People immersed up to the symphysis pubis have effectively offloaded 33-40% of the weight of their bodies, and when further immersed to the umbilicus it increases to approximately 50% (Becker, 2009). Xiphoid immersion offloads bodyweight by 60% or more based on whether the arms are overhead or at the trunk level (Becker, 2009).
Buoyancy has the potential of being a very useful therapeutic utility; for instance, painful hips or knees may not be mechanically stable under full-body loading.
However, during water immersion, gravitational forces and buoyancy provide relative weightlessness, and joint unloading may be partial or complete so that only muscle torque forces act on the joint, allowing active assisted range-of-motion activities, gentle strength-building, and gait training (Becker, 2009;
Schrepfer, 2002).
To sum up, the effect of gravity is decreased by the buoyancy effect of water, allowing for more freedom and comfort of movement of joints and muscles.
Therefore, exercise in water produces less joint compression compared to land exercise, and provides a suitable environment for rheumatic patients to exercise aerobically.
3.5.4 Viscosity and turbulence
Viscosity describes resistance of the internal friction of fluid molecules during motion (Becker, 2009; Salzman, 2003; Schrepfer, 2002). On this basis, a limb
78 moving in water is subjected to the resistance effects of the fluid, and this resistance is correlated positively to the velocity of movement through liquid – this is also called the ‘drag force and turbulence’ (Becker, 2009; Schrepfer, 2002).
Resistance of water viscosity is proportional to the velocity of movement in the water, and this resistance increases when more force is exerted against water (Becker, 2009). Thus during treatment in water when a person feels pain and stops movement, the force drops abruptly as water viscosity dampens movement almost immediately (Becker, 2009). It is important to remember that fluid is more viscous than air, and that as the temperature increases, the viscosity decreases because the molecules are increasingly separated by heat (Salzman, 2003).
Turbulence refers to the eddies that follow in the wake of an individual moving through a fluid (Becker, 2009; Salzman, 2003; Schrepfer, 2002). The production of turbulence is greatly dependent on body shape, and the degree of turbulence is dependent on the speed of the movement. If the movement is slow then the flow of the particles is almost parallel to the object and proceeds in a smooth, continuous curve (Campion, 1997; Schrepfer, 2002). Faster movement produces eddies, and the energy in these eddies is dissipated, reducing the pressure and increasing the drag on the body (Campion, 1997; Schrepfer, 2002).
Turbulence might be used in hydrotherapy to assist movement. Turbulence creates resistance with all active movements, and a long lever arm, for example, results in increased resistance (Schrepfer, 2002). Increasing the surface area of the object moving through water also increases resistance. Moreover, proximally stabilising an extremity during manual resistance exercises in the water requires the patient
79 to perform more work (Schrepfer, 2002). Conversely, distally stabilising an extremity requires the patient to perform less work. A significant clinical implication of this is the possibility that using the physical properties alone to create turbulence results in challenges to balance and coordination, problems that are experienced by RA patients.
3.5.5 Thermodynamics
It has been suggested that water conducts heat 25 times faster than air, and that it also retains heat 1000 times more than air (Becker, 2009; Edl et al., 2004;
Schrepfer, 2002). The use of water for therapeutic purposes depends on its ability to retain heat and transfer heat energy (Becker, 2009). On this basis, the use of water in the treatment is very versatile because water keeps hot or cold and it surrounds the immersed body part (Becker, 2009). However, Table 3.1 (p.81) shows immersion temperatures for variable conditions. For instance, it is believed in some controversial studies that cool plunge tanks at a temperature of 10-15°C are sometimes used to treat overuse injury, to relieve severe muscle pain and speed recovery in athletic individuals (Becker, 2009).
Additionally, therapy pools with temperatures in the range of 27-29°C are used for less active patients such as those with multiple sclerosis or who require vigorous exercise (Becker, 2009). The most common type of therapy pool operates in range of 33.5°–35.5°C (Becker, 2009), they are commonly used in arthritis, spinal cord injury programme, Parkinson’s programming, cardiac rehabilitation and other typical aquatic therapy indications. This temperature will produce therapeutic effects, and despite long sessions in the pool, sufficient
80 exercise will be achieved (Becker, 2009). Similarly, hot tubs are usually kept at 37.5°– 41°C, although 41°C is not recommended for active comfortable exercise, and may be used only for short periods of relaxation (maximum of 5-10 minutes).
Use for long periods in cases of severe/recent bruising or circulation disorders should be avoided (Becker, 2009;Butler, 2005).
81 Table 3.1: Immersion temperatures (in °C) for rehabilitation issues. Reproduced with kind permission from Becker, 2009.
Many types of equipment exist for use in aquatic exercises. Floatation devices are sometimes employed to help the practitioner control and increase exercise intensity by using buoyant support to the body, for example to alter positioning or movement, challenge problems, assist balance and generate resistance to movement (Schrepfer, 2002). These devices have advantages in increasing the buoyancy effect to offer support, reduce compressive forces and lessen impact, increase resistance in movement away from the water’s surface and assist movement to the water surface (Edl et al., 2004; Schrepfer, 2002). They are useful for support and balance and as resistance instrument (Edl et al., 2004; Schrepfer, 2002).