Histological Definition of Skeletal Muscle Injury: A Guide to Nomenclature Along the Connective Tissue Sheath/Structure

Recent years have seen the development of various classifications of muscle injuries, primarily based on the topographic location within the bone-tendon–muscle chain. This paper proposes an enhanced nomenclature for muscle injuries that incorporates histoarchitectural definitions alongside topographic classifications, emphasizing the importance of connective tissue damage characterization. A detailed understanding of the distinct anatomical and histological characteristics of tendon, aponeurosis, and fascia is essential for consistent terminology. Tendons, aponeuroses, and fasciae, while all composed of dense connective tissue, differ in collagen fiber orientation and structural organization. Tendons feature longitudinally aligned fibers suited for high tensile forces and muscle-to-bone connections. Aponeuroses have perpendicular collagen fiber layers, allowing for force distribution and support for both longitudinal and transverse traction. Fasciae exhibit loosely organized fibers providing flexibility and movement between muscles. The continuity and differences within the musculotendinous frameworks, particularly at the musculotendinous junction (MTJ), elucidate the higher frequency of injuries at these sites compared with tendons, highlighting the MTJ's role as a stress-absorbing 'safety valve.'