Tendon Structure & Muscle Matrix

Tendons play an essential role in athletic performance as they enable—and modulate—force transfer from muscle to bone. In rock climbing, tendons and ligaments of the arms and shoulders are exposed to extreme mechanic demands, especially during the dynamic movement common to near-limit climbing. A most severe example is a one-finger pocket pull on steep rock in which much of body weight is carried by the flexor tendons and ligament pulleys of a single finger. Clearly, then, maintaining—and strengthening—connective tissues is paramount for a serious, hard-training climber.

Tendons, ligaments, and muscle ECM are comprised mainly of collagen, a structural protein that accounts for approximately 30 percent of all the proteins in the body. The hierarchical structure of tendons is exquisitely organized with an intricate arrangement of collagen molecules, fibrils, fibers, and finally larger fascicles that, like the core of a climbing rope, provide a robust mechanism for force transfer that won’t be compromised by small internal irregularities (micro-fiber tears). Remarkably, gram for gram the Type-I collagen that comprise tendon tissue is stronger than steel!

At the heart of the collagen is a unique triple helix structure with a repeating sequence of glycine, proline or hydroxyproline, and any other amino acid (Figure 2). The twisting of the helix results in a “crimping” of the fibrils which adds some flex and compliance to the system, while elastin provides recoil during repeated mechanical loading. Surrounding the bundled fibers is a proteoglycan-rich hydrophilic “ground substance” that adds the viscoelastic properties and, importantly, serves as a medium for intercellular exchange and nutrient transport within the tendon.

The tendon core consists of densely packed Type-I collagen matrix that’s sparsely populated with often-quiescent tenocytes (aka. fibroblasts). Recent studies have discovered that while the tendon core changes little between the ages of 17 and 70, the outer part of the tendon (paratenon) is dynamic and metabolically active. Chronic training can lead to tendon hypertrophy as tenocytes extrude collagen that gradually adds material to the outer part of the tendon (vaguely analogous to a tree adding rings). Finally, forceful mechanical loading leads to an increase in enzymatic cross-linking of collagen molecules which increases tendon and ECM rigidity. In aggregate, increased tendon cross-sectional area and cross-linking results in a stronger, stiffer tendon that will elevate performance and reduce injury risk.

The structure of the tendon is tightly coupled to the architecture of the muscle. Optimal function (and injury resistance) of the muscle-tendon system requires an exquisitely tuned viscoelastic structure in which tendon stiffness and muscle strength upscale together. At the myotendinous junction, the tendon fibers fan out like a river delta and extends throughout the length of the muscle in a scaffold-like fashion. Within the muscle, extracellular matrix forms a honeycomb-like structure that groups contractile fibers together and facilitates lateral force transfer to “scaffold” then tendon and bone, while protecting individual fibers from exercise-induced damage (Figure 3).

Whereas the classic textbook model for muscle force transfer is one of longitudinal transfer, from sarcomere to sarcomere through the full length of the muscle, recent research has shown that up to 80 percent of muscle force arrives at the tendon via lateral force transfer through the ECM. Efficient lateral force transfer reduces micro tears in contractile fibers (decreasing muscle soreness) and increases rate of force development and power. Clearly, then, any training and/or nutritional intervention that increases strength and stiffness of the musculotendon system is invaluable to an athlete wanting to increase performance and reduce injury risk.

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