BENEFITS AND USE OF AQUATIC THERAPY DURING REHABILITATION AFTER ACL RECONSTRUCTION -A CLINICAL COMMENTARY

Abstract

Outcomes after long-term injuries such as anterior cruciate ligament reconstruction (ACLR) need improving. One area which has received limited research attention is the use of aquatic therapy to optimize the functional recovery process after injury. There is still limited understanding of what the benefits of the pool can bring for rehabilitation and particularly what and when can be done in the pool after injury. This clinical commentary describes how the application of the properties of water can support the functional recovery process after ACLR. Here it is proposed that the main properties (density, hydrostatic pressure, buoyancy and viscosity) of aquatic therapy, if applied correctly to rehabilitation practices, can be used to achieve six primary goals after ACLR : 1) assist in the reduction of pain and swelling; 2) support the recovery of gait; 3) support the maintenance and/ or development of cardiovascular fitness; 4) help accelerate and optimize motor pattern retraining; 5) allow for earlier introduction of plyometrics and power training and 6) support the between session recovery and optimal load management, particularly in the later phases of rehabilitation. If implemented correctly, the presented phased protocol can support practitioners in implementing or delivering aquatic therapy rehabilitation services to their injured athletes. To support implementation, the authors have provided a specific protocol and supplementary videos for the use of aquatic therapy after ACLR.

Level of Evidence

5

BACKGROUND AND PURPOSE

Despite being perhaps the most discussed rehabilitation topic, there is no consensus on the best way to rehabilitate a patient after anterior cruciate ligament reconstruction (ACLR). It is well established that outcomes after ACLR are not perfect, with 35-45% of patients not returning to sport.^1,2 Of those who do return to sport (RTS), 15% can expect a secondary ACL injury, with 30% of young athletes experiencing a re-injury within the first two years after RTS.^3-5 Furthermore, there are associated long-term complications such as accelerated onset of knee osteoarthristis.⁶

Recently, it has been suggested there is a need to optimize the rehabilitation process after ACLR in order to optimize patient outcomes.⁷ One area which has received limited research attention and may be potentially beneficial in optimizing the ACL rehabilitation process is aquatic therapy. Aquatic therapy does not refer to only different styles of swimming in rehabilitation (e.g., crawl, breaststroke etc.), rather, it includes all utilization of a water-based environment for therapeutic purposes during the rehabilitation process. The aquatic environment can be extremely beneficial for the rehabilitation of athletes after sports injury, if the properties of water are understood and maximized. The biological effects of immersion in water are related to the fundamental principles of hydrodynamics. Understanding these effects and the physical properties of water, such as density and specific gravity, hydrostatic pressure, viscosity and buoyancy, may help the sports medicine team maximize these properties to contribute to an optimized functional recovery program after ACLR. The main advantage of aquatic therapy in the functional recovery process is that activities or exercises can be introduced earlier in the functional recovery process than on land, potentially accelerating the recovery process and reducing the overall recovery time. This includes specific targeted exercises for development or maintenance of the neuromuscular and cardiovascular (CV) systems, at a low risk of injury.

Despite good theoretical relevance, there is a lack actual research assessing whether aquatic therapy could support enhanced rehabilitation outcomes after ACLR. Research has assessed the short and long term benefits of exercise in water in both young and old individuals,^8,9 assessing the benefits for CV training^10,11 and muscle strengthening.^12-14 In addition, a systematic review comparing the effects of aquatic therapy and land-based exercise for people with arthritis found that aquatic therapy was comparable to land-based exercise in terms of functional outcomes.¹⁵ The authors recommended that aquatic physical therapy be used as an alternative to land-based physical therapy when people are unable to exercise comfortably on land.¹⁵ Given the range of limitations seen in athletes after ACLR, there are many benefits of aquatic therapy, which could support improved patient outcomes. Aquatic therapy is used regularly during the rehabilitation process after ACLR, however, there is a lack of published research demonstrating its clinical applicability. As such, this clinical commentary will attempt to bridge the gap between theory and research (based mostly on other injuries besides ACL as well as in un-injured participants) and rehabilitative practice. This commentary will specifically discuss the application and benefits of aquatic therapy to the functional recovery process following ACLR. In addition, the authors will share how aquatic therapy can be applied to the functional recovery of injured athletes, particularly after ACLR, providing a phased aquatic therapy protocol for use during the functional recovery process after ACLR through to RTS.

SIX-APPLICATIONS OF AQUATIC THERAPY PRINCIPLES APPLIED TO THE FUNCTIONAL RECOVERY PROCESS FOLLOWING ACLR

Utilization of the unique properties of water can facilitate optimal planning of rehabilitation sessions to support and optimize a functional recovery program. The six major applied benefits of aquatic therapy include:

1. Reduction of pain and swelling and restoration of joint range of motion

2. Recovery of normal gait cycle

3. Maintenance and/ or development of cardiovascular (CV) fitness

4. Movement pattern/ coordination training

5. Early introduction of plyometric training

6. Between-session recovery and optimal load management

Reduction of pain and swelling and restoration of joint range of motion

Early joint motion is beneficial when it comes to avoiding capsular contractions, reducing swelling and pain, and gaining early full passive and active extension appears to have no adverse effect on joint laxity.¹⁶ Unsatisfactory recovery of joint range of motion appears to adversely affect subjective and objective outcome markers in late-stage rehabilitation.¹⁷ Use of aquatic therapy can support the improvement in both joint swelling and passive and active range of motion. When an individual is submerged in water they are subjected to hydrostatic pressure. This pressure is directly proportional to the density of the liquid (e.g., gravity and depth to which the body is immersed).¹⁸ As such, this means it does not depend on the individuals body shape or the size or shape of the container (e.g., swimming or aquatic therapy pool), or the volume of water, but primarily on the depth of immersion. Water exerts a pressure of 22.4 mm Hg/ft of water depth, which translates to 1 mm Hg/1.36 cm (0.54 in.) of water depth.¹⁰ If an individual is immersed to a depth of 120 cm they are subjected to a force equal to nearly 90 mm Hg, which is slightly higher than the normal diastolic blood pressure.¹⁸

If the water pressure is higher than diastolic blood pressure this will result in fluid shifts forcing fluid out of the joint and supporting optimized venous return and lymphatic system drainage (return). This could thus help with resolution of inflammation/ swelling, facilitate reduced swelling and positively influence (increase) joint range of motion. Normal or optimal gait biomechanics cannot occur without normal or optimal joint motion,¹⁹ so resolving impairments as early as possible after surgery is important to target early gait restoration.

Immersion in water also desensitizes the injured area, as pain perception is diminished, due to an elevated pain threshold, which may be due to the stimulation of sensory nerve endings in the skin and sensory overflow.^20,21 As such, this may facilitate (alongside the reduction in joint swelling) greater possible joint range of motion than possible at the same time-point on land, again facilitating the recovery of both passive and active range of motion.

Recovery of normal gait cycle

Following ACLR, a patient cannot typically fully weight bear or walk without crutches for a period of time, often around four weeks.²² Patients often can develop faulty gait patterns after injury, due to movement compensations to protect the injured joint. Abnormal gait patterns have been associated with joint weakness,²³ decreased functional performance,²⁴ low patient satisfaction with outcome after surgery²⁵ and post-operative complications, including osteoarthritis.²⁶ Re-establishing normal gait early after surgery is a key priority of the functional recovery process.

The properties of water, specifically density and buoyancy, can support maintenance and early normalization of optimal walking gait in ACLR patients, due to reducing the effects of gravity and allowing the practice of walking at lower body weights and joint loads. Density is the ratio between the mass of a substance (kg) and the space in which it occupies (m³). The density of water at 1 atmospheric pressure and 4 degrees C is 1000 kg/m³. The mean density of the human body is less than water at 950 kg/m³, although this varies person to person, with men averaging higher than women.²⁷ Lean body mass, which includes bone, muscle, connective tissue, and organs, has a typical density near 1.1, whereas fat mass, which includes both essential body fat plus fat in excess of essential needs, has a density of about 0.9.²⁸ The relationship between the density of water and human body (with the latter being less) means an individual can float in water (although more muscular athletes may have a tendency to sink). Buoyancy is defined as the upward thrust acting in the opposite direction to the force of gravity. A human with specific gravity of 0.97 reaches floating equilibrium when 97% of his or her total body volume is submerged. As the body is gradually immersed, water is displaced, creating the force of buoyancy, progressively off-loading immersed joints. The greater amount of the body immersed in water, the greater the off-load of body weight. A person immersed to the symphysis pubis has effectively offloaded 40% of his or her body weight; 50% when immersed at the umbilicus and 60% at the Xiphoid depending on if the arms are in or out of the water. Immersion to neck height, would leave only around a 7kg weight (off-load of around 90% body-weight) of compressive force (e.g., the weight of the head), which would be exerted on the body.

Thus, during the early stages of the functional recovery period after ACLR, walking gait should be first trained in the pool at various water depths (progressing from deeper to shallower), in order to enable the removal of a proportion of body weight to facilitate optimal gait patterns, as needed. Furthermore, walking in water can present a challenge to dynamic stability and support the retraining of dynamic movement control in a safe environment. The walking gait re-education program should include selective movement retraining exercises to support the motor re-training process (e.g. standing marches in place, with optimal lumbar pelvic control and hip, knee and ankle flexion).

Maintenance and/or development of cardiovascular fitness

Preserving CV fitness parameters such as maximal aerobic capacity, lactate thresholds and running economy during the rehabilitation process is important for endurance and game sport athletes (e.g. football players) following long-term injuries such as ACLR. Recent research indicates that professional football players report deficits in aerobic fitness still six-months following ACLR,²⁹ indicating a greater need to incorporate CV conditioning during the functional recovery process. It has also been suggested that exercise based CV training can be a useful tool that can allow injured athletes to maintain CV fitness and ultimately running performance.³⁰ For example, over a 22- month period, Burns and Lauder³⁰ examined the effects of deep-water running on 181 active-duty army soldiers with injuries that prevented them from undertaking their regular weight-bearing exercise and suggested that VO₂ max and running performance can be maintained while soldiers are restricted from weight-bearing aerobic training.

Water creates a resistance greater than that experienced during locomotion on land.³¹ This greater resistance is not only due to the water density but also its dynamic viscosity. Viscosity refers to the magnitude of internal friction specific to a fluid during motion. A limb moving relative to water is subjected to the resistive effects of the fluid called drag force and turbulence when present. Viscosity, as well as buoyancy and density of water mean that exercises such as such as deep water running or treadmill running (if carried at sufficient speeds/ intensities and the right depth of water) can allow for the performance of CV exercise, which can result in similar CV responses to running on land.³² There are numerous studies which have analyzed the effects of water-based training, which suggest that improvements in maximal aerobic capacity (VO₂ max) from 12-40% in sedentary or lower physically fit individuals;³³ while it can support the maintenance of CV fitness parameters (running performance, VO₂ max, lactate threshold) in trained athletes.^8,34,35

It is essential to ensure similar intensities that are used during land-based training to ensure an optimal workout and ensure optimal technique during exercises such as deep water running.³⁶ A period of time to focus on technique during deep water running is recommended as part of the rehabilitation process, prior to focusing on CV responses/ intensity. Optimal technique during deep water running would also support the maintenance/ retraining of running gait. Monitoring the athlete with appropriate heart rate monitoring systems can facilitate the analysis of the CV responses (and as such desired exercise intensity). The value of this type of work would support the maintenance of CV fitness in a non-or low-load bearing manner. Cardiac output has been reported to increase by 30–35% when an individual is at rest, immersed in water. This increase has been attributed to an elevated stroke volume,^37,38 which in turn is due to an enhanced diastolic filling.³⁷ During exercise in water, an increase stroke volume has been reported to be higher at any given submaximal or maximal exercise intensity in water when compared with values obtained on land.³⁸ As such, there is typically a reduction in heart rate at equivalent exercise intensities in water versus on land, of around 10-15 beats per minute which should be accounted for when monitoring and reporting exercise intensity and volume in water based training. Thus, it is advised to implement deep water running, while simultaneously monitoring heart rate of the athlete, adjusting the HR parameters (e.g., if the training goal on land is 80% maximum heart rate at 160 beats per minute, then the equivalent intensities in the water would be 150 beats per minute for the same relative intensity) to achieve equivalent intensities in the water as on land to preserve CV fitness during the functional recovery period.

Movement patterns/ coordination training

Targeted movement re-training is important after injury to correct at risk biomechanics which may have either been prospectively linked to the initial injury, have occurred secondary due to changes associated with the injury or a combination of both.³⁹ Altered movement biomechanics and postural control deficits are associated with heightened risk of re-injury upon RTS after ACLR.^40,41 Conventional rehabilitation practices appear to be ineffective at restoring movement quality prior to RTS.^39,42,43 Individualized movement re-training programs targeted at factors which contribute to movement dysfunction such as neuromuscular and biomechanical factors (arthrokinetic dysfunction, synergetic dominance, muscle imbalances, reciprocal muscle inhibition), as well as neurocognitive and sensorimotor factors are required.^39,44,45 Failure to correct movement impairments could be linked to insufficient quality, intensity or volume of movement based training. In particular relevance to this commentary, it appears that neuromuscular retraining to develop strength (e.g. use of isolated exercises such as knee extension) is insufficient at optimizing movement quality, and there is a need to allow for movement practice following or during neuromuscular training in order to relearn appropriate movement patterning and optimize coordination.⁴⁶ It is necessary to undergo a comprehensive movement retraining and coordination program to retrain optimal motor control and ‘integrate’ newly achieved muscle strength into the motor patterns.⁴⁶ Thus, stressing the need for a balance between strength and movement training to optimize movement quality. This movement retraining process after ACLR, without the use of aquatic therapy is typically done in systematic manner in which the patient has a period of strength training to resolve muscle imbalances, followed by coordination training to relearn optimal movements.⁴⁷ This is because during the early and middle periods of the functional recovery process after ACLR the patient is typically load compromised and cannot tolerate the loading parameters which accompany many functional sporting type tasks. The fact that water acts as a counterbalance to gravity, means that functional strengthening/ movement exercises (e.g., squat, step up, lunge) can be introduced earlier than possible on land. This can facilitate earlier introduction of motor training allowing for the simultaneous training of muscle strength (largely in the gym) and motor control (in the pool) during the early and middle stages of rehabilitation. The same holds true for the earlier introduction of running (using buoyancy devices and at various pool depths), jumping, landing and plyometric techniques, which can all be introduced at lower loading parameters than possible on land, thus accelerating the motor relearning process. This would be expected to maximize the progress through mid-stage rehabilitation, allowing for more appropriate restoration of functional performance, and provide a superior movement quality foundation on which to commence late-stage rehabilitation. Exercises such as jumping and single leg balance training in water have been shown to result in similar functional improvements, to the same exercises performed on land.^48,49 The benefit of been able to perform them in water at lower body weight/ impact forces means they can be introduced safely and earlier in the rehabilitation program.

Early introduction of plyometric training

The ability of the neuromuscular system to develop force is important to provide dynamic stability to a joint, as well as for optimal force propulsion. Typically, there is an over-reliance on isolated maximal muscle strength after ACLR, with less consideration of the ability to develop force rapidly.⁵⁰ There is typically limited time to develop force during explosive sporting movements such as sprint running (80-120 ms),⁵¹ or rapid joint stabilization to prevent joint injury after mechanical perturbation ( < 50 ms).⁵² It takes around 300 ms for the neuromuscular system to generate peak force.⁵³ Hence, the capability to produce force during rapid sporting tasks (i.e. explosive efforts) may be more dependent upon the ability to increase force quickly from low levels, termed rate of force development (RFD), than on maximal muscle strength. As such, RFD appears to be an important aspect of neuromuscular function and may require additional consideration in late-stage rehabilitation programs.⁵⁰ From the available evidence, it appears that RFD is not effectively restored after injury prior to RTS.^54,55 Angelozzi et al. identified deficits of RFD of 30% in ACLR patients at six months, despite almost full (97%) restoration of maximal concentric knee extensor strength.⁵⁴ RFD was restored at 12 months following the incorporation of a program based on power development.⁵⁴

As well as RFD, the ability to generate maximal power during complex motor skills is also of major importance to successful athletic performance across many sports.⁵⁶ Mechanical power is typically referred to as the rate of doing work⁵⁷ and is determined by multiplying force by velocity.⁵⁸ Based upon this equation, it is evident that the two central components that impact the athlete's ability to generate high power outputs are the ability to apply high levels of force rapidly and express high contraction velocities.⁵⁹ As such, maximal strength, RFD and high velocity strength are important elements of neuromuscular performance which need to be restored following injury. Power is influenced by numerous factors including slow velocity muscle strength, fast ­velocity muscle strength, RFD, ability to maximize the stretch-shortening cycle, and intra and intermuscular coordination.⁶⁰ It is important that the rehabilitation process utilize a mixed methods approach to developing power, which should include the use of strength training, but also plyometric training, in order to target all the factors which may contribute to explosive power capabilities.⁶¹

Lower extremity plyometric exercises are commonly used by athletes to develop explosive speed, strength, and power. They involve stretch-shortening cycle activity, where eccentric muscle contraction is quickly followed by concentric contraction of the same muscle (or muscles). During the eccentric phase (pre-stretch), the musculotendinous unit is stretched, which stores elastic energy, and the muscle spindles activate the stretch reflex. Plyometric training has been reported to be superior to more traditional resistance training for development of explosive lower limb performance^62-64 and can contribute to improvements in lower limb strength and power, increased joint awareness, and overall proprioception.^62,63,65-67 Performance of high-intensity plyometric exercise often produces muscle damage, due mainly to the eccentric component of the muscle action, and excessive joint loading (ligament, joint structures, tendon), which could result in injury.⁶⁸ Typical impact forces during plyometric exercise on land is between two to six times body mass depending upon the specific plyometric task.^69-71 Deficits in functional eccentric muscle strength of the lower limbs would mean insufficient neuromuscular capacity to eccentrically absorb these forces, with greater reliance on joint complexes (tendon, ligament and joint structures) for force absorption.⁷² This could result in increased injury risk or an overload response to the joint. Plyometric training often forms an important aspect of late-stage rehabilitation/ RTS training, once the necessary functional capacity to tolerate these high forces has been restored. However, in water, the buoyancy force controls the downward (landing) movement of the body, thus generating higher upward (concentric) and lower downward (eccentric) forces. There appears to be a reduction of around 45-60% in peak ground reaction forces recorded from plyometric exercise in water versus on land.^71,73 Furthermore, plyometric training in water versus land appears to result in reduced joint inflammation and perceived pain in the subsequent hours and days after training.^72,74,75 Additionally, similar characteristics in terms of power and explosive force production during the concentric phase have been reported during plyometrics in water versus land,^70,71 thus, mimicking the neuromuscular stimulus for adaptation in power at a lower level of impact forces and muscle soreness/ joint overload. Plyometric exercise in water can offer an alternative to land based plyometrics which can be both introduced earlier in the program, with lower risk of joint injury and/ or overload and use as a supplement to land based exercise to limit the impact forces and overall training load while maximizing neuromuscular training benefits (e.g., introduction to sport-specific on-field training and elevated body loading demands).

There appears to be a large individual variation in terms of the load reduction in water versus land,⁷¹ which can partly be attributed to water depth, participant height, body composition, and landing techniques. Koury⁷⁶ and Miller et al.⁷⁷ recommended performing aquatic plyometrics in waist height water. They suggest that deeper water may impair movement control and coordination, making it more difficult to maintain stability in an upright position, whilst also decreasing the stretch-shortening cycle reaction time, and increasing drag due to arm swing through the water. Clearly, there is a need to use appropriate coaching techniques, observation and where possible filming (with underwater cameras) and providing appropriate feedback (real-time or delayed) on technique. Furthermore, having optimal complexity and load progression as part of a structured program (e.g., squat jump – countermovement jump – drop jump) is essential to ensure optimal learning and adaptation, as well as safe loading progressions.

The typical ground reaction forces during land-based running range from 2-3 times body mass⁷⁸ meaning plyometrics in water may be used prior to (e.g. use of bilateral plyometrics) and during (e.g. bilateral and unilateral plyometrics) the implementation of return to running program, and as preparation for plyometric training on land.

Between session recovery and optimized load management

Deep water running has been recommended for players to accelerate the recovery process between matches.⁷⁹ It also has relevance for recovery during the rehabilitation process, where patient athletes are reconditioning their bodies for athletic activity. Reilly et al.⁸⁰ investigated the benefits of deep water running in accelerating the recovery process after a strenuous bout of plyometric exercise. Deep water running failed to prevent delayed-onset muscle soreness, but appeared to speed the process of recovery for leg strength and perceived muscle soreness. Leg strength was reduced by 20% on average for other groups (rest and/ or treadmill runs) after 48 hours, but only by 7% for the deep water running group, while soreness was also reduced by 40% in the deep water running group.

Optimal loading may be defined as the load applied to structures that maximizes physiological adaptation.⁸¹ Rehabilitative exercise stimulates a series of homeostatic responses and accompanying adaptations of the human body system. It is essential that the rehabilitation program incorporates progressive optimal loading to facilitate functional recovery without overloading the muscle, ligament, tendon or joint and as such potentially compromising the regeneration process. Pool based exercise such as CV training, movement practice including ballistic and plyometric training can allow for a higher volume of training at lower relative loads to if performed on land. Therefore, the pool can be used for supplementary exercise on recovery days (e.g., a non-strength or on-field training day), even during the late-stage and RTS training stage of the functional recovery process after injury. This concept applies especially to the elite athlete or professional player after long-term injury such as after ACLR.

Implementation into practice: Practical use of aquatic therapy during the functional recovery process after ACLR in professional athletes

The properties of water (buoyancy, density, hydrostatic pressure and viscosity) can be utilized to implement an aquatic rehabilitation program which if planned correctly may help optimize patient outcomes after ACLR. The six discussed benefits of aquatic therapy 1) Reduction of pain and swelling and restoration of joint range of motion; 2) recovery of normal gait cycle; 3) maintenance and/ or development of CV fitness; 4) movement patterns/ coordination training; 5) early introduction of plyometric training and 6) between-session recovery and optimized load management should be utilized effectively to complement an existing functional recovery process.

The functional recovery process

To be able to effectively design the right aquatic therapy program after ACLR, it is important to have a well-structured functional recovery process in place. There is no gold standard ACL rehabilitation approach, but having criterion-based rehabilitation with specific criteria to progress through stages or phases is regarded as best practice.⁸² The functional recovery process described here can be broadly defined as a series of phases or stages,⁴⁷ including:

1. Early stage rehabilitation which is focused on resolving pain and swelling, recovering sufficient knee joint range of motion, recovery of activities of daily living including the ability to walk without crutches, and minimization of muscle atrophy.

2. Mid-stage rehabilitation which is focused on restoring strength imbalances (to within 20% of the contralateral limb), basic motor patterning (e.g., functional exercises such as squat and running gait) and physical re-conditioning.

3. Late-stage rehabilitation which is focused on optimising neuromuscular and movement performance.

4. Sport-specific re-training and RTS (consisting of on-field rehabilitation, return to training and return to competition).⁸³

It is important that the aquatic therapy program be aligned to this functional recovery approach and the specific activity fit with the functional recovery stage to optimize its implementation and improved clarity amongst all members of the sports medicine/ rehabilitation team. This ideal program should always be considered as part of a functional recovery approach comprising gym-based, pool-based and on-field rehabilitation.²² The aquatic therapy program is an important element of the process, and should not be considered in isolation. It is essential to ensure that the correct work is undertaken at the correct time and aligned with the activity of the gym based (or field) rehabilitation program. As such, considering the specific activity outside the pool and the functional limitations are needed to prioritize the activity within the pool (see Table 1). Of note although this program can be utilized for all ACLR patients, it is important to recognize that aquatic therapy stages 2-4 are an adjunct to land-based rehabilitative training and a minimum volume of land-based work is needed prior to considering the addition of aquatic therapy. For example, if a patient can only undertake two to three sessions per week then this time should be better prioritized on land-based gym and/or on-field activity, at least within stages 2-4. For these individual's, aquatic therapy may be particularly beneficial in the early stage of recovery (e.g., stage 1– ‘post-op pool’). Those involved in professional athlete schedules, can have good benefit from the incorporation of aquatic therapy throughout their functional recovery programs (Figure 1).

Table 1.

A staged aquatic therapy rehabilitation approach for the athlete after ACL reconstruction. The program involves four stages aligned with the functional recovery status of the athlete after ACL reconstruction. The particular goal, strategy and approach are outlined. The program outlines a typical approach (and allocated time) for a professional athlete, who was able to return to team training at six months after ACL reconstruction. Time lines are dependent upon the injury (e.g., concomitant injury, such as cartilage, medial collateral ligament), and individual healing and progressions. Criteria and not time should be used to transition between phases ensuring entry and exit criteria are achieved. Importantly, aligning the activity in the pool to the activity on land is important.

	Post-op pool	Movement and CV conditioning	High intensity pool and field preparation	Recovery pool
Typical duration	-Weeks 2-4 after surgery	-Weeks 5-12 after surgery	-Weeks 13-18 after surgery	-Weeks 19 + after surgery
Entry criteria	-Medical clearance to commence hydrotherapy - Surgical wounds: no signs of inflammation (soreness, redness, increased temperature swelling) - Surgical wounds: stiches removed by medical staff.	Ready to transition to mid-stage rehabilitation: - Minimal pain (0-1 NPRS) - Minimal swelling (zero or trace effusion) - Knee extension to 0 º - Knee flexion > 120 º - Sufficiently normalized gait outside the water without aids (e.g., no crutches) - No evidence of quadriceps dysfunction (e.g., quadriceps lag test)	- No pain/swelling - Symmetrical/full ROM - Knee flexor and extensor LSI > 80% - Good subjective movement quality on land-based foundation tasks (e.g., bilateral and unilateral squat, step up, lunge and hip hinge) - Ability to run on the treadmill at 8 km/h for 10 minutes with good mechanics	-Be undertaking return to sport training
General goals for the phase	-Support the resolution of swelling - Aid the recovery of ROM - Support the recovery of correct gait cycle - Enhance early recovery of flexibility	-Restoration of basic motor patterning - Introduction and utilization of DWR for motor patterning and subsequent use for CV conditioning - Late recovery of flexibility - Continued fluidity exercises to support active range of motion - Progressive introduction of BL and then UL landing and jumping work to develop eccentric functional control (10 weeks + ) - Sport-specific neuroplasticity exercises (heading, catching, throwing)	-High intensity plyometric exercises - Sports technical gestures - Physical conditioning	-Accelerate recovery between on-field and gym land based sessions - Allow for reduced loading training such as plyometric exercise and CV conditioning at lower body loads
Pool exercises	-Walking, cycling, stretching, basic motor patterning (e.g., standing marching exercises) and gait re-training (see supplementary video 1)	-First half (weeks 5-8)– DWR, squat, step up, standing balance and neuromuscular control exercises, functional strength exercises using additional modalities (e.g., standing press against rubber ring) - Second half (weeks 9-12): Introduction of landing and jumping exercises for neuromuscular control DWR for CV conditioning (see supplementary video 1)	-Running for low-load conditioning - Plyometric training - Coordination drills for preparation for the field (see supplementary video 1)	-Deep water running - Plyometric exercises - Flexibility – dynamic and static stretching
Activity outside the pool	Rehabilitation gym - Bed based isometric strengthening - ROM exercises, - Treatment modalities (TENS, electrical stimulation, ICE, massage)	Rehabilitation gym First half (weeks 5-8) - Resistance training exercises using low-loads for muscle endurance training (12-20 RM range) - Off-feet core corrective training (e.g., transverse abdominus, bridges, clams, side leg raises etc.) - Basic motor control drills (BL squat, marching exercises, gait, walking on treadmill) - Off-feet CV conditioning (bike, cross-trainer) Second half (weeks 9-12) - Progression to land based functional exercises (single leg squat, hip hinge, step up, split squat) - Standing lumbopelvic strengthening exercises (standing clam, lateral band walks) - Moderate load resistance training for hypertrophy and strength development in open and closed kinetic chain exercises (8-12 RM range) - Running re-education with alter-G/ trampoline/ treadmill	Rehabilitation gym / Movement environment - LB high load isolated strength training (3-6 RM) - LB functional strength training (8-12 RM) - Landing drills, jumping drills - Core strength/ stability (load transfer) - Bilateral plyometrics On-field / sand - Linear running (forward, lateral) - Jump and landing drills - Linear acceleration and deceleration - Multidirectional running drills (pre-planned)	Rehabilitation gym / Movement environment - LB high load isolated strength training - LB functional strength training - Landing drills, jumping drills - Core strength/ stability (load transfer) - UB strength, core endurance and aerobic fitness on recovery days On-field - Multidirectional agility drills - Technical based sports re-training - Fitness training - Plyometric training

CV, cardiovascular; NPRS, Numeric pain rating scale; ROM, range of motion; LSI, limb symmetry index; DWR, deep water running; BL, bilateral; UL, unilateral; TENS, transcutaneous electrical nerve stimulation; ICE, ice, compression, elevation; RM, repetition maximum; LB, lower body; UB, upper body

Figure 1.

The use of hydrotherapy in the recovery process after ACLR. Two standard stages have been identified and applied on all the ACLR patients in the first phases of recovery. Two other distinct stages may be applied to the elite athletes to optimize late phase rehabilitation and return to sport training. ACLR: anterior cruciate ligament reconstruction; ROM: range of motion; CV: cardiovascular; OFR: On Field Rehabilitation.

Below it is shown how the aquatic therapy program can be aligned with this recovery process with discussion on important concepts for each stage, providing prescriptive advice for the professional athlete after ACLR for each stage. The authors also share supplementary video of the type of activity which can be undertaken within the aquatic therapy pool.

1. ‘Post-op pool’ (early-stage rehabilitation)

   Post-operative pool typically commences around two weeks after ACLR, when a patient is safe to enter the water. The main contraindications to its use in this stage are wound healing and risk of infection so stiches have to be removed and surgery scars should be free from the phlogosis signs (Table 1). Early post-op aquatic therapy is mainly concerned with over-coming the effects of surgery and complementing the activity in the gym. It is focused predominantly on supporting the resolution of pain, swelling, joint range of motion and recovery of walking gait. During this stage, the athlete often has restricted range of motion, is using crutches on land when walking and is limited in their movement/ ability to perform activities of daily living. It is important to incorporate active range of motion exercises to facilitate active range of motion and avoid joint stiffness. The desensitization in the pool and swelling can facilitate greater active and passive range of motion than possible on land. During this early period, the patient may not be able to use devices such as the stationary bike due to range of motion limitations and thus active range of motion on land may be difficult. The patient can typically develop faulty gait patterns due to limping and avoiding to load the knee. Walking in water at appropriate depth can facilitate restoration of optimal gait and support movement confidence. Thus, key priorities include exercises such as cycling, walking, basic neuromuscular control and passive and active range of motion in this stage. See Table 1 for specific exercises and entry and exit criteria.

2. ‘Movement training and aerobic conditioning’ (mid-stage rehabilitation)

   Following the resolution of basic post-surgical factors (e.g., pain, swelling, joint range of motion and recovery of walking gait) and recovery of activities of daily living, the patient can progress to slightly more intensive rehabilitation. In addition, the patient can now walk on land and typically commence the stationary bike in the gym. Thus, the specific aims of aquatic therapy change. Many clinicians stop aquatic therapy at this stage not realizing the additional uses and benefits of aquatic therapy. The key aims in mid-stage rehabilitation in general are the restoration of strength and basic movement patterning. However, during this stage, the patient is still load compromised for the majority of the stage and so cannot undertake many functional type activities optimally or safely on land (e.g., single leg squat or jumping and landing). The benefits of aquatic therapy can allow for the performance of functional exercises earlier than possible on land, supporting the earlier motor pattern re-training. For example, deep water running can be used to support CV fitness training and restoration/ development of aerobic endurance, but also support running gait re-education with the impacts associated with running on land. This holds true for other exercises such as important functional tasks such as squat and lunging, and high load landing control (jumping and landing exercises). This is highly beneficial psychologically for the athlete as they can commence ‘performance-based training’ in the pool. Introduction of neuroplasticity exercises that can mimic the sport-specific movements (without loading the knee joint) can commence at this stage. These include exercises such as heading a football while seated on a floating device (e.g. rubber ring) to challenge postural control (Figure 2).

   Given the range of complexity of exercises within stage 2, the phase as a whole and the pool program are typically broken down into two sub-phases (stage 2.1, stage 2.2). The first half is when the athlete is undertaking basic muscle endurance in the gym, as well as core corrective exercises performed in a non-load bearing position (e.g., exercises on the bed) due to being too load compromised to practice weight bearing functional exercises (e.g., not strong enough to commence unilateral functional exercises such as single leg squat). During this stage it is highly beneficial to incorporate corrective movement training using foundation exercises (e.g., lunging, squatting etc.) in the pool, as well as deep water running. As the athlete becomes stronger and overcomes many of the aspects of neuromuscular dysfunction following surgery, they can generally commence higher load strengthening in the gym (e.g. transition to moderate load resistance training to restore muscle hypertrophy and strength),⁸³ and weight-bearing exercises on land (generally the second half of this stage), including progressions towards treadmill running. Thus, when they are able to perform these tasks on land, there is less need to perform them in the water. Therefore, during the second half of this stage, patients can practice higher loading impact exercises in the pool (allowing for around a 50% reduction in impact forces),^71,73 such as two-leg and one-leg landing exercises (also using the trampoline as a progression, Figure 3), bilateral jumping exercises to support the development of lower body power (Figure 4), and treadmill running at the appropriate depth. These movements can be performed once the patient is sufficiently strong enough and has the desired level of neuromuscular control (the tasks can still involve around 1.5-2 times body mass). Importantly, the athlete should not perform unilateral plyometrics in this stage, as they involve potentially dangerous knee joint loads, even if in water (ground reaction forces of 2-4 times body mass). See Table 1 for specific exercise content in this stage. Also see supplementary video 1 demonstrating movement-based re-training in the pool.

3. ‘Intense conditioning and field preparation’ (late-stage rehabilitation)

   Late-stage rehabilitation and pool training is an intense program designed to simulate sport-specific training on land to prepare for beginning sporting type movement training on the field. In addition, the athlete transitions to high load movements on land such as running, deceleration, jumping and landing exercises, prior to being able to perform unilateral plyometrics and multi-directional coordination exercises on the field. As such, the benefits of the pool are less dramatic as the majority of activity can be performed on land during this stage and aquatic therapy is less essential in this stage than during previous stages (and generally not required in non-professional athletes). However, the pool can used as a useful adjunct to the program to enable some of the exercises which cannot be done initially on land to be performed in the pool. For example, initially in this stage, the patient can run on land and begin a landing and jumping progression program, but unilateral plyometrics are typically done towards the end of this stage (therefore minimizing the opportunities to train explosive neuromuscular control and power under sport-specific movement demands). Thus, during the initial period of the stage, the patient can perform some of the high load exercises on land (e.g., bilateral plyometrics and unilateral landing drills) and the remainder in the pool (e.g., unilateral plyometrics, Figure 5). Practicing these unilateral plyometric exercises in the pool can support technique learning which can then be applied on land. It can also be used to manage the loading of the athlete when they commence high load exercise on land and support athletic conditioning on land by providing an opportunity to perform some of the work in the pool at lower loads (the pool will reduce impacts loading by around 50%).^71,73 Aquatic therapy is typically used more sporadically in this phase to complement existing programs in conjunction with land based movement training (e.g., a short aquatic therapy session after a land based movement session for specific work, some deep water running and unilateral plyometric tasks). See Table 1 for example activity in this stage.

4. Recovery pool - (RTS training)

   During the on-field rehabilitation process athletes are practicing sport-specific training on the field and are typically involved in re-conditioning work in the gym (e.g., strength and power training, neuromuscular control).⁸⁴ During the on-field rehabilitation process, the pool can be used as a useful adjunct to the standard program to support accelerated recovery and low load conditioning between on-field sessions. During on-field rehabilitation, managing the workloads of the athlete is particularly important, as he/she is introduced to training type activity to prepare them to return to their team environment. As such, there is typically a rapid increase in training load. Optimizing recovery between sessions similar to general athlete training is important. As such, the pool can be used as a recovery tool to support low load conditioning and accelerated recovery between rehabilitation sessions. This is particularly relevant following intense training days on the field and/ or in the gym, which designed to load the athlete to develop their tolerance to increased training demands. See Table 1 for example activity in this stage.

Figure 2.

Neuroplasticity exercise in the pool.

Figure 3.

Running in place (A), and single leg landing on trampoline (B).

Figure 4.

Bilateral jumping for power development with A, countermovement phase, B, flight phase and C, landing phase.

Figure 5.

Unilateral plyometric exercise involving forward A, step off from step, B, landing followed by immediate jump and C, single leg stabilisation on box and lateral drop jumps (D, step off, E, landing and jumping transition and F, lateral landing on step).

SUMMARY

The properties of water (buoyancy, density, hydrostatic pressure and viscosity) can be utilized to implement an aquatic therapy program, which if planned correctly can facilitate the development of an optimized functional recovery program following injury. The six discussed benefits of aquatic therapy 1) reduction of pain and swelling; 2) recovery of gait; 3) maintenance and/ or development of CV fitness; 4) motor pattern/ coordination training; 5) earlier introduction of plyometric training and 6) between-session recovery and optimized load management should be used effectively to complement an existing functional recovery process. It is essential to ensure that the aquatic therapy program is aligned with the ACLR functional recovery approach as a whole. The authors have provided a four-stage aquatic therapy program to complement existing land based ACL functional recovery programs that consists of 1) post-operative pool; 2) movement and CV conditioning; 3) intense conditioning and field preparation and 4) recovery pool.