## 3 Results
### Descriptive Results of the Systematic Review  
A detailed summary of the 61 studies included in this systematic review is available in Supplementary Materials Table S2 ([[https://doi.org/10.6084/m9.figshare.29110022](https://doi.org/10.6084/m9.figshare.29110022)]([https://doi.org/10.6084/m9.figshare.29110022](https://doi.org/10.6084/m9.figshare.29110022)) (accessed on 25 May 2025)). These studies were published between 2003 and 2024, with a marked increase over the past decade. They involved youth handball players aged between 10 and 18 years and were predominantly structured as randomized or quasi-experimental controlled trials. Intervention durations ranged from 4 to 16 weeks and included modalities such as pylometric training, elastic band training, resistance training, and small-sided games. The most commonly assessed outcomes were vertical jump performance (e.g., CMJ, SJ), sprint speed (10-30 m), change-of-direction ability, repeated sprint ability, throwing velocity, and strength measures.  
Assessment of Methodological Quality and Risk of Bias of Individual Studies  
The methodological assessment of the 61 studies included [40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68  
blinding of participants and personnel was among the most common. In many studies, such as those by [PERSON] et al. (2016) and [PERSON] et al. (2017), no clear masking mechanisms were implemented, whereas only a few, such as [PERSON] et al. (2018), reported specific procedures to control for this type of bias. Detection bias, associated with the blinding of outcome assessors, also showed variability: [PERSON] et al. (2017) adequately reported this aspect, while studies such as [PERSON] et al. (2018) failed to mention it.  
On the other hand, attrition bias was generally low. Most studies maintained adequate participant follow-up, and dropout rates were acceptable. With respect to reporting bias, most studies presented their main results; however, some, like [PERSON] et al. (2017), did not indicate whether they had a pre-registered protocol, which made it difficult to fully assess this aspect.  
Finally, in relation to other biases, such as the disclosure of conflicts of interest, some studies were transparent--for instance, those by [PERSON] et al. (2017)--while others, such as [PERSON] et al. (2018), did not provide any information in this regard. Overall, although a portion of the studies demonstrated strong methodological conduct across several domains, key weaknesses remain in areas such as blinding, allocation concealment, and transparency in conflict-of-interest declarations. These elements are described in Figure 2. Although the Cochrane Risk of Bias was used to systematically assess methodological quality, the included studies exhibited heterogeneity across several domains, particularly in randomization procedures, blinding of outcome assessors, and training load documentation. This variability limits the strength of some pooled estimates and could have contributed to residual bias in the overall synthesis. However, the use of a random-effects model, subgroup analyses by training modality, and the cautious interpretation of pooled results helped mitigate these risks. Future trials should aim to standardize key methodological elements and adhere to reporting standards to improve synthesis reliability in this field.  
Figure 2: Risk of bias graph: review authors’ judgements about each risk of bias item presented as percentages across all included studies.