###### Abstract  
This study aimed to determine the criterion validity of the ADR jumping photocell when compared to a motion capture system and to examine the within-session test-retest reliability of the instrument. Thirteen highly trained female volleyball players performed ten countermovement jumps (CM) with two minutes rest in-between trials. Jumps were recorded simultaneously by both devices to compare the outcomes. The main results showed significant mean differences between devices (10.6 cm, \(p\) < 0.001; ES = 0.9), high Spearman's correlations (\(p\) = 0.95), weak CCC (0.25), and SEE = 0.92 cm. [PERSON]'s plot indicated high systematic errors (10.5 cm), heteroscedasticity of error (slope = 0.026), and all values included among the limits of agreement. The within-session reliability (internal consistency) was very high (ICC = 0.96; CCC = 0.94; SEM = 1.18 cm; CV = 4.21%). In addition, the sensitivity of the instrument showed values of SWC = 0.33 cm. In conclusion, _ADR jumping_ is a valid instrument for the estimation of CMJ in highly trained female volleyball players. Furthermore, the instrument is shown to be a consistent tool in the measurement of CMJ. validation; error; countermovement jump; MoCAP; portable devices; infrared barrier; sensitivity; consistency +
Footnote †: journal: _Applied Sciences_  
research and high-performance athletes and sports teams, who can afford these expensive technologies. [12]. In recent years, lower-priced instruments, such as accelerometers, jumping mats, or systems based on video recording, have been developed as an alternative to MoCAP and FP and have enabled physical activity and sports specialists to monitor their athletes [21; 22]. Accelerometers can detect accelerations in all axes, allowing for the estimation of jump height through double integration, similar to force plates [23]. Inertial Measurement Units (IMUs) comprise accelerometers and are a convenient device for obtaining real-time data, but the accuracy of the obtained data may be affected by the sensitivity and error of the accelerometers [24]. The estimation of jump height based on flight time (FT) has led to the development of various instruments, such as jump mats and linear position or electronic transducers, but these methods require further processing and may be expensive or not usable on all surfaces [25; 26; 27; 28; 29; 30; 31]. Jump mats are a low-cost and reliable option for estimating jump height from the time between take-off and landing, but they can only be used on specific surfaces, limiting their ecological validity [18; 20; 32; 33; 34]. Finally, photocells have traditionally been used for the measurement of in-line speed, but they are also being developed for time-of-flight capture, specifically, and their use has become widespread along with that of FP [35], as their logistics and manageability have significantly improved at cheaper prices [36]. In this way, photocells are shown to be valid instruments for V) estimation when compared to MoCAP systems and FP [20; 37]. The main advantage of these systems is that they allow the acquisition of data from several subjects on the same surfaces where physical activity is to be performed, adding ecological value or content validity to the measurement process [38]. These devices consist of two bars: an emitter and a receiver that can be placed on a variety of grounds [39]. The _Optojump_ photoelectric system is a technology that uses an infrared barrier to estimate V) height. It has been shown to be valid and reliable for estimating V) height compared to force plates when flight time is used for estimation [39; 40]. However, when estimating jump height from the double integration of reaction forces using force plates as the reference instrument, the system's validity is limited, and high systematic errors (10.4 cm) have been found [41]. The _Wheeler_ jump photoelectric sensor has been demonstrated to be a valid and reliable device for estimating V) height based on flight time.