## 1 Introduction  
Immersive virtual reality (VR) has grown rapidly over the past several years and is widely used in many areas of life and science. It is used in healthcare, education, engineering, manufacturing, and entertainment, among others [1; 2; 3; 4; 5]. There is also a growing interest in using this technology in athletic training to diagnose and control athletes' motor skills and improve their sports performance [6; 7; 8; 9]. Active virtual reality games (AVRGs), which require the user to do a physical exercise, are also becoming popular [10]. This type of physical activity (PA) offers an alternative to typical computer games, practiced in a sitting position that is unfavorable to health. Potential health-promoting effects of AVRGs can be expected if the accompanying PA is sufficiently high. According to current World Health Organization (WHO) guidelines, the health benefits come from aerobic exercise that is at least moderate in intensity [11]. Therefore, studies have been conducted to assess the intensity of PA during AVRGs and its determinants [12; 13; 14; 15; 16; 17; 18; 19; 20]. Currently, popular consumer VR headsets consist of a head-mounted display (HMD) and a pair of controllers that are held in the hands of the user. The most common ways to use such equipment are to play sitting or standing, with movements performed mainly using the upper limbs. Unfortunately, this form of exercise may not guarantee appropriate PA intensity for health-related benefits [14]. The question arises of how to solve this problem. One solution is to use trainers that are compatible with VR headsets, such as cycle ergometers, rowing ergometers, and treadmills. Recent studies using this type of equipment have shown promising results [21; 22]. Cyclic locomotor movements promote PA intensity and enhance the immersion experience. The first studies on biomechanical analysis of movement in a virtual environment have also been conducted [21; 22]. It was observed that PA using exercise machines in a virtual environment can be more intense than similar traditional training [17]. Another simple way that can increase the intensity of exercise for individuals using AVRGs is to add external load, provided that it does not interfere with the use of the application. Preliminary studies using wrist-mounted Velcro-fastened handheld weights (HHWs) showed that this solution could be comfortable for users and significantly increase the intensity of physical exercise (based primarily on arm movements) from low to moderate [23]. Given the effectiveness of these two solutions, it was assumed that their combination should help reach the high PA intensity needed during AVRGs. Evaluating the effects of such an approach was the inspiration for the experiment described in this paper. However, ankle weights (AWs) were used instead of HHWs, as the AVRGs connected with the omnidirectional treadmill primarily engaged the lower limbs. It should be emphasized that AWs are sports equipment used in jogging, fitness, rehabilitation, etc., and wearing AWs can help improve gait, lower limb strength, and balance [24; 25; 26]. Although AWs are commonly used in training, there is a lack of research into their impact on improving the effectiveness of exercises in VR. It is also unknown whether their use will be comfortable for users playing AVRGs on an omnidirectional treadmill and whether it will reduce the attractiveness of virtual entertainment and, consequently, the motivation to exercise. It is also difficult to determine whether and to what extent this will affect the intensity of PA. Taking into account all the aforementioned issues, the main purpose of the present study was to evaluate the effect of external leg loading in the form of AWs on the intensity of physical exercise in young, physically fit adults playing AVRGs on an omnidirectional treadmill that allows for performing locomotor movements.