## 4 Discussion  
The aims of this study were to examine the short-term and crossover effects of US-guided PNM in patients with unilateral AKP. The main finding of this study was that a single-shot percutaneous femoral nerve stimulation may cause an improvement in pain, knee flexion ROM, and knee functionality, with a significant improvement after 1 week following intervention. Compared to both groups, patients who were treated with neuromodulation in the femoral nerve of the painful knee were more likely to report at least significant change in Kujala and NRS scores at \(1\) week following intervention. However, the therapeutic benefits were produced independently of the stimulated femoral nerve. In line with this [21], these results confirmed the crossover effect of this technique because the US-guided PNM had a great effect on the painful knee when delivered to the contralateral limb. The data of this study showed the crossover effect of the PNM intervention regarding knee flexion ROM, but also pain and functionality. The application of electrical stimulation in the peripheral nervous system has historically required surgical intervention, carrying a risk of nerve damage stemming from implanted devices [29; 30]. Transcutaneous electrical nerve stimulation has been used in an attempt to address this limitation, but it has been unsuccessful [31]. A percutaneous nerve stimulation approach is under clinical investigation, which is designed to address these needs and allow treatment of the nerve. The use of ultrasound has allowed the development of percutaneous peripheral nerve stimulation. On the one hand, some therapists implant a percutaneous peripheral nerve stimulator to manage chronic pain, such as radiculopathy pain [32], neuropathic postamputation pain [33], or low back pain [34]. On the other hand, other clinicians use the US-guided PNM that allows for applying PES in close proximity to the nerve, with ultrasound guidance [13]. US-guided PNM has been found to be an effective technique for the treatment of chronic pain conditions [14; 35; 36] and muscle strength in subjects with previous unilateral knee pain [15] and with short hamstring syndrome [21], as well as for the improvement of healthy dancers' performance [28]. To the best of the authors' knowledge, this is the first study to demonstrate the short-term effects of a PNM intervention in patients with AKP. The relevance of this finding for clinical practice is that a simple single-shot of US-guided PNM in the femoral nerve in patients with a painful knee produces effects that are significant for a week following the treatment (Table 2). These results are in line with a previous study [14] that demonstrated the immediate effects of this procedure on pain and knee flexion ROM in patients with AKP. Therefore, as a methodological recommendation, authors suggest that the technique may be employed once a week. Further research is necessary to elucidate the relative benefits and methodological procedure of US-guided PNM in treating chronic pain. In addition, US-guided PNM is a well-tolerated and safe procedure that may be applied in patients with AKP, and other conditions [36; 37]. In fact, there were no symptoms such as postpuncture pain, and adverse events disappeared within 24 h.  
Finally, to the best of our knowledge, no studies have demonstrated the crossover effect of US-guided PNM in the femoral nerve. These results corroborated the findings from studies that examined crossover effects of other techniques on the contralateral limb [16; 17; 18; 19; 20], including neural therapy [21]. The authors reaffirm that the neural system has a connection throughout the body and it is likely that the neural treatment on one limb may cause crossover effects on the contralateral limb. The current study has some limitations. Firstly, the authors did not take into account the underlying pathology of the patients with AKP. It is possible that different musculoskeletal injuries may provide information of clinical interest.