Biomechanical aspects of aquatic therapy: A literature review on application and methodological challenges

The following research the biomechanical aspects of aquatic therapy is from Anna Severin, Brendan Burkett, Mark McKean and Mark Sayers; School of Health and Sports Sciences, University of the Sunshine Coast, Queensland, Australia; and was published in 2016 by Journal of Fitness Research.


The application of aquatic therapy for health and rehabilitation purposes has been promoted for centuries.

Although used predominantly in clinical settings for the treatment, rehabilitation and management of chronic conditions, the practice is also gaining popularity in athletic settings in such areas as recovery training and for the rehabilitation of acute musculoskeletal injuries.

To date, most studies on the impact of aquatic-based rehabilitation on the human body have focused on physiological aspects.

There is a relative paucity of published research on the biomechanical implications associated with aquatic-based activity.

The published findings have been limited to the influence of the aquatic environment on running and walking gait.

A clear challenge in this field is absence of standardized protocols for assessing the impact of aquatic therapy and its possible role in rehabilitation.

For example, methodologies often differ considerably between studies, and there are no standardized reporting procedures for important variables such as water depth and temperature.

The research knowledge in this area has been questioned, with current medical guidelines highlighting that high-quality research into the roles of aquatic therapy in rehabilitation is warranted.

This review will summarize the current literature on water-based activity and how this can impact human movement and subsequent rehabilitation.

Biomechanical aspects of aquatic therapy.


The health-related benefits of aquatic therapy have been promoted for centuries.

Common uses for aquatic therapy in clinical settings is managing and rehabilitating chronic conditions such as osteoarthritis (OA) and fibromyalgia.

Aquatic therapy is also used for weight management, athlete rehabilitation and recovery.

Despite decades of research examining the roles of aquatic therapy in rehabilitation, many of the results from scientific investigations are conflicting, likely due to differences in applied methodologies (Table 1).

Because of the limited quality of current research, reviews published by the Cochrane Collaboration concludes that aquatic-based rehabilitation programmers are assumed equally effective to programmers performed on land, but highlights that further high-quality research is warranted.

The lack of consensus among previous research regarding the efficacy of aquatic based rehabilitation is resultant from several methodological challenges and a lack of consensus on the most appropriate outcome measures.

This review will briefly evaluate published literature on water-based activity; its impact on biomechanics and current role in rehabilitation protocols.

Existing limitations and challenges in research methodology will also be reviewed along with gaps and limitations in the current knowledge and directions for future research will be recommended.

Aquatic therapy explained

The appeal of aquatic therapy as a tool in exercise, recovery and rehabilitation has increased over recent years.

Previous research has identified several biomechanical and physiological effects associated with exercising in water that must be thoroughly understood by practitioners to prescribe accurate and effective programmers.

These effects occur because of fundamental principles of hydrodynamics and physical properties of water, such as density, buoyancy, hydrostatic pressure, viscosity and thermodynamics.

The physical properties of water


Density quantifies a substance’s mass by volume unit (Kg·m-3).

The density of 4°C freshwater is approximately 1,000 Kg·m-3 at sea level (999.97 Kg·m-3).

Although the temperature of the water affects its density, the change is considered small enough to dismiss (997.05 Kg·m-3 at 25°C).

An average human body consists of approximately 60% water and its density is thus slightly lower than that of water (approximately 974 Kg·m-3).

The specific density of a human body depends on body composition.

Fat free mass, including bone, muscle, organs and connective tissue, has a density higher than water (close to 1,100 Kg·m-3) whilst fat mass has a density lower than water (close to 900 Kg·m-3).

Thus, an individual with a higher percentage of fat free mass has a higher density compared to an individual with a higher fat mass percentage.


A human body with a density lower than water displaces a volume of water that weighs slightly more than the body itself.

By Archimedes principle, an upwardly directed force is exerted on the body equal to the volume of the water it displaced.

This buoyant force, opposes gravity and pushes the submerged body towards the surface of the water.

Accordingly, a human body with a specific gravity of 0.974 (a density of 974 Kg·m-3) will achieve floating equilibrium when 97.4% of the body is submerged due to buoyancy.

As the mass of the submerged body increases, the buoyancy force increases proportionally.

Therefore, an individual immersed to chest level experiences a larger buoyancy force compared to someone immersed to the waist.

Hydrostatic pressure:

In addition to buoyancy, the volume of water surrounding the submerged body also exerts a compressive force on the body – hydrostatic pressure.

At sea level the pressure exerted on the body by the air surrounding it is approximately 1013.0 Pa (7.6 mmHg), a value that is so small that it is basically imperceptible.

However, the proportionally greater mass of water means that immersion in water exposes the body to considerably higher pressure, that like buoyancy, increases with the depth of immersion at a rate of approximately 981.0 Pa (73.5 mmHg) per meter.

Accordingly, standing in water at neck depth will result in approximately twice the hydrostatic pressure on the calf muscles than on the chest.


Viscosity is the magnitude of internal friction a fluid has during motion and is specific to each fluid.

An immersed body moving through water, experiences resistive drag forces opposite to the direction of travel because of viscosity.

The viscous resistance is directly proportional to the force exerted against the fluid.

Therefore, the resistance will increase with increased velocity and surface area of the moving body.

For example, a fully outstretched arm produces a greater resistance when moving through water than a hand only.

As soon as movement ceases and the exerted force on the water disappears, the viscous resistance drops immediately to zero, resulting in no further resistance on the body.


Water has a superior ability to retain heat and transfer heat energy than air and has a heat capacity of approximately 1.0 J·K-1 (1,000 times greater than air).

Water also has a higher heat capacity compared to human body tissues (0.83 J·K-1), resulting in body equilibrating faster than the surrounding water.

Thus, a body immersed in water colder than core temperature will adapt to the temperature of the water and lose heat.

Water warmer than core temperature therefore warms the body and raises its core temperature.

Effects of water-based exercise on the human body

The physical properties of water have large biomechanical, neurological, physiological and hormonal effects on the human body.

Previous research has identified many of these effects; however, to explore them individually is outside the scope of this literature review, thus only those variables implicating on human movement will be addressed.

For additional insight on the effects not included here, see reviews by Becker (2009), Denning et al. (2012) and Mooventhan and Nivethitha (2014).

Biomechanical effects of immersion:

Studies into biomechanical aspects forms a minority of previous research into the effects of immersion on the human body.

Of these, most reported on differences in gait parameters between the water- and land-based settings, (Table 1) thus, insights into biomechanical implications of aquatic therapy remains unreported.

Table 1 Effects of aquatic therapy on gait kinematics

The published research on water-based gait reports several significant adjustments enforced by the aquatic environment, believed to be mainly associated with buoyancy and drag forces.

However, some reports on these adjustments are contradictive, most likely due to the considerable differences in utilized methodologies between studies (Table 1).

Continued table 1 effects of aquatic therapy on gait kinematics.

The inconsistencies in water depth and temperature alone are likely resulting in differences in reported findings as both properties are known to impact on biomechanical variables.

However, as the current understanding of biomechanical adaptations to the aquatic environment is limited to these reports, their findings should still be taken into consideration.

Most studies reported similar joint angles during both aquatic and land-based walking.

A 2012 review on differences in gait mechanics in water and on land concluded similar joint motions at the knee and ankle during water-walking, but highlighted that the activity at the hip joint and pelvis increased.39 Several studies have reported on increased reliance on the hip joint during water-based walking.

Kaneda et al. (2008) reported an increased hip joint range of motion (ROM) during water walking and suggested that it was a consequence of buoyancy allowing an increased hip flexion motion during swing phase.

It is possible that these adaptations in hip joint kinematics may influence other movements performed in water, such as squats and lunges.

Miyoshi et al. (2003) further noted a hip extension moment throughout the entire stance phase during walking in water that was not present during land-based walking.

A similar study reported decreased joint torques about the knee and ankle during water-walking compared to overland, but highlighted that no decreases were noted at the hip joint.

Perhaps this is because of the increased resistance supplied by the water as the hip joint attempts to translate the leg forward through the viscous fluid.

These studies on gait has concluded that kinematical adaptations occur in aquatic settings, and highlights the need for future kinematic research conducted on exercises used for aquatic-based rehabilitation.

One study highlighted that although drag forces of water might be advantageous for rehabilitation, they may be a contra indicator against water-based exercise if not properly understood.

The added, and abnormal resistance supplied by the water element may result in compensations or prove too much for an injured tissue and should be considered when programming for rehabilitation.

Biomechanical research has also been conducted into vertical ground reaction forces (GRFZ) during aquatic activities compared to land-based equivalents, and shown significant differences between the two environments.

These differences have been attributed to the decreased loading associated with buoyancy and drag forces.

Further, research comparing jumping actions in water and on land, reported increased force production, rate of force development, and power output during water based jumping actions.

It was also noted the aquatic environment produced lower impact forces.

These studies inferred that the aquatic environment is ideal for plyometric training as it reduces potentially harmful impact forces.

Similarly, Martel et al. (2005) suggested that aquatic-based plyometric training improves land based plyometric performance and potentially reduces muscle soreness.

Although these studies were not performed in a rehabilitation context, they have provided further evidence of biomechanical implications in the aquatic environment, which should be considered in the application of aquatic therapy.

Further, authors have suggested that the aquatic environment might be beneficial for static and dynamic balance training.

However, although studies have reported significant improvements in balance following aquatic-based exercise, the improvements were not significantly different from those achieved with land-based programmers.

The aquatic environment is often considered a safer environment than land, as it provides increased stability and reduces the risk of injury in case of a fall.

Consequently, performing some exercises in the aquatic environment offers clear advantages over the land-based equivalent for populations with a high risk of falls such as older adults and post-surgery patients.

Although previous kinematic research is limited to gait, it seems the aquatic environment has the potential to affect several parameters of human movement.

Future research should include other activities common in everyday life, exercise and rehabilitation.

Aquatic therapy in rehabilitation of human movement

Buoyancy and viscosity are the two physical properties of water believed to have considerable effect on the biomechanical aspects of rehabilitation.

Buoyancy opposes gravity and thus decreases the loading on joints and muscles.

Becker (2009), reported that immersion to the pubic symphysis offloads approximately 40% of the body weight, immersion to the umbilicus offloads 50%, and immersion to the xiphoid process offloads 60%.

Reduced joint and muscle loading during immersion to these depths may allow a patient to perform exercises and activities earlier than may be possible during full gravitational loading.

Decreased loading of joints and early rehabilitation could be beneficial across several acute and chronic injuries, and for several different populations including athletes, elderly and patients with various chronic conditions as it facilitates movement.

The viscosity of water provides resistance to movements and may therefore be helpful for building muscle strength and endurance following musculoskeletal injuries or surgery.

However, research has shown the improvements in strength achieved with water-based training are significantly less than improvements achieved with similar exercises performed on land.

The ability of the aquatic environment to build strength with decreased joint loading constitutes the rationale for the use of aquatic therapy in improving the quality of life for an elderly or obese population, or as a part of a general weight-management programmer.

Current rehabilitation protocols for ligamentous injuries recommend early functional treatment.

These protocols aim to control inflammation during the acute phase and limit subsequent loading stress.

The hydrostatic pressure and decreased joint loading supplied by the water caters to both these aims and constitutes the use of aquatic therapy in rehabilitation of musculoskeletal injuries.

Kim et al. (2010) reported that aquatic-based rehabilitation produce superior rehabilitation outcomes at two and four weeks’ post-injury compared to a land-based programmer for ligamentous injuries in the knee.

Previously, Bartels et al. (2007) highlighted the low quality of past studies in their meta-analysis on the use of aquatic-therapy as a rehabilitation regime for OA.

It was suggested that aquatic-based rehabilitation exercise protocols offer some short-term benefits in rehabilitation of knee OA, but that further research is needed before any definitive conclusions can be drawn.

Clearly, current knowledge on the roles of aquatic therapy in rehabilitation is lacking, and future research should aim to settle protocols and guidelines to ensure best outcomes.

Current methodological challenges in aquatic therapy research

The growing attractiveness of aquatic-based rehabilitation among medical professionals is likely based on suggestions that the aquatic environment allows for an earlier commencement of rehabilitation and reduces joint and muscle loading.

However, despite being a common part of many rehabilitation programmers, there is a paucity of high-quality scientific literature on the efficacy of aquatic-based rehabilitation training regimens.

The different context offered by the aquatic environment provides several challenges to researchers rending it difficult to conduct high-quality research projects.

Motion tracking in the aquatic environment

Most previous research into kinematical effects of water-based motion have relied on video analysis capturing the sagittal view only and operating at 30 or 60 Hz. (Table 1)

Researchers used video cameras placed along an underwater walkway and recorded participants as they walked past.

Caution is advised when performing kinematic analysis using video footage because of the risk of parallax error (Figure 1).

Figure 1 Diagram depicturing the parallax error in biomechanical research.

Parallax error denotes a distortion of the image because of an angle of inclination between the subject and camera.

Further, by limiting the analysis to sagittal view due to camera positions, data on frontal and transverse plane movements are not recorded.

Collecting video footage from a sagittal and frontal view allows for a more comprehensive analysis, however, the capacity of video analysis to accurately assess data on frontal and transverse plane movements have been questioned.

Further, the reliance on video analysis for kinematic parameters in gait research has been questioned as the surrounding water induces differences in basic kinematic descriptors such as stride frequency and length.

The author thus recommended that electromyography (EMG) would provide valuable additional information during kinematic gait studies.

A literature review on surface EMG during aquatic-based exercise concluded that muscle activity generally is lower in during activity performed in water compared to land.

However, the review highlighted that the included studies were low in number and that more high-quality research is needed to fully understand the implications of this.

Further, authors have reported that the use of EMG underwater requires caution as it constitutes further challenges including water interfering with the signals and safety considerations when using electrical components in water.

Current practice considers motion capture technologies the gold standard for analyzing human movement.

Motion capture using infrared cameras to track reflective markers on participants are capable of capturing at frequencies of up to 50 KHz.

However, motion capture systems are expensive, complicated, and limited to laboratory settings.

Therefore, their availability and application in clinical and practical settings are restricted. In addition, as the refractory index differs between air and water, light travels differently in the two mediums.

Thus using systems relying on infrared cameras in water remains challenging.

The use of isoinertial sensors, such as accelerometers and gyroscopes, is gaining popularity amongst researchers in attempts to track human motion in non-laboratory settings.

These sensors are small, inexpensive and portable, thus allowing for testing in various settings.

Studies have confirmed the accuracy of these sensors during walking, the timed-up-and-go test, and the sit-to-stand test.

However, only sagittal plane data, peak velocities and power were reported.

Thus, future research should aim to examine the use of isoinertial sensors in nonsagittal plane human motion, as this could further establish their role in biomechanical research.

In addition, isoinertial sensors rely on measurements from within the sensor itself and so can therefore be used to track human movement in water.

Research into the effectiveness of isoinertial sensors for tracking human movement in aquatic environments would provide valuable and exciting additions to current knowledge and research methodologies.

Lacking protocols

To date, the consensus on the biomechanical and physiological effects of aquatic based activity are lacking.

A likely reason for reported contradictions is differences in methodological protocols, including differences in water depth, temperature, activity and intensity (Table 1).

These factors are all known to impact biomechanical and physiological responses to exercise.

Caution is therefore warranted when comparing studies reporting on effects of aquatic based exercise, and target population and exercise specifications should be considered.

Establishment of guidelines for water temperature and depth would also be beneficial for aquatic-based exercise and research.

Comparative studies – land versus water

There are numerous systematic reviews and meta-analyses published assessing the differences in water- and land-based rehabilitation for patients with lower limb OA, fibromyalgia, chronic obstructive pulmonary disorder, asthma and stroke.

However, these reviews agree that previous research is of poor quality and fails to show significantly different outcomes between the two environments.

These reviews highlight the need for high quality comparative studies in this domain.

Much of the research in this domain relies on outcome measures, typically including subjective pain scales, functional tests with hand-held stopwatches, isolated muscle strength testing using non-specific hand-held dynamometers and isolated ROM tests.

Although scientifically validated in clinical settings, research has questioned the application of these measurements in comparative research.

Hatfield et al. (2011) highlighted that subjective reports are insensitive and likely produce skewed results.

Further research has reported that pain is not necessarily reflective of functional outcomes and so the use of pain scales as an assessment tool may not be a valid measure of performance.

The reliability of ROM tests has also been questioned following total knee replacement (TKR) surgeries, as ROM may be affected by several factors including the prosthetic design, preoperative motion and surgical technique.

An objective alternative to the outcome measures in question is the use of motion capture systems to determine pre- and post-intervention changes in kinematics.

Although motion capture testing comprises several known limitations, it provides objective information on human movement that subjective data cannot provide.

A recent literature review by Komnik et al. (2015) showed that motion capture is a common method to identify differences in kinematics following TKR.

The review highlighted several lingering alterations in kinematics following surgery, including asymmetries between the limbs, and have provided useful information on the use of rehabilitation programmers following TKR surgery.

However, this review was limited to include studies assessing kinematics following only land-based rehabilitation protocols.

Surprisingly, despite providing no empirical evidence to support these claims, highly regarded medical research foundations such as the Cochrane Collaboration and BioMed Central have indicated that aquatic-based rehabilitation is comparable to conventional land-based protocols.

However, at the time of this review no published studies have investigated biomechanical differences between the two-media using empirical methods such as motion capture.

Research comparing pre- and post-rehabilitation kinematics of individuals following land- or water-based rehabilitation programmes would provide new information on the roles of aquatic therapy in rehabilitation and its effect on human movements.


Aquatic therapy can aid in the rehabilitation of musculoskeletal, cardiovascular and neurological conditions as it offers a safe and social alternative to common land-based protocols.

The physical properties of the water including buoyancy, viscosity and hydrostatic pressure has beneficial effects on joint loading, pain perception and blood flow.

Studies have assessed the effectiveness of water based rehabilitation programs for management of various medical conditions.

However, these studies relied on subjective or clinical outcome measures.

Although the subjective experience is an important aspect of rehabilitation, its scientific validity has been questioned.

Further, previous research into the biomechanical and physiological effects of water-based rehabilitation present contradicting results and a consent on practices such as water depth and temperature have not been established.

Additionally, the current limitations in motion tracking methodologies adds further complexities to this research area, as it is possible that exercises performed in the aquatic environment has biomechanical implications that remain unknown.

This literature review identifies several gaps in the current knowledge and highlights possible pathways for future research.

By bridging the gaps and gaining new knowledge in the roles of aquatic therapy in rehabilitation, we can establish protocols and procedures to ensure optimal recovery for individuals with injuries and pathologies.

People doing exercise in the pool and analyzing biomechanical aspects.

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