The gracilis muscle (GM) is located on the medial side of the thigh. While it belongs to the adductor muscles of the hip joint, it differs from the other members of the group due to its superficial position, relatively long and narrow muscle belly, and long distal tendon. The GM extends from the symphysis pubis, ischial ramus, and inferior pubic ramus to a site located medially to the tibial tuberosity, composing the pes anserinus. Its blood supply is provided mainly by branches from the deep femoral artery and medial circumflex femoral artery, and innervation by the anterior branch of the obturator nerve. The GM provides hip joint flexion and adduction, as well as knee joint flexion and internal rotation (Dziedzic et al., 2018, Magden et al., 2010, McKee et al., 1990, Meyers et al., 2005, Morris and Yang, 1999).

Although some studies describe the anatomy of the GM, including its innervation (Crystal et al., 2005, Fattah et al., 2013, Kumar et al., 1998, Kwon et al., 2009, Magden et al., 2010, Won et al., 2012), the importance of exhaustive and precise anatomical examination with a sufficient sample size is highlighted by some researchers to assess morphological variability of anatomical structures reliably (Olewnik et al., 2022, Olewnik et al., 2021, Olewnik et al., 2020, Olewnik et al., 2019, Olewnik et al., 2018). An anatomical description of innervation may consist of inter alia the localization of the main nerve trunk, the number of its branches, their correlation to other structures and sites where nerve branches enter the muscle (Albert et al., 2000, Crystal et al., 2005, Fattah et al., 2013, Kumar et al., 1998, Kwon et al., 2009, Magden et al., 2010, Parratte et al., 2002, Seidel et al., 1996, van Campenhout et al., 2010, Won et al., 2012). The location where the terminal nerve branch pierces the muscle belly is known as the motor point (MP). A few studies have tried to identify the position of the MPs within the GM (Crystal et al., 2005, Kwon et al., 2009, Magden et al., 2010). It is possible to localize MPs based on classical anatomical dissection (Crystal et al., 2005, Kwon et al., 2009, Magden et al., 2010) or electrophysiological procedures based on scanning the muscle surface (Botter et al., 2011, Gobbo et al., 2014, Gobbo et al., 2011); however, all of aforementioned studies lack the sufficient sample size (Botter et al., 2011, Crystal et al., 2005, Gobbo et al., 2011, Kumar et al., 1998, Kwon et al., 2009, Magden et al., 2010).

Spasticity of the adductor muscles group of the hip may be a symptom of a damaged central nervous system in such medical conditions as multiple sclerosis, traumatic brain injury, spinal cord injury or cerebral palsy. The range of spasticity can be reduced by botulinum neurotoxin (BN) injections to the muscle belly (Graham et al., 2000, Molenaers et al., 2006, Ward, 2008, Wissel et al., 2009), a procedure applied also to the GM. The effects depend on the applied dose, but it should not be so high as to cause side effects (Park and Rha, 2006, Stone et al., 2011). Although the remains no agreement on where to perform the injection, some anatomical studies propose that the best targets are the MPs of a muscle (Crystal et al., 2005, Kim et al., 2005, Sook Kim et al., 2002). However, it has been proposed that the best results are achieved by delivering a large dose to a synaptic connection between the axon of a motor nerve and striated muscle fiber, called the motor end-plate (MEP) (Lepage et al., 2005, Stone et al., 2011, van Campenhout et al., 2010).

An effective method of rehabilitating or maintaining muscle function and mass in conditions characterized by long-term muscle disuse (e.g., multiple sclerosis, spinal cord injuries, sports injuries) is electrical stimulation (ES) (Botter et al., 2011, Chae et al., 2009, Enoka et al., 2020, Gobbo et al., 2014, Gobbo et al., 2011, Maffiuletti, 2010, Shimada et al., 1996, Wahls et al., 2010, Yamashita, 2017). In this procedure, muscle contractions are induced by electrical impulses delivered to the muscle belly. The electrodes can be attached to the skin surface or implanted intramuscularly (Botter et al., 2011, Daly et al., 2001, Gobbo et al., 2014, Gobbo et al., 2011, Popovic et al., 1991, Shimada et al., 1996). The best place to deliver such electrical stimulation is widely believed to be the MP. Several anatomical and electrophysiological studies have been performed to identify and localize the MPs present in human skeletal muscles, and their results indicate that most muscles have more than one MP. Their location is a crucial consideration when carrying out ES (Botte et al., 1991, Botter et al., 2011, El-Din Safwat and Abdel-Meguid, 2007, Gobbo et al., 2014, Gobbo et al., 2011, Liu et al., 1997, Yamashita, 2017).

The aim of our study is to identify the localization of MPs of the GM based on a statistically sufficient sample size. The obtained results can help to understand the correlation between MPs and MEPs. Moreover, they may be used to identify the most suitable sites for ES electrodes placement or BN injections, or general research on MPs positioning in other muscles.