78]. Buckthorpe’s study [78] supports aquatic therapy and emphasizes the role of low-impact conditioning exercises in reducing the stress on the knee while enabling continued muscle strengthening. This approach is particularly beneficial for athletes as they progress into sport-specific training since aquatic exercises can help maintain conditioning without compromising joint integrity, making it an effective option for sustaining gains made during earlier rehabilitation stages.

Once pain and swelling are well controlled, muscle strengthening should begin. Following ACL reconstruction, quadriceps muscle atrophy and arthrogenic muscle inhibition (AMI) are common and should be promptly addressed [79]. Weakness in the knee extensor muscles is associated with altered biomechanics, instability, persistent pain, and a risk of early-onset osteoarthritis [80,81,82,83]. The study by Lewek et al. [80] points out that managing pain and swelling is crucial for early mobilization and knee extension recovery. It emphasizes that strategies such as cryotherapy, elevation, and compression should be prioritized immediately post-surgery to set a solid foundation for later rehabilitation stages. This is especially important as extension deficits can lead to long-term biomechanical issues and affect gait patterns, potentially complicating further rehabilitation. Additionally, quadriceps weakness can impact long-term outcomes and the return to sport, with greater initial weakness leading to more difficulty in regaining strength post-surgery [84,85,86,87]. In particular, Amin’s study [82] suggests that quadriceps strength is not only essential for knee stability but also crucial for slowing osteoarthritis progression. This supports the view that rehabilitation programs should include targeted quadriceps strengthening from the early stages to minimize long-term joint deterioration and improve functional outcomes, particularly for athletes wishing to resume prior sports levels. The degree of muscle weakness depends on various factors, such as the type of surgery, the graft used, the severity of AMI, swelling, pain, and inflammation [81,87,88,89]. Initially, recruiting the quadriceps may be challenging. This can be facilitated by using cryotherapy before training [89,90,91,92], transcutaneous electrical nerve stimulation (TENS) [93,94], neuromuscular electrical stimulation (NMES) [95], blood flow restriction (BFR) training [96,97], and fatigue exercises [98,99]. These methods should be combined with active training, starting with isometric exercises [63]. Once sufficient quadriceps recruitment is achieved, progression to open and closed kinetic chain exercises is possible. There are some concerns regarding open kinetic chain exercises as they have been thought to overload the new ligament [100,101]. However, recent evidence suggests that this risk is minimal [100]. Nonetheless, it is recommended to start these exercises with a reduced range of motion (45–90°) and low load [102,103,104,105,106]. In the early stage, strengthening exercises should always be performed with a low load to the point of fatigue [63]. This is crucial to respect the biological healing timeline of the new ligament, which can take several months, progressing up to two years post-surgery [107,108,109]. The new ligament is particularly vulnerable between 6 and 12 weeks post-operation due to its remodeling process and may not tolerate overloading during this phase, posing a risk of re-injury [102,103,110,111]. As with the extensor apparatus, hamstring weakness following ACL reconstruction is very common when using autograft, as well as hamstring strains, and can persist for years [112,113,114,115,116], with the failure to recover increasing the risk of re-injury [87]. Therefore, the recovery of hamstring strength and function should be initiated promptly. If the hamstring tendons have been used as grafts, selective inhibition of the semitendinosus is very common and also, the neo-hamstrings tendon takes a considerable amount of time to form, if at all [117,118,119,120]. In many cases, it just scars into the fascia of the semimembranosus, and all of these complications contribute to making recovery more challenging [121]. In such cases, some authors recommend delaying strengthening exercises for up to 6 weeks post-surgery to allow for proper healing [102,122,123]. However, to avoid impairing muscle recovery, it is considered safe to start low-intensity isometric exercises even in the early stages, after an initial rest period [124]. Alongside strengthening the quadriceps and hamstrings, it is also crucial to diagnose the weakness of the stabilizing muscles of the proximal and distal joints to the knee [63]. Therefore, part of the rehabilitation should be dedicated to strengthening the triceps surae and plantar flexors, as well as hip stabilizing muscles [63], which are important in preventing dynamic knee valgus [125].

Finally, it is recommended to introduce non-weight-bearing hip and core muscle strengthening exercises during the early stages of rehabilitation [63,126,127]. Restoring muscle strength and function is crucial for recovering proper movement quality and neuromuscular control, which are often compromised following an ACL injury [62,128,129] due to its significant proprioceptive role [86]. This loss leads to abnormal movements and compensatory mechanisms that disrupt normal walking patterns [130]. It is important to immediately instruct patients in the correct use of crutches to regain a normal gait pattern, perform knee mobilization exercises focusing on full extension recovery, and introduce proprioceptive exercises to improve neuromotor control of the knee [63]. Initially, selective retraining exercises should be introduced in a swimming pool [78]. Crutches should not be dismissed until the period of protected weight bearing on the operated limb has been completed and physiological gait and complete knee extension are achieved, with no swelling, pain, or quadriceps lag [63]. In the early stage, it is also recommended to incorporate both land- and water-based gait, balance, and foundational movement retraining, such as bilateral squats and step-ups in the pool [63,78]. This should include specific coaching on technique and movement practice, ideally using biofeedback to monitor limb loading strategies (such as asymmetries in ground reaction forces) and kinematics, with an emphasis on maintaining an external focus of attention [63,131].

3.2. Mid-Stage

The mid-stage of rehabilitation can begin only when all the objectives of the early stage have been achieved, including restoration of the joint ROM, swelling control, pain management, joint stability, and the athlete’s psychological state [62].

The primary goal of mid-stage rehabilitation is to restore knee extensor strength, targeting strength within 20% of the contralateral limb [62]. Surgery reduces the neural responsiveness, strength, and volume of the quadriceps, making it crucial to restore its function during rehabilitation [132]. However, most patients fail to achieve this target within 6 months [132], and over 50% of athletes do not reach strength parity with the contralateral limb even after returning to sports [85,132,133]. Inadequate achievement of this goal compromises knee biomechanics, reduces functionality, and increases the risk of re-injury and progression to osteoarthritis [83,86,134,135]. The predominant cause of this failure is AMI, which is triggered by injury and/or surgery at the neuromuscular level [86,132,136]. Various strategies to address AMI include pain and swelling management [137], BFR training [62,138,139], cryotherapy [89], NMES [95,140,141,142], and active exercises [79,136]. In the mid-stage, it is recommended to incorporate both open and closed kinetic chain exercises in addition to functional tasks [62]. Functional exercises improve coordination among muscle groups by simulating real neuromuscular tasks, reflecting how muscles function during athletic movements [143,144,145]. However, they do not prevent compensatory mechanisms wherein one muscle compensates for the weakness of another [62,144,145]. Conversely, open or closed kinetic chain exercises isolate individual muscles, enhancing their ability to generate force while minimizing these compensatory behaviors [146]. Given the controversy surrounding open kinetic chain exercises after ACL reconstruction [147], it is advisable to introduce them gradually with low loads and at safe flexion angles between 45° and 90° [102,148]. Generally, these exercises should also be applied to the uninjured limb to preserve its strength and also to improve the injured limb itself, thereby ensuring an accurate limb symmetry index (LSI) [149,150,151]. This can be accomplished by performing the same exercises for the injured limb but at reduced volumes for the healthy limb [62].

The second goal is to restore the strength of the knee flexors, particularly in patients where the hamstrings have been used as a graft for ACL reconstruction [117,118,119,120]. Strength deficits in the hamstrings are common and can persist for years post-surgery [112,113,114,115,116], and residual hamstring weakness is associated with an increased risk of re-injury [87]. This is because the hamstrings counteract anterior tibial translation and external rotation of the knee. The rehabilitation protocol for the hamstrings is similar to that for the knee extensor apparatus. However, if the hamstrings have been used as a graft, it is recommended to delay strengthening exercises for those muscles beyond 6 weeks to allow for tendon healing [102,122,123]. Following this period, isometric exercises should be introduced first, followed by a progression to open and closed kinetic chain exercises, as well as functional tasks [62]. By the end of the mid-stage, at least an 80% LSI should be achieved [62].

Simultaneously, when strengthening the knee flexor and extensor muscles, it is also important to reinforce the muscles that stabilize the joints proximal and distal joints of the knee [62]. These include ankle periarticular muscles, which generate a significant amount of force during running and jumping [152], and the triceps surae, which is a crucial stabilizer of anterior tibial translation, acting as an agonist to the ACL [153,154,155]. Similarly, the stabilizing muscles of the hip and pelvis, particularly the gluteus medius, can be weakened after surgery [126,127]. Weakness in these muscles promotes dynamic knee valgus during closed kinetic chain exercises, which is a significant risk factor for reinjury [125,127]. Therefore, it is strongly recommended to strengthen the core, pelvic, and hip muscles, as well as the posterior compartment of the lower leg [62].

The ACL is rich in mechanoreceptors and serves as a crucial proprioceptive organ of the knee, essential for neuromuscular and somatosensory control [156]. The rupture of the ACL disrupts these mechanisms, resulting in instability and reduced movement quality [156,157]. Movement quality depends on proprioception, proper neuromuscular coordination, and adequate and balanced strength. Poor movement quality following an ACL rupture is associated with an increased risk of both ipsilateral and contralateral reinjury [158,159]. Therefore, during the mid-stage of rehabilitation, it is important to restore proper movement quality through postural and joint control exercises, proprioception exercises, reactivation or inhibition programs to re-establish muscular synergy, and strengthening and flexibility exercises [62]. Understanding the exercise is as important as performing it because, following trauma, cognitive effort is required to recall the neuromotor pattern for executing a specific task that must be re-learned [145].

Since the late stage of the physiotherapy program focuses on the return to sport, it is also important that the transition from the mid to the late stage occurs only after achieving good athletic preparedness for the subsequent phase. Therefore, physical fitness reconditioning plays an important role even during the mid-stage. This primarily involves athletic reconditioning through strength exercises targeting the upper body and low-impact or no-impact exercises such as deep water running in a swimming pool and stationary biking [62].

3.3. Late Stage

The late stage is the final phase of the rehabilitation process and prepares the motivated patient to return to sports activities [61]. This phase focuses on optimizing muscle strength, neuromuscular control, and movement quality and then introduces sport-specific exercises [61]. Various criteria for return to the sport have been described [160]; however, only 20% of professional athletes return to their previous level of sport [161,162], and this percentage rises to 40% when considering non-professional athletes [163] with a risk of reinjury ranging between 15% and 30% [84,164,165]. The low return-to-sport rate highlighted by Grindem et al. [84] underscores the need for thorough late-stage rehabilitation. It implies that athletes should meet multiple criteria, including muscle strength, psychological readiness, and neuromuscular control, before resuming high-risk activities. This finding suggests the importance of a tailored approach to sport-specific training that can help bridge the gap between rehabilitation completion and readiness for competitive play. The main components of post-NC-ACL rehabilitation are summarized in

Table 2. Rehabilitation stages following NC-ACLs.

Rehabilitation Stage	Main Contents	Objective
Early stage	Pain medication, cryotherapy, compression, and elevation [64,66]	Solve pain and swelling
	Active and passive mobilization exercises prioritizing the recovery of full extension [63,83]	Recover complete ROM
	Ice, NMES, TENS, BFR, fatigue exercises for the hamstrings, and active exercise of the quadriceps [79,89,90,91,95,142]	AMI resolution and muscle reactivation
	OKC exercises should be safe with a reduced ROM (45–90°) and low load [63,110,148]
	If hamstrings were chosen as grafts, their functional recovery should be delayed until 6–8 weeks after surgery [63,110,124]
	The resumption of weight bearing depends on the procedures associated with ACLR, such as meniscal repair, extra-articular ligament reconstructions, and bone procedures [63]	Weight-bearing and walking gait recovery
	Re-education to normal gait [63]
Mid-stage	OKC, CKC, and functional exercises [63,80]	Restore quadriceps and knee flexor strength
	Strengthen the muscles that support and stabilize the joints both above and below the knee (triceps sura, hip, and core muscles) [62,125,127]	Stabilize ACL agonists, prevent dynamic valgus, improve movement quality
	Postural and joint control exercises, proprioception exercises, reactivation or inhibition programs to re-establish muscular synergy, and flexibility exercises [62]	Restore neuromuscular control and balance and movement quality
	Strength exercises targeting the upper body, deep water running in a swimming pool, stationary biking [62]	Fitness reconditioning
Late stage	Explosive development of force with isometric tasks, ballistic exercise (jumping), full OKC and CKC exercises [166,167,168]	Restore explosive neuromuscular performance
	Solve muscle strength imbalances/agonist weakness [86]	Optimizing the quality of sport-specific movement
	Optimize neuromuscular activation [61,62]
	2D visual movement assessments [169]
	Engage in sport-specific tasks by simulating realistic environments and stimuli [170]
	On-field rehabilitation [171]
	Cardiovascular training [61]	Neuromuscular conditioning whilst fatigued
	Prolonged exposure to effort [172]
	Meet the criteria for RTS (psychological readiness, muscle strength, graft healing at MRI, functional tests, etc.) [173,174,175,176]	Optimizing the RTS and reducing risk of reinjury

Abbreviations: ACL = anterior cruciate ligament; ACLR = anterior cruciate ligament reconstruction; AMI = arthrogenic muscle inhibition; BFR = blood flow restriction; CKC = closed kinetic chain; MRI = magnetic resonance imaging; NMES = neuromuscular electrical stimulation; OKC = open kinetic chain; RTS = return to sport; TENS = transcutaneous electrical nerve stimulation.

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4. Conclusions

Non-contact anterior cruciate ligament injuries (NC-ACLs) are prevalent in sports, causing significant physical and socioeconomic impacts. Effective prevention is the implementation of tailored programs focusing on proprioception, neuromuscular coordination, and biomechanical corrections. Rehabilitation following an ACL injury is critical for restoring function and preventing re-injury. A criterion-based approach addressing pain, range of motion, muscle strengthening, and neuromuscular re-education is essential for optimal recovery. Future research should explore innovative, personalized strategies to further reduce NC-ACL rates and improve recovery outcomes.