Piezoresistive strain sensors based on polymer nanocomposites are presented as promising candidates for human motion monitoring due to their higher flexibility, conformability, and sensitivity than conventional strain sensors. The fundamentals behind the use of conductive nanoparticles embedded within a flexible polymer matrix are investigated. This design is explained to allow changes in strain to translate into variations in electrical resistance, enabling accurate motion detection. Crucial performance parameters for human motion monitoring, including sensitivity, linearity, and response time, are discussed. Recent advancements in designing these nanocomposite sensors for human motion applications are highlighted. Specifically, the use of 1D nanoparticles, such as carbon nanotubes (CNTs), and 2D nanoparticles belonging to the graphene family, specifically so-called graphene nanoplatelets (GNPs), has been studied for the formation of electrical percolation networks in different flexible matrices, with a high capacity for elastic formation. The potential for the integration of these sensors into comfortable and wearable platforms for real-time monitoring of joint movement, muscle activity, and gait analysis is emphasized. For these reasons, various proofs-of-concepts with developed polymer nanocomposites are presented. By exploring the exciting potential and ongoing advancements in piezoresistive strain sensors based on polymer nanocomposites, this presentation aims to spark further developments in human motion monitoring technology.