**1.2. Why knowledge of muscle activity during exercise in a water environment is important?**

When exercising in water, the benefits of buoyancy and water resistance mainly occur be‐ cause these qualities act directly on the exercise motion. For example, weight reduction, due to buoyancy,maycontributetotheupwardordownwardmotion,andadditionalloadingbywater resistance would strongly affect the motion in any direction. Therefore, the instructors of water exercise need to consider the effect of water specific qualities can play on the human body.

As a method of understanding the loading or offloading effect of water specific qualities on human body, investigation of muscle activity is considered an important tool. This provides information regarding the degree of muscle loading during exercise in water, and also provides a basis to apply health and rehabilitation exercise in a water environment more effectively. It is easily hypothesized that the muscle activity would be changed during exercise in water, compared with the same exercise on land. However, there is a challenge in measuring muscle activity in a water environment.

In consideration of these points, firstly, this chapter focuses on the methodology and its validity of collecting muscle activity data in a water environment, with special mention regarding a waterproofing method. Following this, the muscle activity modality for walking, running and many other exercise forms in water are also featured. Walking and running exercise in water are the most popular and basic exercise forms of water exercise [9]. This chapter focuses on walking, running and other water exercise forms which is based on gait, for example, walking with long step, walking with twisting, walking with kicking and so on. Finally, this chapter provides an informative suggestion for exercise participants and instructors.
