Optojump

The experimental devices to measure GCT were the inertial measurement sensor PARTwear (PW; HuCE-microLab, University of Applied Sciences, Biel, Switzerland) and the optical measurement system Optojump (OJ; Optojump Next, Microgate, Bolzano, Italy). As gold standard, a high-speed video camera system (HSC; Camera Marathon Ultra CL600, Videal AG, Niederönz, Switzerland) was used. Data assessment took place on a completely even indoor track and participants wore their own running spikes, alike during competition. To ensure measurement system accuracy for a range of speeds, each subject was asked to perform three sets of four runs over a distance of 40 m at individual maximal sprinting, intense, and normal training speed. Speeds were self-selected to allow for natural running technique. For each run, one step was analyzed 30 m from the start. To test the PW's and OJ's validity, GCT were simultaneously registered by all three measurement systems worn by the same subject.

For RSI, participants were instructed to drop from a 0.3 m platform and upon landing, jump maximally, whilst minimising ground contact time and maximising vertical jump height. The RSI values were calculated automatically using manufacturer software (Optojump, Microgate, Italy) through the sum of jump height (cm) divided by contact time (ms). Following a 30-s rest period, participants performed a CMJ with hands on hips, feet shoulder width apart and when prompted, descended into a squat (~60°) and jumped vertically with maxima l effort. Jump efforts were measured using a portable optical measurement system (Optojump, Microgate, Italy) and were separated by a 30-s passive recovery period with the mean of three jump being used for analyses (Field et al., 2020) .

Participants started in a press up position, with their hands placed on the floor 70 cm apart.
Participants then rapidly flexed their elbows to approximately 90 degrees before maximally exploding off the floor, clapping their hands together, and landing with their arms fully extended. This was repeated three times within quick succession using an Optojump timing system (Optojump, Microgate, Microgate S.r.l., Bolzano, Italy). Flight time for each push up was recorded, and the total flight time was used for comparison. After completing one submaximal plyometric push up as a warm up, participants performed two maximal RPP efforts, with one minute recovery after the warm up, and in-between each effort. Flight time for each push up was recorded, and the total flight time was used for comparison. The coefficient of variation (CV%) for this measurement with the same group of players was 5.5%.

The CMJ was executed as previously described. 14 Jump height was recorded using an optoelectric system (Optojump, Microgate, SRL, Bolzano, Italy). A rest period of 1-min was allowed between trials. The best out of three trials was retained for further analysis.
For the SLJ test, the protocol of Negra et al. 15 was followed. The horizontal distance between the starting line and the heel of the rear foot was recorded using a tape measure to the nearest 1cm. A rest period of 1-min between trials was allowed. The best out of three trials was recorded for further analysis.
For the assessment of reactive strength, participants executed five repeated bilateral maximal vertical hops using an Optojump photoelectric system (Microgate, SRL, Bolzano, Italy) for performance assessment. Before testing, youth athletes were instructed to maximize jump height and to shorten ground contact time. The first jump was not counted and the four remaining jumps were averaged for the calculation of RSI using the following formula: RSI= jump height (mm) / ground contact time (ms). A rest period of 5-min between trials was allowed. The best out of two trials was recorded for further analysis.