Skeletal muscles play a central role in human movement by generating active force. The force generated by sarcomeres, the smallest functional unit of a muscle, is conventionally thought to be transmitted to the joints through myotendinous connections. However, recent research has provided a more comprehensive perspective (Huijing, 1999, Siebert et al., 2014, Yucesoy and Huijing, 2012), challenging the traditional notion of force transmission. Notably, a substantial amount of force produced within the muscle was shown to be transmitted through the extracellular matrix (ECM) to the adjacent structures (Huijing, 2009, Huijing, 1999, Maas et al., 2005).

A contracting muscle can exert a considerable amount of pressure on adjacent muscles orthogonal to its line of pull (Reinhardt et al., 2016), and it was shown to influence the muscle’s force output (Siebert et al., 2012). Furthermore, myofascial loads resulting from changing the relative position of a muscle to its surrounding structures have been shown to impact muscle force production capacities (Maas et al., 2003, Yucesoy et al., 2005). This is evidenced by variations in force distribution along muscles, as observed through unequal forces measured at the proximal and distal ends across different muscle lengths (Maas et al., 2004, Tijs et al., 2018). The hypothesized mechanism for such loads often involves the shearing of connective tissue layers, as described by Sharafi and Blemker (2011). Moreover, myofascial interactions mediated by the epimysium play a critical role in determining transmitted forces in the longitudinal direction (Finni et al., 2023). Modeling studies have further elucidated that such myofascial force transmission may lead to increased heterogeneity in sarcomere lengths (Yucesoy et al., 2005), a phenomenon corroborated by in situ muscle experiments (Moo et al., 2017).

The myofascial interactions in muscle and compartment levels were characterized with three-dimensional finite-element models (Blemker and Delp, 2005, Yucesoy et al., 2003, Yucesoy et al., 2002) and observed in vivo in human lower leg muscles using magnetic resonance imaging (Karakuzu et al., 2017, Pamuk et al., 2016, Yaman et al., 2013). Clinical relevance of lateral force transmission, particularly in conditions like cerebral palsy (e.g. Ateş et al., 2014, Kaya et al., 2018), and in treatment methods such as muscle lengthening surgeries (e.g. Ateş et al., 2013), botulinum toxin administration (e.g. Ateş and Yucesoy, 2018, Ateş et al., 2018) or Kinesio taping applications (Yildiz et al., 2023) underscores the importance of understanding these mechanisms.

The majority of the recent literature on muscular interactions has focused on force transmission through the epimysium (e.g. Huijing, 2009). However, emerging research indicates the continuity of ECM from epimysium to endomysium and compartmental fasciae, suggesting potential intramuscular sources of lateral force transmission, which may interact with contractile components (Bloch and Gonzalez-Serratos, 2003, Rode et al., 2016, Williams et al., 2013). This becomes particularly crucial in conditions like muscular dystrophies, where muscle degeneration occurs due to genetic mutations in dystrophin, a protein connecting the intercellular cytoskeleton to the ECM (Gao and McNally, 2015, Klingler et al., 2012, Sun et al., 2008). A better understanding of these diseases requires deeper knowledge of intramuscular lateral force transmission (e.g. Monti et al., 1999). While some earlier studies have suggested that the spatial arrangement of the perimysium supports the pattern of lateral force transmission from myofibers to tendons and neighboring muscles (Passerieux et al., 2007), information regarding the endomysium remains limited. Previous investigations, utilizing both scanning (Purslow and Trotter, 1994, Sleboda et al., 2020, Trotter and Purslow, 1992) and transmission electron microscopy (Street, 1983, Trotter and Purslow, 1992), have depicted the endomysium as a complex collagen network. This network facilitates shear-mediated force transfer across fibers (Purslow and Trotter, 1994, Trotter and Purslow, 1992), underscoring its potential influence on muscle function across different vertebrate species (Sleboda et al., 2020). For example, in frog myofibers, Street (1983) demonstrated the potential of fascial connections to transmit forces in the transverse direction. Additionally, Sharafi and Blemker (2011) developed a model showing that fibers within physiologic length ranges can transmit parts of their peak isometric force laterally through the endomysium.

Despite these investigations, no study has quantified the impact of endomysium on force production. Therefore, the objective of our present research is to address this knowledge gap. We hypothesized that force production by two muscle fibers in the presence and absence of the endomysium shows significant differences.