## 4 Discussion
### Results Interpretation  
EMG-based metrics such as mean frequency or median frequency are commonly used biomarkers for measuring muscle fatigue in patients and athletes [12; 13]. Using EMG-related measurement for inpatients has already proven to be a valuable methodology. In fact, the onset of fatigue has been already observed in previous studies, reporting that during dynamic contractions in healthy younger and hospitalized patients, there was a statistically significant difference between MF at initiation and termination of contractions, indicating that subjects' muscles did truly fatigue [24].  
In this study, we found very similar fatigue levels before and after a 2-weeks rehabilitation intervention, even when testing different thresholds for fatigue detection. While the choice of the threshold impacts on the number of EMG channels showing muscle fatigue, our analysis showed that, when comparing PRE and POST therapy findings, this induced no alteration of statistical tests, suggesting that the results are inherent to the dataset and are not related to the adopted threshold for fatigue. However, similar fatigue patterns are coupled with higher MVC, which translates to more demanding physical training during our evaluation trial. It also suggested that patients underwent functional improvement as higher MVC is not associated with more muscle fatigue, thus patients could hold significantly higher forces with no significantly higher fatigue levels.  
A distinguishing feature of our study was that five muscle heads were monitored simultaneously. While the exercise was designed to involve as a primary muscle the vastus lateralis (proximal head), we immediately noted that the whole quadriceps muscle was involved in force exertion. A multi-channel recording might be useful to detect if, for various reasons (positioning, partial adherence to the protocol, compensatory strategies, etc.), subjects were experiencing fatigue, but on other muscle heads, which it happened indeed. In fact, muscle heads of quadriceps are non-uniformly activated during fatiguing exercises [38; 40].  
A relevant result that we found was that for some subjects, fatigue was not detected in the measured muscle heads. This can be explained by admitting that some patients might not have exerted their maximum MVC, or because of their good training level. Non-adherence to the exercise guidelines was limited as multiple trials were performed and graphical and vocal feedback were given by sanitary operators and experimenters to motivate patients. It is also possible that for some patients the exercise was not challenging enough. Lastly, while patients were in general compliant to the task, their motivation and effort could not be perfectly uniform throughout the trials and the sessions.  
In general, it was found that EMG metrics did not improve (i.e., fatigue was not reduced). This is, however, explainable with the increased MVC: while fatigue did not increase, functional performance was higher. However, it may also indicate that a 2-week protocol could not be long enough to detect significant changes on EMG-based metrics. The results might also be related also to the training level of the patient before the hip fracture. The acquisition protocol was based on MVC to provide challenging conditions to patients; however, if MVC in the post-rehabilitation was set a priori equal to the pre-rehabilitation value, it is likely that muscle fatigue might decrease in more patients as the exercise would be less challenging. Our results could be the basis for a selection of the length of clinicalprotocols to maximize recovery reducing hospitalization times and consequently costs and unpleasant time spent in the hospital for patients.