Studies report that 30–40% of people older than 65 will fall at least once per year and about 10–20% of these falls will result in hospitalization [1, 2]. The number of people aged 65 and older will increase due to the demographic developments worldwide, which will further increase the total number of falls . Major risk factors for falling are an age-related decrease in functional capabilities, especially in balance control and muscle strength [4, 5]. Multidimensional training programs have been shown to ameliorate these risk factors and reduce fall risk in older adults. This is especially the case when strength training and sufficiently challenging balance exercises are provided for at least 3 h per week [1, 2, 6, 7]. However, ongoing participation in a training program is needed to prevent fading of the benefits due to the progressive strength and balance decline caused by aging [2, 6]. As long-term, structural supervised training is costly, home-based training appears most promising for long-term effects. Sadly, adherence to traditional home-based training programs is low due to the repetitive nature of the exercises, lack of perceived usefulness and therefore motivation [8, 9].
The use of computer games to aid in balance training for older adults, also called exergames balance training, receives increasing attention [10,11,12]. In this study, exergames are defined as computer games using commercial consoles as the Wii and the Kinect console and that are controlled with body movements. Different commercial games are already available that might have a balance training potential [10,11,12]. Potential benefits of exergames over conventional training are: an increase in motivation and thereby adherence , the option to offer dual task training , the option to provide different forms of feedback  and to adapt the training intensity to the skill level of the player so that individualized progression is possible. However, the latter is not always possible in commercial games. Despite these promising features, systematic reviews report varying results on balance [10,11,12], possibly due to the wide variability in games that have been studied and the fact that these games were not specifically developed with the aim to improve balance in older adults. In conventional balance training, strength and specific balance training were shown to be key elements in preventing falls [2, 6, 16, 17]. It is recommended that balance training is sufficiently challenging by requiring weight-shifts to the limits of stability, by reducingthe base of support (BOS) , or by adding a cognitive task. For strength training, it is recommended in literature that the muscles are sufficiently challenged by increasing the intensity of the exercises or the number of repetitions, so that the muscles will fatigue . The American College of Sports Medicine defined the threshold for hypertrophy and strength gains to be 60% of the one-repetition maximum . However, exercises with external weights are unpractical in VR training, which is often performed at home. Recent research showed that strength exercises at low loads, but with high velocities, can induce muscle activations comparable to training with high loads . Furthermore, these low-load exercises also seem to induce benefits for strength and balance in older adults . Finally, ongoing participation in the training program is recommended to prevent fading of the gained benefits . A study that analysed the challenge of balance provided by off-the-shelf games showed that balance is challenged to a varying extent, but that ample room for improvements is left. Moreover, it was found that adaptation to or learning the game, as trials advanced, resulted in a decreasing challenge in some games [22, 23]. From the analysis of muscle activity in seven off-the shelf games, it was concluded that overall muscle activation was low and that longer periods of muscle activation were scarce . Only the games that required faster movements elicited some muscle activity that seemed challenging enough to be considered as a training impulse .
The motivational pull of exergame balance training with off-the-shelf games, was assessed in older adults and results showed that playing exergames can lead to strong intrinsic motivation . Especially games that provide positive feedback resulted in high intrinsic motivation. Furthermore, physically active games containing variation seemed to be the preferred game mechanics .
Based on the above summarized recommendations for balance training (e.g. sufficiently challenging balance tasks and strength exercises that lead to muscle fatigue), an exergame package for balance training for older adults was developed [2, 4, 6]. The aim of the current study was to evaluate whether the novel set of exergames (called Virbal), which are controlled with off-the-toy-shelf technologies, are feasible and well-suited from a content perspective for balance training in elderly. The novel games were evaluated to see whether they were more challenging in terms of balance movements and muscle activity than existing off-the shelf games. Furthermore, the novel exergames were evaluated on how motivating they are for older adults. Games were compared regarding the challenge imposed to balance in terms of magnitude of center of mass (COM) displacements and regarding the muscle activation elicited in terms of intensity and duration of muscle activation. Motivation was evaluated using questionnaires on motivation.