**1. Introduction**

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#### **1.1. Why exercise in water has become popular?**

The physical qualities of water are well established and include buoyancy, water drag force, hydrostatic pressure and thermal conductivity [1]. The large difference in these physical qualities, compared to land-based activities, affect the human body in both physiologic and biomechanical aspects. An example of this is buoyancy, which acts vertically against gravity on the immersed object thus decreasing weight of the human body. The buoyancy level is equal to the mass of water displaced by the immersed object and is based on the accepted Archime‐ dean principle. When a human is immersed in water up to the level of pubis around 40% of weight is accounted for, 50% at umbilical, 60% at xiphoid, and almost 80% at the level of axillary [1, 2]. When immersed to their lower limb joint and waist in a water environment, humans can easily move, without gravitational overload, due to the buoyancy effect.

Water drag force is composed of three types of drag; surface drag, form drag and wave drag [3]. Surface drag is affected by viscosity of water and the surface quality of the immersed object. For example, the roughness of the surfaces of the object might increase surface drag, however, this depends on the speed of movement of the object as seen in the decreased drag on a golf ball due to the dimples [3]. Form drag depends on the shape and size of the immersed object, whilst wave drag directly opposes the object's movement through water. Water drag force increases proportionally by the frontal projected area and square of moving velocity [4]. Therefore, humans need to exert a much greater force to overcome water drag force when moving in water due to a greater frontal surface area [3].

Hydrostatic pressure is related to water density and depth. Water density is about 800 times greater than air. When at a water depth of 1 meter, hydrostatic pressure increases about

73.5mmHg [1], which is similar to normal diastolic blood pressure. Hydrostatic pressure causes blood to shift from the lower extremities to the thoracic region [5], and also affects lung, renal and other endocrine functions [6]. Thermal conductivity, which is about 25 times faster than air, also affects the human circulatory system [1]. When a human is immersed in water with a lower than thermo-neutral temperature, peripheral vasoconstriction occurs and increases the blood shift to the thoracic region [7]. This thermoregulation effect has been used for a range of therapeutic practices [1].

this risk, most research has used silver-silver-chloride surface EMG electrodes as passive surface electrodes [10, 11]. Some researchers who have investigated muscle activity using EMG devices developed their own waterproofing methods [10, 11] of which there are two types. The first type attached a waterproof seal onto the electrodes placed on the local muscle of interest [10, 11], and the second wears a dry suit on the whole body created for EMG collection [10, 12]. Waterproofing the seal onto the electrodes usually involves attaching transparent film and/or a foam pad (Foam Pad, 75A: Nihon Kohden, Tokyo, Japan) and this method has been used successfully in previous research [10, 11]. For even better waterproofing, transparent film and/ or foam pad are attached in combination [10, 11]. This is achieved by sealing a small piece of transparent film over the electrodes which are attached on the muscle belly to be studied. Then, fill up the slight gap between electrodes leads and transparent film with putty. Finally, use a

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large piece of transparent film to cover over the small size film and putty (Figure 1).

Even when this strict waterproofing is applied the water can sometimes immerse under the films cause interference with the electrodes. Therefore, when using this method, the EMG signal has to be monitored continuously throughout the experiment to ensure the data is not affected by water intrusion. When water intrusion under the films or electrodes occurs, a high frequency noise and/or baseline fluctuation on the wave data would be observed. If an abnormal data reading is confirmed due to water intrusion, the electrodes attachment has to be removed and re-applied and the experiment procedure should be started over again. In addition to the waterproofing to the electrodes, the code of each electrode should be taped along with body segment to avoid unexpected tension by water resistance and swinging by exercise motion in water which may prevent or interfere with the normal motion (Figure 2). Due to the series of extensive electrode attachments, waterproofing procedures and establish‐ ing the settings for collecting data of underwater EMG, the procedure for underwater EMG

takes considerably longer to perform compared with testing in the land environment.

A whole body dry suit has been developed for the collection of underwater EMG, specifically for water sports activities to allow full range of motion [10, 12]. This suit consists of waterproof material with the arm, leg and neck openings tightly sealed. Once the electrodes are attached on the subject, the electrodes and its leads are well covered from the water due to the fully enclosed suit. The advantages of this suit include less set up time than waterproofing and

A B

**Figure 1.** Waterproofing of surface electrodes. [Pictures were taken by the authors]

A: doubled by transparent films with putty. B: covered by foam pad.

As briefly described above, the water physical qualities can be very beneficial for the human body. In addition to those effects, exercising in a water environment is a safe environment and may reduce the incidence of falling [8]. A wide range of people including those who have difficultymovingonlandduetoobesity,alower-limbdisorder,throughtothosewhoarehealthy or want to improve their fitness may benefit from utilizing the water environment [8, 9].
