## 3 Muscle Synergies in Sports: Limitations
### Methodological Limitations  
In this section, a specific list of the limitations that apply to the studies conducted so far when using muscle synergies is provided. They are also summarized in Table 1.  
Synergies extracted using the spatial model onlySo far, only Non-negative Matrix Factorization (NMF- based approaches [21] based on the spatial model have been used. While this is not a limitation \"per se\", this approach limits the interpretative potential of synergistic approaches, as several other available models could capture specific features not emphasized with the spatial model [22]. Synergies used mainly for evaluationThe vast majority of the available studies used muscle synergies only to provide a picture of the motor control available to athletes at a given stage [5]. However, as preliminarily suggested by some studies, synergies might \"enter the loop\" of training, rehabilitation, and injury prevention by providing decisional elements rather than descriptive analysis only [13]. Synergies used to examine the neural drive onlyThe standard formulation of muscle synergies covers only neural variables derived from surface EMG. This makes it not trivial to map EMG activations to the motor output, the so-called \"task space\", limiting their range of application and interpretation. Synergies extracted without using non-linear modelsThe synergy theory assumes a linear combination of synergies. This assertion is supported by findings on animals and humans. However, especially when combining synergies with task space variables, non-linear relationships might be expected and tools for modeling such non-linear relationships might be needed [23]. Synergies not extracted on separated EMG componentsRecent studies have demonstrated that phasic and tonic synergies, associated with motion-related and postural/anti-gravity components, respectively, are different in composition [24]. Thus, separating synergies that originate from these two components might lead to more refined models and a deeper understanding of the biomechanics of gestures. Synergies not extracted from simulationsWhile using experimental data allows us to work on real data, a more extended use of models and simulations would allow us to test conditions that are hard to reproduce experimentally and extend the range of the investigations, for example, by incorporating more muscles in the analysis [25]. Synergies extracted in a few fields of applicationMany popular sports have never been tested in the framework of muscle synergies, thus providing many opportunities for further investigations. The suggestions outlined in the table represent advancements achieved in the application of synergies within the biomedical field. The novelty of this review is to propose the \begin{table}
\begin{tabular}{l l} \hline \hline
**Limitation** & **Suggestion** \\ \hline _Synergies extracted using the spatial model only_ & To extend synergistic models to spatial, temporal, and spatiotemporal models (and more) [22] \\ \hline _Synergies used mainly for evaluation_ & To incorporate synergies in extensive training and rehabilitation protocols [13] \\ \hline _Synergies used to examine the neural drive only_ & To incorporate task variables into synergistic models \\ \hline _Synergies extracted without using non-linear models_ & To incorporate non-linear models [23] \\ \hline _Synergies not extracted on separated EMG components_ & To separate phasic and tonic EMG components and extract phasic and tonic synergies (when applicable) [24] \\ \hline _Synergies not extracted from simulations_ & To promote the use of simulations and digital twins [25] \\ \hline _Synergies extracted in a few fields of application_ & To expand the range of applications, especially to sports not analyzed yet \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Limitations of the systematic application of muscle synergies in sports and suggestions on how to explore novel directions in the field. application of these advancements also in sports where the application of synergy analysis is limited.