## 2 Materials and Methods
### Virtual Reality Gaming Development  
The development of this system required exploratory research based on open interviews with physiotherapists who work directly with professional soccer players. After a qualitative analysis of the transcribed interviews, a list of physical rehabilitation exercises emerged and was validated among all stakeholders. The list of combined exercises makes up for an entire session of musculoskeletal injury rehabilitation. The session was designed and developed in a soccer-themed virtual environment, integrating games mainly for soccer players (Table 1). A set of five customized VR games (i.e., \"Weight Transfer\", \"Military March\", \"Side Squat\", \"Progressive March\", and \"Walking along a Straight Line\"; see descriptions below) for soccer players were used to provide a complete rehabilitation session after a musculoskeletal injury. The VR games contain specific soccer motives and other gamification techniques that help keep users engaged in the process. Although each VR game has been designed to cover the main training domain (e.g., aerobic endurance, upper/lower strength, and motor ability), several game parameters were used to aid the content personalization towards meeting the player's needs. Weight Transfer: The weight transfer exercise requires the user to put one foot forward and one foot behind. The purpose of the exercise was to transfer the weight forward and back by raising the toe of the leg that was in front, transferring the weight of the user backward, lowering the toe, and raising the heel of the foot of the leg that was behind to transfer the weight of the user forward. Considering this, the game developed required the user to lift the left toe to select a color to \"shoot\" signaled in the goal. After selection, the user had to rest the toes of the foot and finally lift the right heel. Raising the right heel activates the laser (used to aim) and the timer (time the user has to select one of the circles). When the user selected the right circle by pressing the trigger from the command, they gained points, and if they chose the wrong one or did not select it in the required time, they lost points. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{c} Balance and \\ coordination \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of each VR game in terms of gamification, movement purpose, and the main functional fitness capacities involved. Military March: The exercise requires the user to raise the left knee until it forms a 90-degree angle with the belly and stretches the right arm simultaneously, then repeat for the opposing members. Considering that aspect, a dartboard was created that the user had to point with their knee and only shoot when the hand was lifted. To implement this idea, two balloons in the air were put in the environment responsible for knowing if the user raised the requested hand. To know how high the balloons had to be, we added the height of the user with the size of their arm when calibrating the avatar. To make the user raise their knee, there was an integrated laser that only appeared when the user lifted their knee to the height they wanted. When the user accomplished these two movements, the user could aim at the dartboard and press the hand's controller, which was lifted and earned points. To penalize the user when losing balance, the knee was raised, and the user lost points and received audio feedback. Side Squat: The side squat requires the user to step sideways while pushing the hips back and bringing the torso toward the floor, then stand on the other side and prepare to repeat the back-and-forth movement. The development of this exercise was divided into two parts: the first part was necessary to create something that obligated the user to move to the left and right sides. For that reason, a ball was implemented that went to the left and right sides. The second part was implementing something that obligated the user to crouch down as necessary for the exercise.