Objective: Muscle weakness is an important risk factor for falls in older adults. Intensity and duration of muscle activity are important determinants of exercise effectiveness in combating muscle weakness. The aim of this paper was to assess the intensity and duration of muscle activity in Virtual Reality (VR) balance games. Materials and Methods: Thirty young and thirty healthy older adults played seven different VR balance games. Muscle activity of the Vastus Lateralis, Vastus Medialis, Soleus and Gluteus Medius was obtained using surface EMG. The processed EMG signals were divided in 200ms blocks, after which each block was categorized by its average normalized EMG activity i.e. >80%, 60-80%, 40-60% or <40% of maximum voluntary contraction (MVC). We calculated the total number of blocks in each category to score intensity, as well as the maximal number of consecutive blocks (MCB) >40% MVC, to identify prolonged muscle activity. Results: Muscle activity during game-play was mostly below 40% MVC and prolonged activation was lacking. Only the games that included more dynamic movements showed activation blocks of higher intensity and resulted in more MCB. Conclusion: Our method allowed us to analyze the overall muscle activity and the distribution of activity over a trial. Although the activation levels during these VR games were low in general, we identified game elements that could potentially provide a strength training stimulus. Future research should aim to implement these elements, such that the intensity, prolonged activity and rest are optimized to sufficiently challenge lower limb muscles in VR training.