As this is still a time of biological healing of the injury, strength training during this part of rehabilititation is usually increased isometrically or dynamically using low loads to activate muscles and lay the groundwork for building muscular endurance (< 50% 1 RM) and strength (60–80% 1 RM) later.15 Subsequently, muscles, tendons, ligaments and cartilage should be progressively prepared to withstand the higher loads to which joints are subjected during sport-specific skills (SSS). This is known as the creation of structure (specific adapatations to imposed demands, SAID principle).16

During this phase, the creation of structural adaptations (muscles, tendons, ligaments and cartilage) through strength training becomes especially important. Practitioners should try to make strength training functional (eg. dynamic, fundamental movement) while considering the role of the neuromuscular system on active joint stabilization. As a player becomes stronger and their structure improves, they will also advance in adaptations aimed at sport movements.1 Classically, strength training with the aim to target hypertrophy is worked between 3, and 5 sets of 6–12 repetitions (per muscle group and session) and at an intensity between 65, and 85% of the one maximum repetition (1RM). As a guideline, and depending on the individual adaptations of each athlete in this phase, it is recommended to progress from lower to higher intensity and volume. Regarding the training volume, Schoenfeld and Grgic (2018) suggest that a good starting point to build hypertrophy would be to work about 10 sets per muscle group per week. This volume can be increased progressively and over time (eg. 20 weekly sets per muscle group).17 At the end of this stage athletes should be able to tolerate intensities close to 80–85% 1RM (6–8 maximum repetitions) during the fundamental strength movements of the lower limb.

At this level of progression, tasks 0+ are prioritized. The focus is on strengthening fundamental movement patterns (e.g. squat, lunge or Romanian deadlift). In the case of knee injuries, knee-dominant exercises (e.g. squat, lunge, Nordic hamstrings, etc.) and hip-dominant exercises (e.g. Romanian deadlift, glute bridge, etc.) are particularly relevant. The combination of these patterns facilitates progress toward sport-specific movements. Similar to the previous stage, it will be very important to continue focusing on the correct alignment of the hip-knee-ankle complex during triple flexion exercises of the lower limb, as well as proper trunk mechanics (Figure 1) (e.g. maintaining a slight curve in the lumbar spine during execution of a squat). Neither technique nor quality of movement should be sacrificed for the intensity of the exercise.

Figure 1.

Tasks created to optimize low limb biomechanics (triple flexion of the hip-knee-ankle complex). This neuromuscular activation strategy improves quadriceps-hamstring coactivation and reduce dynamic knee valgus. This fact reduces joint load to which the knee is subjected during high-risk ACL injury actions (changes of direction, decelerations, and jump receptions). In addition to triple flexion of the low limb, flexion of the trunk is also important while maintaining the physiological curvature of the spine.

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While limbs should be worked both bilaterally and unilaterally, unilateral tasks should be progressively prioritized because they are commonly performend in team sports. During bilateral work, it is important to emphasize limb symmetry despite the large asymmetries that are likely present. Unilateral work is key to reducing the neuromuscular asymmetry typical of the first stages of rehabilitation.18,19 During this time the injured leg will determine the load for both legs.20 In addition, unilateral work on the uninjured leg allows the athlete to take advantage of the intermuscular coordination mechanism known as the crossover effect.21 The crossover effect occurs when the benefits from work done by one (uninjured) limb “crossover” to benefit the contraletal “uninjured” limb.

Most actions in team sports are related to the application of force in unstable situations. In fact, strength training on unstable surfaces (eg. squat on a bosu) has been shown to facilitate, among other things, greater co-activation of the quadriceps-hamstring muscles and greater activation of the core compared to the same exercises performed on a stable surface.22 In addition, stability training allows for high neuromuscular activation with little joint load or stress, which makes it very useful during the early phases of rehabilitation. However, unstable strength training does not allow athletes to achieve the levels of power or strength that can be achieved with stable strength training. Although there is no consensus on this, some research recommends combining training heavy loads under stable conditions with the aim of gaining greater power and strength with training lower loads under unstable conditions to obtain greater neuromuscular adaptations that promote joint stability (e.g. coactivation between quadriceps and hamstrings).22,23

In addition to prioritizing strength training of functional movements or the aforementioned fundamental movement patterns, it is also important to analytically approach muscles that have a more stabilizing function and that are associated with risk factors for ACL injury. This type of work is often called compensatory work (level of approximation 0-) and ecompasses tasks that reduce the risk of injury during functional movements (Figures 2 and 3). In the case of readaptation following ACL injury, the following muscles are especially important to knee stability, strength and precise muscle activation (neuromuscular control):24 (1) Quadriceps, in eccentric activation, (2) hamstrings, especially the medial part, (3) external rotators and hip abductors, (4) trunk, and (5) vastus medialis (related to the typical patellofemoral syndrome after knee surgery). In addition to the aforementioned muscles, decreased dorsiflexion25 can also be considered an area for compensatory work for ACL injury prevention since it promotes dynamic knee valgus, one of the main mechanisms of ACL injury.

Figure 2.

Example of strength and neuromuscular control tasks according approaching levels to RTC post-ACL injury in a volleyball player. The figure presents three level of approaching levels (0 - / 0+ / I) to improve strength and neuromuscular control in three areas: Lineal displacement, lateral displacement and bilateral jump. Level I examples are strength coordinative exercises performed through inertial equipment (A. Forward lunge with inertial device simulating forearm pass, B. Lateral step with forearm pass, C. Yo-yo squat reproducing a volleyball block), level 0+ examples are fundamental strength exercises with gravitational resistance (D. Unilateral hip trust, E. Lateral lunge with vibration, F. Hexagonal bar squat) and level 0- includes compensatory exercises (G. Single leg death lift with bosu, H. Running stork, E. Fitball kneeling with ball control).

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Figure 3.

Example of the main part of a post-ACL RTC session organized in three chained exercises in a handball player. This session focus on coadjuvant training (emphasizing conditional and coordinative structure) and is organized by three training objectives (stations): 1 / Jump strength (A. Landmine squat, B. Core lateral clam, C. Jump simulating a defensive block); 2 / Change of direction strength (D. Crossover step with inertial device, pass and receive, E. Assisted Nordic hamstrings, and C. V-cut at maximal speed) and 3 / Fight strength (G. Unilateral pull press with partner perturbations, H. Unilateral row while bouncing, and C. Pivot position fighting and simulating a gain attack position.

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Neuromuscular control depends on the sensorimotor system. It is described as precise neuromuscular activation that ensures dynamic joint stability and, in turn, enables coordinated and effective action.26 One of the main objectives during this phase is to achieve good neuromuscular control during the main fundamental motor skills (FMS) that compromise the injured structure. The main FMS following ACL-R in most team sports are running, jumping, and change of directions, which all involve the stretch-shortening cycle. These actions are associated with increased risk of ACL injury. Striking a balance between coactivation and reciprocal inhibition of the quadriceps and hamstrings is key to both protecting the knee and facilitating sport performance.27 To ensure proper mechanics (alignment and triple flexion of the hip-knee-ankle complex) during FMS (e.g. landings, change of direction), accurate neuromuscular activation of the hip abductors, trunk muscles and medial hamstrings are really important.28

Next, the bases on which to progress the difficutly of neuromuscular control tasks are described. Start with core work and lower limb dynamic stabilization and progress towards the retraining of the FMS, such as running, jumping (especially landings), and change of direction.

The stability of the central part of the body (core), is essential to maintaining stability of the spine and proper lower limb mechanics (e.g. avoiding dynamic valgus in the jump landing) during sport actions.29

Our experiences and a review of the current literature suggest that dynamic core work should begin immediately following ACL-R with basic stability, mobility, and body awareness exercises. At this time it is very important to activate the deep muscles of the trunk (e.g. transverse abdominis and multifidus) and hip (e.g. gluteus medius) because low activation of these muscles combined with hyperactivity of the superficial muscles (e.g. rectus abdominis) is associated with injury, especially spinal injury.30,31

Once a good base of proprioceptive control of the core has been created, athtletes can progress towards more stable exercises to improve muscular endurance and capacity to react. It will then be possible to progress towards more specific exercises that load the core in the same what that training and competition do – during rotation, acceleration and deceleration actions.32

The current literature describes 3 categories of progression in dynamic stability training: a) static balance; b) dynamic balance and c) dynamic stabilization.33 Static balance is defined as the ability to maintain the center of mass on a fixed support base.34 This ability is highly influenced by the sensory information received, which is obtained through the somatosensory (especially proprioception), visual and vestibular systems.35 Once a good base of the proprioceptive-visual-vestibular system is created, athletes can progress towards improving dynamic balance. Dynamic balance is ability to maintain the center of mass on a fixed support base while experiencing disturbances, such as by balancing on an unstable base or by manipulating and/or controlling objects with other limbs.36 The disturbances effected in this second level of progression begin with controlled stimuli and progress towards unpredictable or unexpected situations.37 Finally, and as a guiding criterion to move on to RTS, the athlete should be able to tolerate dynamic stabilization tasks; that is, they should be able to maintain stability during/after dynamic actions, such as landing after a bilateral or unilateral jump in multiple directions with perturbations.38

When performing both the balance and dynamic stability tasks described above, it is recommended to focus on the correct alignment and triple flexion of the hip-knee-ankle complex (Figure 1). Previous research has shown how knee flexion during simple tasks, such as single leg balance (30°), or more demanding tasks, such as changes of direction, reduce valgus and increase coactivation between the quadriceps/hamstring muscles, both of which reduce the risk of ACL injury.6,39 In addition, it is also important to flex the trunk and maintain the physiological curvature of the trunk (lumbar lordosis), which will facilitate the activation of the hamstring muscles and further reduce the risk of ACL injury.

Training using vibration plates can also be useful in improving the dynamic stability of the injured complex.40 Coactivation of the quadriceps-hamstring muscles and the activation of the myotatic reflex (one of the main reflexes that works directly to facilitate balance and the stretch-shortening cycle) increase as a result of the vibratory stimulus. In addition, the vibratory stimulus produces a pain-inhibiting effect via the gate-control mechanism. This is very useful to improving neuromuscular control following damage to the cartilage, which occurs commonly following knee surgery.40 Several studies exist that demonstrate improvements to stability and neuromuscular control of the lower limb with vibration plate training following injury and during rehabilitation.41-43

The main objective in this phase is to support proper mechanics of linear (front and back), lateral, diagonal and rotational movements. Table 3 shows a progression of tasks that aim to retrain the different movements based on approaching levels.

First, it is important to achieve good symmetry in linear running. Some reserach suggests starting running around 12–18 weeks post-surgery, although this will depend greatly on the individual and especially if there are concomitant injuries.6,7,44 Training gait mechanics on soft surfaces (e.g. sand pit and/or graded treadmill) can be useful before starting running on solid ground.

Once an athlete has mastered a low speed straight run, they can progress towards a low speed lateral shuffle. The athlete can also progress to movements in multiple directions that include low impact changes of direction (e.g. 45°, 90° and 180°) at a controlled speed. It is important to train different angles of direction change to mimic sport movements and to train different types of maneuvers (e.g. side-step or cross-step). In Australian football, it was observed that ACL ruptures occurred during side-stepping maneuvers (37%), in landing (32%), land and step (16%), stopping/slowing (10%) and cross-over cut maneuvers (5%).45 Therefore, it is important to properly train these movements to mitigate ACL injury risk.

Useful tools to improve running and low impact multi-directional displacement include classic running technique exercises (e.g. skipping), coordinative exercises using ladders or something similar, and movements with resistance (e.g. incline treadmill or displacements with resistance bands).44

As the athlete tolerates the different movements, situations of deceleration (braking) and acceleration (starting) can be introduced at a controlled speed. Then, speed and complexity can be progressively increased.

In most team sports, one of the keys to success is the ability to repeat high intensity bouts. This ability depends on capacity of the neuromuscular, metabolic and cardiorespiratory systems to adapt to fatigue. Fatigure-resistance plays an important role not only in performance, but also in injury prevention because neuromuscular fatigue is considered one of the biggest risk factors for sports injuries.46 Recently, high intensity interval training (HIIT) has become one of the most popular and effective methods for improving the cardiorespiratory, metabolic and neuromuscular systems.47 As long as the injured area allows, HIIT can begin in the initial stages of ACL rehabilitation. The conditioning program can progress from low-impact (eg. swimming, cycling, elliptical, etc.) to high-impact, such as running or change of direction drills.

During this phase, it is especially important to exclude the athlete from tasks that involve opposition and collaboration (1 vs 0), but to still include them in group training sometimes. For example, in basketball, injured athletes can participate in individual technique work with and without the ball, in a varierty of movements, like dribbling, changes of directions, changes of rhythm, reversals, etc. The athlete can also start making layups with changes of pace and direction. An important rule in this phase is that the technique must be prioritized over intensity. Once the technique is mastered (eg. good neuromuscular control during triple flexion of ankle-knee-hip), the athlete can progress towards an increase in the intensity of the action.

In order to progress to higher specificity levels, the injured athlete should be free of knee edema and have full range of motion (some degrees of knee flexion deficit may remain), good neuromuscular control of the FMS, and inter-limb strength asymmetry close to 70%3,7,8,48 When creating tasks closer to the competition, it is important to consider that the actions that determine the specific movements of team sports are changes of pace (accelerations and decelerations), the speed of movements, changes of direction, stride length and frequency; and all this adapting to the ball/implement, the teammates and / or opponents.49

Table 2 shows some useful tools and criteria to monitor and control the athlete's progression during the RTT period.7,31,50–52

Until now strength training has focused on hypertrophy and general coordination. When the structures are already prepared, the work of the coordinative strength can begin to be emphasized, that is, that strength directed to the sporting gesture and that the athlete must apply at each moment of the game (Approach level I → II → III). And, consequently, progress in specific and competitive strength training, that is coordinative work (technique of the sport itself) under the increase of external resistance.58

When following the structured training framework1 the main areas (basic skills) to reatrain post- injury are displacements (linear, lateral and rotational) and jumps (multiple directions, unilateral and bilateral) (Figure 2). These should begin in a controlled environment and progress towards a chaotic environment, which is specific to team sports. Improvements in strength and neuromuscular control during these actions will be key to enabling the athlete to tolerate sport-specific work.

As previously mentioned, team sports are characterized by wide variability in muscle activation patterns, which is why strength training aimed at sport movements must follow the path outlined here. Consequently, Moras (2017) proposes the construction of a variable training environment (3-dimensional and 4-dimensional) to compensate for the lack of specificity in classical strength training11, especially in the approach levels furthest from the competition (0-II). For example, in a change of direction exercise with a conical pulley can be varied by changing the distance, the bending angle, the number of steps, adding the ball, adding an unstable platform, or adding opposition, collaboration, shot actions or decision-making.

Most ACL injuries occur during the deceleration phase of jumping and/or direction changes59, which is why eccentric overload training takes an special relevance. From our experience, we observed that control of dynamic knee valgus during deceleration depends on the athlete's ability to both generate eccentric force in the extensors and activate the stabilizing muscles (e.g. hip abductors). Recently, several isoinertial devices that allow for eccentric overload during sport-specific tasks have become available18,60 (Figures 2 and 3).

To organize tasks in RTS sessions, it may be useful to use Seirul·lo's triseries (fundamental, compensatory and application exercises).1 This format is represented from the design of a fundamental exercise followed by a complementary or compensatory exercise and, finally, an application exercise. Fundamental exercises are polyarticular exercises that involve global movement and affect the main musculoskeletal system. Depending on the specificity of the session, the fundamental exercises will mimic the actions of the sport. The complementary exercises are characterized by recruitment of secondary muscle groups within the sport movement. Compensatory exercises help to minimize the risk of injury, and are aimed at correcting imbalances. Finally, the application exercises are those that facilitate muscular actions similar or identical to the technical gesture, reproducing both movement patterns and execution speeds. Depending on the levels of approximation of the exercises, actions that require decision-making can be included.61Figure 3 shows an example of the main part of a post-ACL RTS session that aims to improve strength and neuromuscular control (fight, change of direction and bilateral jump areas) organized in three chained exercises in a handball player.

Finally, it should be noted that in this phase training aimed at reducing neuromuscular asymmetries between legs during FMS and SSS is particularly important.62 Additionally, training should address asymmetries in both physical abilities (e.g. foot dorsiflexion, balance and/or strength) and motor skills (e.g. jumping and/or changes of direction in multiple directions). Research has shown that unilateral training is key to reducing inter-limb asymmetries resulting from injury.19,63 Once again, practitioners should monitor not only the magnitude of the asymmetry (with the Asymmetry Index), but also the direction of the asymmetry (eg. left vs right leg).62 Asymmetry becomes more important to address as the athlete approaches the end of the RTC process.

When retraining jumping and other movements in team sport athletes, it is important to keep in mind that using a knee-predominant strategy in the frontal plane has been associated with an increased risk of ACL injuries.64 In addition, the load on the ligament during changes of direction or landings increases with the decision-making, unexpected actions and fatigue65 typical of team sports.

In team sports, the intensity of actions depends not only on conditional and coordinative abilities (e.g. strength and power), but on perceptual and cognitive abilities. For this reason, good motor control strategies during FMS that involve the knee (displacements and jumps in multiple directions) in a closed and planned way must first be acquired. Then, open tasks, including decision-making, unexpected situations and fatigue conditions in game-specific situations, can be introduced progressively.66 In other words, the athlete must be prepared for the reality of the game, which includes adapting to the opposition, collaboration, fatigue and the pressure of competition. In this case, it can be very useful to have the so-called conditional-coordinative circuits. This kind of circuit training is widely used in the physical preparation of team sport athletes, and the nature and organization of the load resemble those of competition (Table 3). This type of work includes specific coordinative elements and nonspecific decision-making. From the conditional perspective, Circuit training allows for variations in work and rest times, which can place the emphasis on resistance to high intensity (longer duration with shorter rest) or on the speed of the action (shorter duration with longer rest). In addition, circuit training also allows sport-specific work on one or more specific motor skills through modification of the space and duration of the circuits. For example, to enhance high speed running in soccer players with circuits, we should introduce high intensity 20–30 m runs in a linear or near-linear path. Conversely, if what we want to enhance eccentric components of the actions, or increase the number of maximum accelerations and decelerations, we should introduce short and intense actions (e.g. stopping, changes of direction).

In order to prepare the injured athlete for unexpected game events, it is essential to train anticipation or pre-activation mechanisms (feedforward).26 Feedforward mechanisms have been associated with a greater ability to stabilize lower limb joints during risky situations, such as landings or changes of direction.67,68 In order to develop these protective mechanisms, it is important to introduce a wide variety of tasks and to gradually introduce unexpected actions.33

Finally, it is important to strike a balance between neuromuscular control (eg. correct technique and/or biomechanics) of the susceptible structures and the coordinative, perceptual and cognitive demands of the proposed task.

In line with the previous section, in team sports one of the keys to success is the ability to repeat high-intensity bouts of effort.69 This ability is not only important for performance, but also plays an role in reducing the negative effects of fatigue (one of the major risk factors associated with ACL injury).66 In recent times, high-intensity interval training (HIIT) has become a popular method to improve respiratory, metabolic and neuromuscular capacity in team sport athletes.47 There are many potential applications for HIIT, including progressing from more basic skills in controlled situations to more specific skills in more unstable environments. For example, a possible progression of tasks from less to more demanding could be the following: 1) Classic HIIT with short intervals (eg. 15 s at high intensity - 30 s of active pause × 7 min × 2 sets), 2) repetitive sprint training with various changes of direction (eg 10 sets of 30 s - recovery 30 s) with minor decision-making, and 3) small games (eg. 5 sets of 3 min of half-court basketball).70 The possibilities for progressions are endless. It is important to consider integrity of the structure being retrained when progressing from a controlled environment to a chaotic, game-like environment.71

Once the athlete is able to tolerate high-intensity SSS in a controlled environment, it is time to prepare the athlete for the reality of the sport: Fatigue + hyper-complex environment (unexpected situations where the athlete must make decisions based on their opponents, teammates, the ball/implement and decision-making with combined conditions). At this time, the athlete can begin to progressively train with the rest of the group. The athletes’ load should be closely controlled. When progressing tasks, it is important to consider the following aspects: 1) fatigue (e.g. training time), 2) number of high-risk motor actions (e.g. jumping and/or high intensity changes of direction), 3) decision-making (from simple to complex), 4) environmental disturbances (e.g. tasks with contact, and/or opponent, and/or the ball/implement), 5) collaboration (teammates), 6) and the degree of opposition. An example of task progression would be to start with situations without an opponent (1 vs 0, 2 vs 0…), progress towards situations of attach/defense (2 vs 1, 3 vs 2…), then reduced games (1 vs 1, 3 vs 3,… 5 vs 5), and end with real game (5 vs 5, 7 vs 7, 11 vs 11, etc. depending on the team sport).

Although there is currently no evidence on which factors succesfully predict RTC following ACL-R, it is recommended that practicioners use an extensive battery of tests in order to reach a consensus on RTC.3Table 2 shows some useful criteria and tools that can help determine how to progress from rehabilitation to competition.8,72,81,73–80