## 4 Discussion  
The aim of this study was to quantify the within-individual elbow valgus torque variability when pitching fastballs in elite youth baseball pitchers. The results showed that within-individual elbow valgus torque variability is present in pitchers and differed among pitchers. In the majority of these pitchers, the variability can be described as a Gaussian distribution. Pitching at full effort produces slightly different elbow valgus torques between fastball pitches within a pitcher. As expected, and in line with other studies, the magnitude (average) of the elbow valgus torque was different between pitchers, showing the between-pitcher variability [8; 11; 21]. The group average elbow valgus torque in this study population was 53.19 Nm (SD 9.34; range: 36.35-68.5). Similar valgus torques were reported in other studies with comparable ball speeds and study population [8; 11]. In these studies, the valgus torque magnitude differences between pitchers has been regularly reported, but the within-individual valgus torque variability has mostly been neglected. In our study, we show that this within-individual valgus torque variability is present in pitchers and differs between pitchers ranging from 1.34 Nm to 4.40 Nm (Figure 1). All pitchers were instructed to throw 25 full effort fastballs, and some pitchers were able to throw with less valgus torque variability compared with others. It is known that within-individual variability is smaller compared with between-individual variability [21]. The magnitude (average) external valgus torque between pitchers can be explained mainly by personal characteristics (body length, mass), motivation, well-being, fatigue, and technique (kinetic chain, energy flow), whereas the within-individual valgus torque variability can mainly be explained by motivation, technique, and fatigue. So, in addition to the magnitude (average) valgus torque, the within-individual valgus torque variability is also different between pitchers. A higher valgus torque is considered to lead to an increased UCL injury risk, according to previous research [8; 10; 22]. Within a homogenous group of professional pitchers, 25% sustained a UCL injury [2]. The valgus torque magnitude on its own cannot completely explain this percentage, as all professional pitchers show high valgus torques. To explain this prevalence, the within-individual load variability and the injury threshold are both  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**Participant**} & \multirow{2}{*}{**Analyzed**} & \multirow{2}{*}{**Mean**} & \multirow{2}{*}{**Median**} & \multirow{2}{*}{**SD**} & \multirow{2}{*}{**IQR**} & \multirow{2}{*}{**Skewness**} & \multirow{2}{*}{**Kurtosis**} & \multicolumn{2}{c}{**Shapiro–Wilk Test**} \\ \cline{5-10}  & & & & & & & & & **Sig. ** \\ \hline
**1** & 24 & 58.91 & 59.86 & 4.40 & 6.55 & \(-\)0.07 & \(-\)1.27 & 0.93 & 0.11 \\ \hline
**2** & 25 & 53.40 & 53.61 & 2.07 &  
important metrics to include [13]. The injury threshold in this model is the maximal load the UCL can sustain before it strains or tears. The demonstrated within-individual valgus torque variability and the fact that this differs between subjects is a step towards validating the injury model of [PERSON] et al. (2020). However, it is suggested that pitchers who throw with a higher magnitude and within-individual variability of valgus torque might have--but do not necessarily have--a higher chance of sustaining an injury, because the injury threshold is also of importance [13] (Figure 2). It is important to mention that we did not investigate the relationship between valgus torque variability and injuries. In the model of [PERSON] et al. (2020), a load distribution is formed with a load average and standard deviation. The results showed that the elbow load distribution in nine out of eleven pitchers could be described as a Gaussian distribution. The standard deviation of this Gaussian distribution can be used as a parameter to quantify the within-individual load variability.