Layer-Specific Architecture and Nerve Innervation of the Popliteus Muscle: Neuroanatomical Basis for Precision-Guided Interventions for the Knee Joint

Background/Objectives: The popliteus muscle (PM) plays a crucial role in stabilizing the posterolateral aspect of the knee. However, its layered structure and innervation are not well understood due to its location, size, and proximity to neighboring anatomical features. This study aimed to clarify the layered morphology, intramuscular innervation, and fiber-type composition of the PM, providing anatomical insights for clinical interventions. Methods: We examined 32 lower extremities from sixteen formalin-embalmed cadavers using a multimodal approach that included gross dissection, Sihler’s staining, ultrasonography, and histochemical analysis. Results: On average, 2.8 ± 1.1 branches of the tibial nerve entered the PM, with a consistently high-density entry zone located at 56–64% of the muscle length. Sihler’s staining and ultrasonographic analyses revealed a distinct separation between the superficial and deep layers across the central tendon, each exhibiting compartmentalized intramuscular branching territories. The superficial layer was primarily composed of type IIx fibers and exhibited a larger pennation angle, while the deep layer was richer in type IIA fibers with a smaller pennation angle. These findings illustrate that the PM functions as a dual motor unit rather than a uniform structure. Conclusions: The PM exhibits a distinct compartmentalized organization, functioning as a multifunctional motor unit. The identification of specific intramuscular entry zones and the organization of muscle layers provide strong anatomical evidence for improved targeting in neuromuscular-modulating interventions. This enhances the precision, safety, and efficacy of clinical strategies aimed at addressing posterior knee stability and pathologies related to the posterolateral complex (PLC).