Structure, Composition, and Multiscale MechanicsTendons are remarkable structures. They quietly manage the high volumes of low‑level, cyclical strain that keep everyday movement efficient. When the task shifts to explosive effort, they can amplify muscle power to meet the demand [1-2]. This versatility is central to human performance, but the mechanisms that make it possible remain widely misunderstood. This raises an important question: what features of tendon structure make these behaviors possible? Structure determines function. When we rely on oversimplified assumptions about tendon shape, composition, or the way forces move through the tissue, we risk misunderstanding the system and, more importantly, misapplying our interventions. The basics are never truly basic in tendon science, and returning to them with clarity is essential for improving practice. Stig Peter Magnusson has been paramount in advancing our understanding of tendon structure, loading, and adaptation for the last four decades. His work provides a natural starting point for this installment, which lays the structural foundation for the rest of the series. From Whole‑Body Function to Tissue DemandsFrom a distance, tendons appear simple. They connect muscle to bone and transmit force. But their functional significance becomes clearer when we consider how they behave during real tasks. Roberts and Azizi showed that during eccentric actions, tendons act as mechanical buffers by absorbing energy while fascicles shorten or remain nearly isometric [1]. This reduces peak power input to muscle fibers, limits damaging lengthening velocities, and decouples fascicle behavior from joint motion. Sawicki and colleagues demonstrated the complementary behavior during propulsion, where tendons release stored energy more rapidly than muscle can generate it, allowing the muscle–tendon unit to exceed the muscle’s isotonic power capacity [2]. These task‑level behaviors reflect the underlying mechanical properties of tendon tissue. Tendons behave as viscoelastic structures with nonlinear stress–strain characteristics and strain‑rate–dependent stiffness [3-5]. These properties allow the muscle–tendon unit to function as a motor, a damper, or a spring depending on the demands of the task. They influence force transmission, rate of force development, and elastic energy return during activities such as running, jumping, braking, and change of direction. Together, these findings reveal tendons as structures that modulate timing, velocity, and force [1-2]. They absorb energy quickly and release it more slowly, store energy slowly and release it rapidly, reduce peak fascicle strain rates, and shift the timing of muscle work relative to joint motion. These functions are the “why” behind our pursuit of tendon expertise. If we want to influence them with precision rather than trial and error, we have to examine the structures that make them possible. The clearest place to start is with how individual fascicles share and transmit load. Rethinking Load Sharing at the Fascicle LevelOnce we move inside the tissue, the picture becomes more complex. Historically, tendons were conceptualized as composite ropes [5,8]. The prevailing assumption was that load was shared relatively evenly across fascicles through interfibrillar shear and the surrounding matrix. This model implied that local disruptions would be compensated for by neighboring structures. Human studies have challenged this view. Bojsen‑Møller and Magnusson isolated adjacent fascicles in patellar and Achilles tendons and selectively disrupted them to test how much force transferred through surrounding tissue [6]. Cutting one fascicle caused a noticeable drop in stiffness, and cutting both resulted in only minimal force transmission through the interfascicular matrix. These findings suggest that fascicles behave as partially independent force‑transmitting units with limited load sharing. This reframes how we think about tendon injury, adaptation, and regional strain patterns. Instead of a homogeneous load distribution, tendons may experience localized mechanical environments that vary significantly across the tissue. If these findings hold true, they offer a potential mechanical framework for why tendon pathology often localizes, how unequal load distribution or stress shielding may emerge within the tissue, and why higher strain magnitudes are typically required to drive meaningful adaptation. This shift in understanding sets up the next level of complexity. To understand where these localized strain patterns originate, we have to look below the fascicle and examine how fibrils themselves transmit load. Fibrils, Continuity, and the Emergence of Local Strain PatternsAt the microscale, another long‑standing assumption has been challenged. Older imaging techniques suggested that collagen fibrils were short, discontinuous segments that transferred load primarily through interfibrillar shear [8]. Magnusson and colleagues provided evidence that many fibrils are structurally continuous across long distances. Using serial block‑face electron microscopy, they observed fibrils that span substantial regions of tendon with branching, kinking, and weaving patterns that produce local variations in strain [7]. This matters for several reasons. Continuous fibrils can carry load directly along their length. Branching and weaving create heterogeneous strain fields, and these local variations influence cellular signaling. As a result, remodeling may occur along specific fibrillar pathways rather than uniformly. Taken together with the fascicle findings, a clearer picture emerges: tendons are mechanically complex structures where load does not distribute evenly but instead follows pathways shaped by continuity, branching, and local architecture. Composition, Hydration, and Molecular Contributors to MechanicsAt the molecular level, structure alone cannot explain tendon behavior. Composition exerts its own strong influence [10]. Hydration affects how closely collagen molecules sit next to each other, how they interact, and how stiff or flexible the fibrils become [3-4]. Well‑hydrated fibrils behave more compliantly and absorb energy more effectively, while dehydration causes fibrils to pack tightly and become significantly stiffer. Because water content is regulated in part by matrix proteins, hydration becomes a direct link between composition and load‑bearing characteristics [5,10]. Non‑collagenous matrix proteins such as decorin, biglycan, elastin, tenascin‑C, lumican, fibromodulin, and COMP contribute to fibril formation, matrix organization, viscoelasticity, and remodeling [10]. These molecules influence how tendons handle tensile, compressive, and shear forces and add further layers of complexity to the evolving picture of tendon function [5,10]. However, architecture alone does not explain the behaviors we see in vivo. A deeper look at the molecular contributors that govern how tendons sense and respond to load is needed. This perspective helps refine our understanding of the inputs that shape tendon function and points toward where targeted interventions may have meaningful impact. The Challenge of Modeling a Multiscale SystemAt every level we have examined, from whole tissue to molecules, the picture becomes more intricate and less predictable. The physiological environment is too complex to model perfectly, and many interactions remain difficult to measure in vivo. This is not a limitation of any single scale. It is a defining feature of tendon biology. This complexity is also why multiscale research, including the work of Magnusson and many others [8], is essential for advancing our understanding. How Load Shapes Tendon Behavior Across ScalesThe structural and compositional features described above are not passive. They respond to load in quantifiable ways. Magnusson and Kjaer showed that tendon cells up-regulate growth factors, collagen transcription, and cross‑linking enzymes when exposed to repeated, appropriately dosed tensile strain [9]. Too little strain suppresses collagen expression, down-regulates tendon‑specific markers, and accelerates proteolytic activity. Too much strain shifts the tissue toward inflammatory signaling and matrix breakdown. These responses occur across scales. Molecular interactions change with hydration and strain. Fibrils slide, stretch, and reorganize. Fascicles experience localized strain environments. Whole tendon stiffness and energy storage capacity adapt over time [9]. Together, these multiscale responses determine tendon capacity, resilience, and performance. They also define the loading environments clinicians must create if they want tendons to adapt rather than deteriorate.
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