The participants of this study were five able-bodied and five transradial amputee subjects with a mean age of 26.3 years (SD 4.47), who were recruited from University Malaya Medical Centre (UMMC), Kuala Lumpur, Malaysia. Two out of the five transradial amputee participants had congenital amputation, while the other three had undergone amputation owing to traumatic injury at a mean of 3.7 years (SD 1.69) at study inclusion. One participant was a bilateral amputee; therefore, only one residual limb was considered in this study. The rest of the amputee participants had unilateral amputation (right side for all), and the amputation level ranged from long to short transradial amputation. All amputee participants had never used any prosthetic device since amputation and had no experience with an EMG system. The amputee participants were independent, could perform all activities of daily living on their own, and received no other treatment or rehabilitation throughout the study period. In this study, the amputee participants represented the test group and the able-bodied participants represented the control group.

The EMG signal generated by each participant’s forearm was evaluated before the training session. Four video games (Crate Whacker, Race the Sun, Fruit Ninja, and Kaiju Carnage) and their respective control variations were assessed for suitability to be used in the rehabilitation protocol. All participants had normal vision and were guided throughout the study. Two questionnaires were administered (modified version of the IMI and the user evaluation study). The IMI questionnaire was administered at the end of the session, whereas the user evaluation study was administered at the end of each game on the 10th training session. The test was approved by the Medical Research Ethics Committee and the Ministry of Health Malaysia (approval ID: NMRR-16-2106-32880). The Medical Research Ethics Committee considered that the data collection for this study would only involve physical evaluation. Participants were required to sign a written consent form prior to the tests.

The following four different games were used in this study: Crate Whacker, Race the Sun, Fruit Ninja, and Kaiju Carnage (Figure 1). The games were selected because of the suitability of the main player control to prosthesis usage and the compatibility with the Myo armband (Thalamic Lab). The main control used for Crate Whacker and Fruit Ninja is “make a fist,” which can improve the overall strength of the forearm muscle. The amputee participant’s residual muscle must be in good condition [29,30], and precise EMG control ability is required to control the prosthesis naturally [31]. Meanwhile, Race the Sun and Kaiju Carnage were included because of their ease of control. The game control methods are summarized in Table 1.

The keyboard mapper utility available in Myo Connect was used to assign keyboard and mouse commands to any of five gestures to control the game. The gestures used to control the games included fist, wave in, wave out, finger spread, and double tap. Pan motion from highly sensitive motion sensors embedded in the Myo armband is important as it allows the player to move in the game setting. The custom configuration for each game is exported as connector scripts. In Crate Whacker, the player repeatedly smashes the crate with a mace and destroys them with grenades. The player controls the mace position with the acceleration and gyroscopic data of the armband. In Race the Sun, the player controls a solar-powered spacecraft, dodging various objects on the way, such as laser beams, other ships, and other stationary obstacles, while collecting pick-ups that can be used in the game, with the sun slowly setting on the horizon. The game ends either when the player hits an object and the ship gets destroyed or when the sun sets. Meanwhile in Fruit Ninja, the player needs to slice the fruit into half to collect point. The Kaiju Carnage game enables the player to claw, smash, and shoot nuclear lasers on buildings.

The set up for this study includes a monitor that displays the game that the participant plays with the Myo armband connected via Bluetooth. The system utilized commercially available equipment of the Myo gesture control armband. It consists of eight plastic pods held together by a rubber lining that measures 7.5 inches (19.05 cm) in circumference and can be extended to 13 inches (33.02 cm). There are three medical steel EMG sensors on the bottom of each pod, which are responsible for reading the muscles’ electrical activity and arm movement. With the use of a machine learning process, the armband can recognize the gesture perform based on the electrical activity of the arm in real time. Amputee participants wear the Myo armband on the affected limb. Able-bodied participants wear the armband on the dominant hand. Ten 1-hour sessions were carried out within a 4-week period. Participants were initially instructed to perform the VR BBT designed by the authors, and provisional MVC levels were recorded before the four Myo-controlled computer games were introduced to each participant in a randomized order. Randomization of the games was performed by the study facilitator. At the end of the training session, the MVC value was recorded again, and a modified IMI questionnaire and a simple user evaluation survey regarding the gaming experience, similar to the approach in the study by Prahm et al [32], were administered to the participants. Follow-up MVC values were taken 2 days after the last intervention session.

In order to investigate the effect of the video game rehabilitation protocol used in this study, VR BBTs and basic EMG assessments evaluating approximate coordination and muscle strength were performed. The participants were asked to perform the BBT in a virtual environment (Figure 2). The test was made up of the following two phases: an initial 15-second phase, where the user gets a trial run, followed by a scored 60-second phase. Participants were told to move as many blocks as possible in a span of 60 seconds (one by one) from one compartment to another. The participant’s hand must cross over the partition in order for a point to be given, and blocks that drop or bounce out of the second compartment onto the floor are still rewarded with a point. The instructions are scripted, and they were read to each user according to the manual for the test. The experiments were performed according to the World Medical Association Declaration of Helsinki [33]. The player needs to move the virtual hand near the cube, contract the forearm muscle to pick the cube up, and then stop contracting the muscle to release the cube into another compartment. The score generated on the top left of the screen refers to the amount of successfully transferred blocks, and the red line on the top right corner of the screen refers to the linear timer (Figure 2).

The MVC was recorded for each participant before, immediately after, and two days after playing the game. Participants were instructed to maximally contract their forearm muscle and to hold this contraction for 10 seconds, and the highest value was recorded as the MVC value. The assessment was carried out with the Myo visualization tool (Figure 3) on a laptop with a discrete graphics processing unit (NVDIA GeForce GTX 1060) for signal acquisition, filtering, calibration, training, and prediction. The Myo visualization tool is an open-source Myo armband data reader created using Myo SDK, Qt Creator, and QCustomPlot that streams and displays the EMG signal and accelerometer, gyroscope, and orientation data of the Myo armband.

Two questionnaires were provided for each participant to be completed at the end of the training session, which included a modified IMI questionnaire [3] and a user evaluation survey [32]. The participants’ experiences with each game were evaluated using the IMI questionnaire, which has previously been used with a virtual environment for motor rehabilitation [22-24]. A modified version of the IMI questionnaire made up of five selected subscales was utilized to evaluate participants’ experiences with the protocol. The subscales included were interest/enjoyment, perceived competence, perceived choice, pressure/tension, and value/usefulness. The questionnaire included statements such as “I found this activity very interesting,” and they were rated on a 7-point Likert rating scale from 1 (not true) to 7 (true).

A brief questionnaire about the gaming experience was presented after the completion of each game. This short survey included questions about the gameplay, fun factor, motivation, and input and control methods, and it included the following questions: (1) Did you like this game in general? (2) Did you have fun playing the game? (3) How would you rate the input mechanism? (4) How would you rate the game control? (5) Did this game motivate you? Question could be answered on a 5-point Likert scale ranging from 1 (“do not agree”) to 5 (“agree”). The participants were asked to focus on the differences between each game and to avoid giving the same answer to one statement for all the games if possible.

An important finding from this study lies in the usage of the platform in 10 1-hour sessions within a 4-week period. The BBT is a standard test to assess hand functionality [34,35]. In this study, the test was used in a virtual setup in order to check the participants’ ability to control and operate a myoelectric hand, as the mechanism utilized to pick and place an object in the virtual setting is similar to that for a myoelectric prosthesis. Compared to the mean conventional BBT score for a healthy person as reported by Mathiowetz [34], which is in the range of 78.9 (SD 15.3) to 75.1 (SD 11.1) per 60 seconds, the score for the virtual setup was very low (approximately 74% lower score). However, the mean score increased from session 1 to 10.

The MVC test is a standardized method for measurement of muscle strength [36]. The results obtained demonstrate improvements in muscle strength from pregaming to follow-up assessments. A high increment was observed among amputee participants after playing the game, which is consistent with the finding in the study by Prahm et al [32]. This result may be associated with the electrode resistance that may be affected by a change in body temperature or sweat, which, at the same time, proves that the use of video games in upper limb amputee rehabilitation is not fatiguing for the participants. It can be safely said that certain improvements in EMG control could be detected by the MVC test. Besides, from the results, MVC values recorded for able-bodied participants were greater than those for transradial amputee participants, with the lowest MVC value recorded for an amputee participant with congenital amputation, which may have been associated with muscle atrophy. Atrophy of the remaining muscle occurs when the muscle is not used for long periods of time, and the EMG signal becomes very weak after amputation [37]. Male participants recorded greater MVC values compared to female participants.

Based on the IMI questionnaire and user evaluation survey conducted, the motivational aspects of training gamification were traceable. Participants agreed that the activity can induce desirable physiological changes that are consistent with the findings in the study by Lohse et al [10]. Pressure (psychologically) could be felt by amputee participants while playing the Race the Sun game because of its difficulty level. Nevertheless, the participants were still motivated to play the game according to the survey conducted. It has been reported that enjoyment, control mechanisms, music, direct feedback on scores, and ability to receive upgrades influence a player’s motivation [32], and choice, rewards, and goals lead to increased engagement [10].

Aside from game suitability and main player control, the four games were included in this study because each of the games involved a different engaging element. Crate Whacker is continuous without a specific time limit, scoring system, and level upgrade. Race the Sun and Fruit Ninja have both a scoring system and specific time to complete the game, but the player can receive upgrades only in Race the Sun. Meanwhile, in Kaiju Carnage, a specific time limit is available, but the player receives no score or upgrade. All games, except for Kaiju Carnage, have no background music. The majority of the participants reported that they enjoyed and wanted to continue playing Race the Sun and Fruit Ninja after the training period, which indicated increased participant motivation as reported in previous studies [25-27]. Both of these games provide direct feedback for the player interaction, and the player can see the timer and score obtained clearly while playing, which is supported by the findings of Liepert et al [15]. According to question 3 and question 4, participants preferred the control in Fruit Ninja, as they felt that they were slicing the fruit in the game with their own hands. A significant difference in the score distribution was observed in Crate Whacker and Kaiju Carnage under game control input (P<.001), indicating that amputee participants had control difficulty in these games. In terms of motivation (question 5), all of the participants only felt less motivated while playing Kaiju Carnage owing to lagging in EMG control and long loading time. Participants also reported that this game showed no score while playing, had a low level of difficulty, and was not interesting. In addition, all amputee participants appeared to be struggling to imitate the clawing gesture, but this was not the case with able-bodied participants. This part of the study showed that engaging elements contribute to increasing a player’s motivation during rehabilitation.

This study demonstrated that a video game–based rehabilitation protocol can be used as a complementary method for upper limb amputee rehabilitation. Participants showed greatly improved muscle strength, coordination, and control. A good muscle condition and induced neuroplasticity enabled better control in the BBT, which is related to readiness to use a myoelectric prosthesis. Participants agreed that this activity is beneficial to them both behaviorally and psychologically. According to the questionnaire responses, it could be shown that the use of video games maintains patient interest during training, therefore improving patient motivation and exercise intensity and enhancing adherence throughout the rehabilitation period. The engaging elements in the game could be identified with the questionnaire. Video games with high intensity and difficulty create pressure for the player, but not all Myo-controlled video games are suitable for transradial amputee rehabilitation.