The success of combat sports is often determined by fast motor actions in response to a given stimulus [1-3]. Response time, defined as the time required to perform a voluntary movement after a stimulus [4,5], is an important variable that can be improved and monitored in combat sports conditioning [6-9]. A greater success during competition could be associated with faster response time value [10,11].

To measure response time from a given visual or audible stimulus, different systems have been proposed to determine the time to perform a specific task or motor action. Most systems use cameras with high sampling frequency [12,13] or accelerometers [14,15] to obtain spatial and temporal information of a determined motor gesture or task. Moreover, some systems measure response time based on visual stimuli shown on a computer screen in time intervals as previously defined. In these cases, response time to visual stimulus is measured from the moment that the user presses the computer keyboard [16,17].

Despite the quick results provided by those systems in which computer keyboards are used, they lack specificity (specifically motor gestures) and ecological validity, as combat sports actions are not performed during the task. In addition, systems involving kinematics (motion analysis) or accelerometry to measure response time usually require more time to prepare, collect, and analyze the data. Moreover, these systems are expensive and often have low applicability to athlete training routines.

To minimize the aforementioned limitations, the use of a specific mobile phone app (TReaction app, ETS4ME, São José, SC, Brazil) has been proposed to determine the response time of different motor actions (strikes) with a low-cost, highly applicable, and ecologically valid solution. The TReaction app measures the motor response time between the flash fire and sound waves produced from contact with the target through the mobile phone microphone. However, as far as we know, the response time measurement validity obtained by the TReaction app has not been tested. In fact, the use of mobile apps has been growing exponentially in fitness, health, and sports science, which raises the need for scientific evidence [18]. Therefore, this study aimed to test the validity and reliability of the TReaction app in measuring motor response time in combat sports.

Data collection for this study was performed in 2 sessions. In the first session, a response time test was performed using the TReaction app simultaneously with a high-speed camera (visual system). In the second session, the TReaction app measurement reliability was validated with a repeated measures design using a simulator. This simulator was programmed to verify the accuracy of the visual stimuli and the response time with previously programmed fixed intervals.

The 2 subjects of this study were selected intentionally. These individuals were male black belt athletes with significant experience in regional and national taekwondo competitions. Furthermore, each selected subject had to have at least 10 years of experience in a modality of combat sport. Before participating in the study, all participants were informed and provided written consent about the specific aims and methods of the study along with possible study risks. The local research ethics committee approved this study (protocol number 145.882). All procedures were performed in accordance with the Declaration of Helsinki.

Descriptive statistics (mean [SD]) were used to present the results, and normality was verified using the Shapiro-Wilk test. Pearson linear correlation was used to verify the relationship between response time obtained by the TReaction app and high-speed camera. A paired Student t test was used to test differences between both methods. The magnitude of the differences was verified from the effect size (g), and it was then scaled for trivial (0.25), small (0.25 to 0.50), moderate (0.50 to 1.0), or large (1.0) effects [19]. The typical error of measurement (TEM) was estimated by dividing the SD of the difference score by √2 [20]. The Bland and Altman [21] analyses were used to test agreement between the methods. The intraclass correlation coefficient (ICC) was established to verify the accuracy of repeated measures. Data processing and analysis were performed using Microsoft Office Excel 2007, SPSS 17.0 (IBM Corp) and GraphPad Prism 5.01. The statistical significance level was set at P<.05.

This study aimed to verify the validity of the TReaction app, which was designed to evaluate the motor response time of combat sports athletes. In addition, this study aimed to verify if the setup used by the TReaction app to determine the beginning of the task (flashlight stimulus) and the time of contact with the target (time to perform the task) would reach acceptable levels of accuracy. To the best of our knowledge, this is the first study to investigate the validity of the TReaction app. The main results showed no differences between the mean response time of all strikes evaluated using the TReaction app and those obtained by the high-speed camera, which is supported by the effect size observed (trivial). The response time results showed a strong correlation between both TReaction app and high-speed camera (gold standard) methods. Besides, the TEM in our study (9.2 ms) is lower than the value (30 ms) observed previously with single-beam photocells across short-sprint studies [22].

When using the TReaction app, considerations must be given to the values of differences by the inference and the degrees of agreement of the Bland and Altman plot. Our results show an average difference of 13 ms for response time between TReaction app and high-speed camera. Response time presented a bias of 13.05 ms, and the 95% limit of agreement for the differences between the high-speed camera and TReaction app was 27.06 ms with 3 individual samples slightly outside the limit of agreement. Although the response time observed in our study presented bias higher than the value (0.35 ms) detected by [23], the variance of the magnitude of different values can be considered similar to the values observed by the authors (95% limits of agreement −11.78 to 12.39 ms). In the above-mentioned study, the authors also observed 3 individual samples slightly outside the limit of agreement, and the difference between systems to measure response time during the task was higher than 17 ms for an average time of 246 ms.

This study demonstrated that the difference between the TReaction app and high-speed camera observed is associated with the distinct method that the TReaction app uses to calculate the time between the visual stimulus (flash fire) to perform the task and the moment to determine the motor response time. The TReaction app calculates response time based on the sound wave generated by the impact with the target object. This measure presented a high ICC and no differences between the audio output and the previously programmed computer software.

The kinematics is among the most accurate systems used to measure motor gestures associated with performance such as vertical jump [24,25], especially when the goal is to accurately measure the performance of motor gestures in combat sports [13,26]. However, high cost and low applicability of these kinematics systems make it challenging to control essential parameters in combat sports. Therefore, the cost-effectiveness and high applicability of utilizing the TReaction app make it a viable alternative for accurate response time measurements in combat sports.

One limitation of the TReaction app may be associated with the small variability observed in the firing time of the flash over repeated measures, which increases the probability of incorrectly identifying the moment of the initial stimulus to perform the task. However, in this study, a high-speed camera (1000 Hz) was used to establish the response time with maximum accuracy, and no differences were observed when compared with the response time values obtained by TReaction app. Another aspect to consider when interpreting our findings is regarding the limited sample size, as a higher variability can be expected from international and national level athletes (coefficient of variation 2.9% and 6.1%, respectively) in response time when repeated measures of kicks are performed [27], which means that our perspectives are limited for this group. Although we believe that it did not reduce the strength of our findings, future studies should be encouraged to investigate TReaction feasibility in different modalities and subjects’ characteristics, such as competitive levels.

Finally, the reliability observed for computer screen color change time, previously programmed to occur simultaneously to the auditory stimulus at predefined times, was also verified. Results observed allow the use of the screen color change as an alternative to be used as a visual stimulus to initiate a motor task. Therefore, we suggest that the TReaction app can accurately measure response time during different techniques used in combat sports. This is relevant because response time is an important variable for combat sports [8,10,15,26]. As the TReaction app is a viable alternative to obtain and control the response time for combat sports athletes, it can possibly be used by athletes in other sports, physical trainers, and coaches to obtain time interval measurements between stimulus onset and the response time required for contacting a target with a strike.

The results of this study suggest that TReaction app is a valid and reliable tool for measuring response time in combat sports. However, the TEM and observed interference of agreement should be considered when comparing the response time values obtained by TReaction app with other kinematic measurements.

The TReaction app provides an easily executable test that provides relevant sport-specific data regarding motor response time. Outcomes from the TReaction app suggest that athletes and coaches involved in combat sports can obtain reliable continuous motivational feedback leading to accurate diagnostic and performance criteria. The TReaction app can achieve and better control training-induced adaptations as well as help choose interventions that will help achieve maximum combat sports success. In addition, the TReaction app showed to be a valid tool that is both cost-effective and highly applicable to combat sports practitioners.