## 1 Introduction  
Stand-up paddle boarding (SUP) is a Hawaiian paddle sport derived from surfing. Unlike traditional surfing, during which participants sit on a board until a wave arrives, SUP requires that boarders stand on a board and use a paddle to push themselves on water. The variations of SUP include flat-water paddling for outdoor recreation, fitness, or sightseeing, paddle-board racing, and paddle-board yoga. By 2003, the Encyclopedia of Surfing had never reported on SUP [1]; however, a few years later, according to the SUP industry statistics, the sales revenue of stand-up paddle boards exceeded US$720 million [2]. SUP has become the world's fastest growing water leisure activity [3, 4]. This water activity can be performed in different environments, such as calm lakes, rivers, or open occans, and people typically participate in SUP for one or more hours. Participants must learn how to use the power of various body parts to paddle in different environments [5] and maintain dynamic stability at all times. To maintain ideal paddle posture, muscles in trunks, upper limbs, and buttocks and knee and ankle joints are activated and coordinated throughout movement and constantly stimulate balance perceptual receptors to maintain stable paddling when standing on an unstable board. Continually and correctly performing a correct paddle action when balancing the interference from the board buoyancy and the blade resistance is challenging. Contracting core muscles is necessary to establish a stable basis for paddle movements that require constant muscle activation and sufficient strength to maintain long-term activation. The goal of SUP is using a kinetic train to transfer the core strength to upper limbs to enhance propulsion power.  
Many websites and reports claim that long-term participation in SUP can improve muscle strength, fitness, core muscle stability, and balance as well as reducing lower back pain. Although SUP is also generally regarded as an effective full-body workout, few studies are in progress, and little scientific evidence can confirm these benefits. Studies related to SUP have included only research on topics such as sports injury epidemiology [6; 7; 8], body composition analysis, and respiratory metabolism [9; 10; 11; 12]. Other studies have analyzed the effectiveness of exercise training intervention [13; 14]. One biomechanics study [15] used surface electromyography (EMG) to observe muscle activity during SUP on a training machine and at sea, demonstrating that muscle activation during the water-based test started sooner and was maintained longer than that during the dynamometer test. They concluded that the training effect on water was more beneficial than that on the training machine because of longer myoelectric signal activation [15]. These studies have mostly focused on professional paddle boarders and SUP competitions rather than recreational paddle boarders or beginners.  
During performing long-distance leisure paddling, paddlers must often alternate between kneeling and standing positions in accordance with personal physical conditions. Until now, the biomechanical understanding of paddling in these two postures remains limited. Regardless of whether SUP is considered from the perspective of exercise training or exercise fatigue, understanding the difference between the two postures is necessary. Therefore, this study examined the muscle group activation in various parts of the body by using EMG signals during SUP in kneeling and standing positions.