The underwater treadmill (UWTM) is a popular tool in human, canine and equine rehabilitation. However, research into canine and equine use of this tool is limited (Barnicoat and Wills, 2016) and therefore rehabilitation programs are often based on anecdotal experience rather than actual research (Tranquille et al., 2022). Information is also taken from the human field to try to inform best practice. Although this poses some issues, as humans are clearly not quadrupeds (an animal with four feet), there are some important points which warrant consideration. Human studies evidence that the response to exercise on an UWTM varies greatly, depending on a person’s age, level of fitness, current health and their previous training (Tranquille et al., 2017). Therefore, it would be sensible to assume that age, level of fitness, general health and current training will also influence the response of a dog or horse. Water depth, water temperature and speed of the belt are also important factors when planning a training or rehabilitation program in human physiotherapy and therefore, also need consideration in the canine and equine sector.
Habituation
It may be fair to conclude that humans take less habituation to exercising on the UWTM than dogs and horses because they have a better understanding of the situation, the apparatus and what is required of them before they begin. Animals do not have this awareness and will be experiencing a new environment for which habituation will be required. Both dogs and horses require time to habituate to the moving belt, to moving through water and to the adaptations in speed. Anecdotal evidence suggests it takes two sessions for sound, uninjured dogs to habituate to the UWTM (Tranquille et al., 2017). It may be presumed that dogs with injury may take longer due to possible instability or reduced proprioception. Similarly, sound horses took two, fifteen minute sessions on the UWTM before their heartbeat remained steady (Nankervis et al., 2006) and four sessions before their gait pattern became regular ( Scott et al., 2010). Therefore time for habituation needs to be built into any rehabilitation or training plan.
Effort Required
It may be considered that walking or running on the UWTM requires more effort than walking or running on the land because you are moving through water rather than air. However, evidence suggests that it is a bit more complex than simply stating that training on the UWTM requires more effort than land exercise. Research has shown that walking slowly on the UWTM requires the same amount of energy as walking slowly on land – it is only once the speed begins to increase that the UWTM has an increased metabolic cost (Dolbow et al., 2008). Human studies have also evidenced that walking in deeper water requires more energy as both oxygen uptake and heart rate increase (Hall et al., 1998). However, once water reaches a certain depth, heart rate is seen to plateau (Linder et al., 2012), possibly due to the buoyancy of the water which reduces the amount of weight the body is having to carry (Tranquille et al., 2017). This suggests that although increasing the water depth does increase the energy consumption compared to walking or running on land, continuing to increase the water depth does not mean that exercise intensity will continue to rise.
Temperature is another factor which impacts heartrate. Both human and equine studies evidence that increases in water temperature result in increases in heart rate, as the need to dissipate the heat increases the load on the cardiovascular system (Gleim and Nicholas, 1989; Nankervis et al., 2008). All of the above research indicates that UWTM exercise is an aerobic form of exercise which provides cardiovascular conditioning and increases oxygen uptake to fuel the body (rather than anaerobic, which breaks down glucose to fuel muscles).
Therapeutic effects of water
Water has a number of therapeutic effects which support it being an effective tool in rehabilitation and training.
Viscosity describes the friction between the water molecules, which create resistance (Monk, 2007). This resistance not only contributes to the additional effort required on the UWTM in deeper water, it also means that movement is slower. Slower movement can reduce the risk of injury (Tomlinson, 2013) and is often more desirable for the rehabilitating patient.
Surface tension is a force which is exerted by molecules at the surface of an amount of liquid (Monk, 2007). This means that it takes more effort to break through the surface of the water. This needs to be taken into consideration when using shallower water, which an animal may continue to step into. Each time they step back into the water they will be moving through the surface where the resistance is greater- this is good if you want the effort to be increased but may need careful consideration if the limb which is continually re-entering to break the surface is weak.
Hydrostatic pressure is explained by Pascal's law, in which fluid pressure is exerted on all surfaces of a body immersed into that fluid (Tomlinson, 2013). This pressure has an effect similar to light massage, which can help to reduce pain as well as reduce oedema through venous return.
Water temperature has an effect on the therapeutic properties achieved. While canine guidance suggests temperature should be between 26- 30 degrees Celsius, as this increases cell elasticity, promotes relaxation and helps to reduce pain (Prankel, 2008), equine research provides no guidance. Anecdotal reports suggests water temperature is 'shed temperature', which will differ depending on weather and time of year. The impact of this on muscles and circulation needs to be considered as does the warmer temperatures in the canine UWTM, bearing in mind the information above regarding the increase in cardiovascular effort required when temperature increases.
Buoyancy and Drag both increase with water depth. Like viscosity, drag causes humans, dogs and horses to move more slowly on an UWTM than they do on land because it impedes forward movement of any body part immersed in the water. Buoyancy creates an upward thrust equal to the weight of the water which has been displaced (Edlich, 1987). This offloads body weight which means that vertical ground reaction forces (this is the force which travels through a limb once it makes contact with the ground) are reduced in water. As buoyancy increases with water depth, ground reaction forces decrease. Immersion up to the pelvis offloads 40% of body weight in humans and 60% of body weight in quadrupeds (Devine et al., 2010; McClintock et al., 1989). Canine research reports a 9% weight reduction when water is at tarsus (hock) level, 15% at stifle level and 65% at hip level (Tranquille eta al., 2018). However, it was also reported that a greater percentage of this weight was taken by the forelimbs when water was at hip level compared to water at stifle and tarsus level, or when there was no water at all. This is worth consideration for dogs with any forelimb injury. The reduction in weight through buoyancy reduces the load to limbs, which impacts limb kinematics. For example, it is estimated that, as water depth increases, fetlock extension decreases as there is less weight through joint (Tranquille., 2022).
Buoyancy and drag have been evidenced to impact the features of the stride in both equine and canine studies. Equine research reports that water at carpal (knee) depth reduced stride frequency but increased stride length (Scott et al., 2010), while canine research reports that stride length increased with water depth but frequency reduced (Baricoat and Wills, 2016). This is because drag impedes forward movement, slowing the frequency. Flight arc and swing phase are reported to increase (Baricoat and Wills, 2016) because buoyancy assists with the upwards movement, increasing the height of step (Edlich et al., 1987). This means that walking quickly on the UWTM requires more intense muscle activity than trotting because greater drag is experienced when limbs move forwards, compared to trotting, which requires more vertical displacement (limbs are elevated upwards in the step). This is not only helped through buoyancy, it also reduces drag if more of the limb is elevated out of the water (Tokuriki et al., 1999).
Buoyancy and drag are two of the elements of UWTM exercise which make it popular for training and rehabilitation. Human and canine research both evidence that UWTM exercise can be targeted to support weight loss and, what’s more, the increased muscular forces required to combat drag resulted in the development of leaner muscle mass compared to land exercise (Greene et al., 2009). The reduced ground reaction forces which buoyancy achieves means that load put through joints or load bearing muscles is reduced, something which is desirable if planning exercise or rehabilitation programs when joints are injured or diseased. Buoyancy can also provide stability and support. Muscles which are responsible for propulsion however, experience an increased load to overcome drag.
Limb Kinematics
Water depth has been shown to influence forelimb and hindlimb gait patterns in horses, with greater depths resulting in patterns which are considerably different to those seen on land (Nankervis and Lefrancois, 2018). As mentioned above, fetlock extension decreases during the stance phase of the stride and, because of drag, there is an increase in stance duration. Retraction in both the fore and hind limbs has also been shown to increase, and this has been contributed to both the belt and to the increase in drag pulling the foot backwards (Tranquille, 2017).
Another evidenced alteration of gait when exercising on the UWTM is the increase in joint ROM (Tranquille et al., 2022). Canine studies have concluded that maximum tarsal, stifle and hip ROM were seen when dogs were walking with water at stifle height (Kathmann et al. 2006; Monk et al., 2006). Equine studies have shown that the greatest forelimb fetlock flexion was seen when water was at fetlock height and greatest carpal (knee) flexion was seen when water was tarsal height. In the hindlimb, greatest tarsal flexion was seen when water was at stifle level. However, both maximal carpal and tarsal flexion plateaued once water reached a certain depth. This depth seemed to be just below or just above the tarsus, with slight differences between each horse dependent on confirmation, flexibility and muscle strength; if horses were less able to flex the joints of their hind limbs (either through reduced joint ROM or weaker hindlimb musculature) they adapted their gait pattern earlier. Nankervis and Lefrancois (2018), reported individual differences in the horses they studied, depending on their ability to protract the hindlimb in the increasing depth of water. Horses lifted their limbs above the water in lower depths (which increased the flight arc – the height which the toe of the hoof reaches when leaving the ground). Once water depths increased, there was a switch in their response, with a greater reliance on pelvic flexion to aid limb movement through the water. However, the point of the switch from one movement to the other was individual to the horse and influenced by their ability to flex either the distal (lower) limb joints, proximal (higher) limb joints and lumbosacral spine. Lumber flexion has also been seen to increase at the point of switch between the gait patterns (Nankervis et al., 2016).
Back and Pelvis Movement Patterns
As we have seen in the evidence referred to above, the UWTM impacts limb kinematics. We know from research into lameness studies that altering gait patterns has an impact on movement patterns of the back and pelvis (Gomez-Alvarez et al., 2008). Tranquille et al., (2022) investigated the impact different depths of water had on back kinematics to quantify the effects. They concluded that an increase in flexion and extension ROM was evidenced in the thoracic spine and cranial lumbar (pelvic) region, which results in the increased stride length mentioned above. However, a decrease in ROM was observed in the caudal (later part) lumbar spine. They also reported that as the water depth increased, pelvic roll also increased, which they considered to be a compensatory movement pattern in response to the deeper water, and possibly an indicator that the horse had reached its capacity of tarsal flexion.
Another significant finding on the impact of water depth on equine back kinematics is the increase in extension seen in the cranial thoracic spine (near the withers) (Tranquille et al., 2018). This is possibly seen as an attempt by the horse to keep their head above the height of the water and is therefore going to increase with water depth. This is an important point to consider as would not be a desirable movement pattern to train, especially with horses rehabilitating from kissing spines.
Tranquille et al., (2022), used the outcomes of this and previous studies to conclude that the UWTM is beneficial exercise for horses who require limb flexion as part of their rehabilitation program, and, as maximum tarsal flexion did not change between water depths of 34-47cm, report that deep water is not essential, unless reducing the load on joints is also an aim. However, they did conclude that UWTM programs should be designed on an individual basis, with responses closely monitored so that compensations can be quickly identified. If rehabilitation of both the limb and back is required, a protocol which helps both sites of injury needs to be designed, to ensure that rehabilitation of one site does not cause compensatory movement elsewhere.
Key Points
All of the evidence referred to above suggests that exercise and rehabilitation using the UWTM is complex. They are many positive contributing factors which make it an effective tool for canine and equine rehabilitation and training. However, there are also some important points for consideration, especially as it is often used alongside other therapeutic modalities and therefore knowing the direct impact of the UWTM alone is difficult to determine.
If you are thinking about using the UWTM for your dog or horse, here are some key points taken from the above you may wish to consider:
Habituation- make sure your animal has enough time to habituate to the UWTM
Current level of fitness/stage of rehabilitation- be honest with the therapist and ensure that the program provides the right amount of challenge. Consider things like temperature and water depth and how this will increase the effort required
What are the therapeutic effects of the water which are going to be beneficial to your animal- be clear on these and how they will provide the support you seek
What injury/disease are you rehabilitating? Consider how drag and buoyancy will impact this area and discuss this with your therapist.
Finally, work with your therapist to monitor your animal's response - your therapist will monitor their response while using the UWTM but you need to monitor their response once you get them home and between treatments so that you can work together to ensure the program is tailored to their needs.
Here you can see that the horse on the treadmill is still flexing the joints of the hindlimb to 'step over' the water rather than push through. The hamstring muscles are working hard to lift the limb through the water to make the forwards step.
References
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