*2.2. Procedures*

All participants were familiarized with SJ, CMJ, and CMJAS techniques one day before testing at the same place where the testing was conducted. Assistants also have introduced the participants

with the proper technique before testing by video and live demonstration and the explanation of the correct technique.

Before testing, they carried out a standardized 10 min warm-up that consisted of lower-body dynamic stretches, jogging, skipping, and vertical jumps based on similar jump warm-up protocols used in previous studies [15,17]. Their body mass was measured to the nearest 0.1 kg with electronic scale TANITA BC 540 (TANITA Corp., Arlington Heights, IL, USA) and body height with a stadiometer (SECA Instruments Ltd., Hamburg, Germany) to the nearest 1 cm. The leg length and height with bended knees at about 90◦ were measured using a measuring tape to the nearest 1 cm. Leg length was measured from the anterior iliac spine to the tiptoe in the laying position. Height at 90◦ was measured vertically from the anterior iliac spine to the ground in an optimal jump performance position (the angle at approximately 90◦). Then, each participant performed three SJs, three CMJs, and three CMJs free arms with the instruction to jump as high as possible. For all jumps, it was recommended that the participants leave the floor at take-off with the knees and ankles extended and land in a similarly extended position [18]. Between the trials, there was a two-minute passive rest. The highest jump of each technique was taken into analysis. The jumps were recorded with the Optojump photoelectric cell system (Optojump photocell system; Microgate, Bolzano, Italy) and with an iPhone X (Apple Inc., Cupertino, CA, USA) through My Jump 2 app at the same time. The participants repeated the testing procedure after two weeks with the same conditions and in the same order as during the first testing.

#### Squat jump performance [19]

Participants were instructed to start the jump in the position of 90◦ knee flexion with the feet shoulder-width apart and with their hands on their waist. They were asked to jump for maximum height and maintain their hand on the waist. Counter-movement was discouraged, and in case of any mistake, the jump was repeated.

#### Counter-movement jump performance [20]

The CMJ starting position was a standing position with a straight torso and knees fully extended with the feet shoulder-width apart. Participants were asked to keep their hands on their waist throughout the whole jump. They were instructed to perform a quick downward movement (approximately 90◦ of knee flexion), and afterward a fast upward movement to jump as high as possible.

#### Counter-movement jump free arms performance

The CMJAS technique is similar to CMJ with the exception of arm movement. Participants were instructed to swing back with their arms during downward movement and forward during upward movement.

#### Optojump photoelectric cell system

The Optojump system consists of two parallel bars placed approximately 1 m apart and parallel to each other (see Figure 1). The bars are equipped with 33 optical light-emitting diodes (LEDs) with continuous communication of the transmitting and the receiving bar. The LEDs are positioned 0.3 cm from the ground level and at a 3.125 cm interval. The height of the jump is calculated as follows: h = 0.5g × t2, where h is the height of the jump, g is the acceleration of gravity, t is half of the flight time.The Optojump achieved strong concurrent validity for jump height in comparison with the force platform (ICC = 0.99; 95% CI (confidence interval) = 0.97; 0.99; *p* < 0.001) and was recognised as an reliable instrument for field-based vertical jump assessments [18].

#### My Jump 2 app

The app My Jump 2 for iPhone X was used to calculate the jump height by manually selecting the take-off frame and landing frame (Figure 1) of the video. The app determines the jump height using the equation h = t2 × 1.22625 described by Bosco et al. [21] where *h* stands for the jump height (in meters) and *t* for flight time (in seconds). All collections were made with the same phone and by the same evaluator with no professional experience in video analysis. The evaluator was always recording from the same position (approximately 1 m height) and with the same distance from the participants (approximately 1.5 m), enabling the clear view of participants lower limbs. We used the sagittal plane because it showed that identification of the exact take-off and landing frames was more easily viewed, compared to a frontal plane view [22].

**Figure 1.** Take-off and landing phase frames on My Jump 2 app.

#### *2.3. Statistical Analysis*

Descriptive statistics were presented using means and standard deviations. Shapiro–Wilk test was used to check the data normality. Systematic bias between sessions and tools was evaluated using the paired samples *t*-test [18]. Standardized differences in mean (with 95% confidence intervals; CI) were calculated to determine the magnitude of the change across and between tests. According to Hopkins et al. [16], Cohen d effect size (ES) magnitudes of change were classified as trivial (>0.2), small (0.2–0.5), moderate (0.5–0.8), large (0.8–1.60), and very large (>1.60). Reliability between test-retest was analyzed using intraclass correlation coefficient (ICC), typical error (TE) expressed as coefficient of variation (CV%), and smallest worthwhile change (SWC) according to Excel spreadsheet provided by Hopkins (2007) [23]. Regarding the ICC analysis, a single measure, two-way mixed, absolute-agreement parameter was used [24]. The highest jump from each subject on both testing sessions, retrieved from the My Jump 2, was used. ICC was interpreted as <0.1 = low, <0.3 = moderate, <0.5 = high, <0.7 = very high, <0.9 = nearly perfect, and <1.0 = perfect. A good reliability was considered if following criteria was fulfilled: CV < 5% and ICC > 0.69 [25]. Test usefulness was determined based on the comparison of SWC (0.2 multiplied by the between-subject SD, based on Cohen's ES) to TE [26]. The following criteria were used to establish the usefulness of tests: "Marginal" (TE > SWC), "OK" (TE = SWC), and "Good" (TE < SWC).

The concurrent validity of the app was tested with Pearson's product-moment correlation coefficient (r). Additionally, the agreemen<sup>t</sup> between Optojump and My Jump 2 data was then examined

graphically using Bland and Altman's plots in which the difference between both devices was plotted against the mean of the two devices [27].
