## 2 Materials and Methods
### Testing Procedures  
All athletes were already familiar with the DJ assessment procedure. The measurements were performed 6 weeks before the Tokyo 2020+1 Olympic Games. The jumping protocol consisted in performing three DJ attempts from increasing heights (20, 30, 40, 50, and 60 cm) until the point at which the RSI value decreased. Attempts above 60 cm were not considered to protect the athletes for the Olympic Games.  
At the beginning of the testing session, data (age, sporting experience (years), anthropometric data) were obtained and the 100 m sprinters and 400 m sprinters were divided into two groups of 5 athletes each. Before performing the jumping protocol, the athletes completed a standard warm-up, which included the following: three laps of the track (400 m), 2 \(\times\) 20 m skipping A, 2 \(\times\) 20 m skipping A with right leg, 2 \(\times\) 20 m skipping A with left leg, 2 \(\times\) 20 m skipping B, 2 \(\times\) 20 m skipping C, 2 \(\times\) 40 m progressive sprint, and 3 jumps in counting movements. After 5 min of recovery, a test drop jump from a height fall (HF) of 20 cm was performed. Athletes were instructed to perform maximal effort, aiming for lower GCT and higher FT. Each athlete's take-off and landing were standardized following the jump protocol [18]. Using a force plate (Kistler Model 9260 AA6, Winterthur, Switzerland), data were collected and displayed in real-time at a sampling rate of 1000 Hz with the interface box provided by the brand. The data were analyzed using Bioware version 5.3.2.9 (Winterthur, Switzerland) following the manufacturer's instructions. Verbal stimuli were provided during all attempts. Attempts of DJ20 were performed with the force platform at a separation of 10 cm from the jump box. For the DJ30 and DJ40 cm, the force platform was 15 cm away from the jump box, while the DJ50 and DJ60 were 20 cm away. The recovery time between attempts of the same height was 1 min and between different heights 4 min. The RSI was calculated using the equation described below [8]:  
\[\text{RSI}\ =\ \text{JH}/\text{GCT} \tag{1}\]  
where RSI is the reactive strength index, JH is the jump height, and GCT is the ground contact time.  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Variable** & **100 m Sprinters** & **400 m Sprinters** & **Total** \\  & **n = 5** & **n = 5** & **n = 10** \\ \hline Age (y) & 26.23 \(\pm\) 1.84 & 23.58 \(\pm\) 3.01 & 24.9 \(\pm\) 2.42 \\ Height (cm) & 1.74 \(\pm\) 7.2 & 1.76 \(\pm\) 2.8 & 1.75 \(\pm\) 5.0 \\ Body weight (kg) & 75.53 \(\pm\) 1.96 & 73.32 \(\pm\)2.08 & 73.42 \(\pm\) 2.02 \\ BMI (kg/m\({}^{2}\)) & 24.94 \(\pm\) 1.76 & 23.66 \(\pm\)1.88 & 24.3 \(\pm\) 1.82 \\ Sports experience (y) & 11.03 \(\pm\) 2.40 & 12.83 \(\pm\) 3.66 & 11.93 \(\pm\) 3.03 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics (mean \(\pm\) SD) of the Olympic dash sprinters (n = 10).  
RC was evaluated using Equation (2):  
\[\mathrm{RC}\ =\ \mathrm{JH/HF} \tag{2}\]  
where JH is the jump height and HF is the fall height.  
HF and JH have already been described above. These formulas can be considered the most practical since their evaluation can be performed with any validated jump interruption equipment or different accelerometer devices that provide this variable [19; 20].  
To integrate the effects of HF into the RSI formula, a new variable was developed, which we call the integral reactive strength index, obtained by the following equation:  
\[\mathrm{IRSI}\ =\ \mathrm{RSI}\ -\mathrm{RC} \tag{3}\]  
where IRSI is the integral reactive strength index, RSI is the reactive strength index, and RC is the reactive capacity.  
Anthropometric assessment: height was measured on a stadiometer close to 0.1 cm (Bodymeter 206, SECA, Hamburg, Germany). Body mass was assessed (scale provided by the platform) with an accuracy of 0.1 kg. This protocol coincided with the week of low training load with the commitment of the coaches not to perform pyometric sessions in the 5 days before the test.