## 1 Introduction  
Anterior knee pain (AKP) is characterized by a nonspecific pain around the patella. AKP is one of the most common problems of the knee, mainly affecting active people [1]. The underlying cause of AKP has been suggested to be multifactorial [2; 3], such as patella abnormalities, muscle disbalance, or malalignment of the lower extremities [4; 5]. However, some researchers have suggested that AKP may be caused by slight nerve damage or altered mechanosensitivity of the femoral nerve, like other musculoskeletal pathologies [6; 7]. [PERSON] et al. [6] found a decreased excitability in the common fibular nerve in patients with functional instability of the ankle and [PERSON] et al. [7] identified a decreased excitability of the radial nerve in patients with chronic unilateral epicondylalgia. In relation to the knee, [PERSON] et al. [8] suggested a \"neural model\" to understand AKP. They found that a hyperinnervation in the lateral retinaculum, mainly nociceptive substance P-positive nerves, is involved in the pathogenesis of AKP. [PERSON] et al. [9]found an alteration in the mechanosensitivity of the femoral nerve, measured by a slump test, in patients with AKP. Therefore, the authors suggest that AKP may be the result of sensitization of the neural system and not only a local mechanical problem. Determining an effective intervention that helps to treat the neural condition is very important. To the best of the authors' knowledge, there is limited evidence in regard to \"neural therapy\" in patients with AKP. The electrical stimulation of the peripheral nervous system is a therapeutic procedure that applies the electric current on the peripheral nerve to treat pain. The underlying mechanism of this therapeutic effect is caused by the stimulation producing a modification of the input to the central nervous system, known as neuromodulation. Since its first description by [PERSON] in 1967 [10], the peripheral nerve stimulation (PNS) efficacy for chronic neuropathic pain limbs has increased significantly. [PERSON] et al. [11; 12] demonstrated that percutaneous femoral nerve stimulation may be effective in providing analgesia following anterior cruciate ligament reconstruction or total knee arthroplasty. Clinically, an invasive technique has appeared, known as ultrasound (US)-guided percutaneous neuromodulation (PNM). [13] This minimally invasive intervention consists of the application of a percutaneous electrical stimulation (PES) through an acupuncture needlelike electrode that is placed in close proximity to the nerve or motor point of the muscle with ultrasound guidance. [13] In this way, [PERSON] et al. [14] observed that a single PNM intervention on the femoral nerve may cause a decrease in pain and increase the ROM in patients with AKP, which was greater at 24 h after the stimulation. [PERSON] et al. [15] demonstrated that US-guided PNM on the femoral nerve may reestablish the quadriceps strength in inhibited muscles in subjects with knee pathology. Different studies provide evidence of the crossover effect, understood as the response of a nonexercised contralateral limb. There is extensive conflicting evidence in the literature. Several authors have showed that unilateral stretching [16; 17] and foam rolling [17] may have a crossover effect on the contralateral limb ROM; others [18; 19] suggested that there were crossover fatigue effects in the contralateral knee extensors; and [PERSON] et al. [20] did not support the existence of crossover effects after exhaustive stretch-shortening cycle exercise. Regarding the crossover effect after neural intervention, [PERSON] et al. [21] showed that PNM in the sciatic nerve produced benefits in the hip flexion ROM, measured by a straight leg raise test, in the nonintervention limb. The purpose of this study was to evaluate the short-term effects of the US-guided percutaneous femoral nerve stimulation, regarding the pain, knee flexion ROM, and functionality in patients with unilateral AKP.