Abstract
The Achilles tendon (AT) is comprised of three distinct sub-tendons bound together by the inter-subtendon matrix (ISTM). The interactions between sub-tendons will have important implications for AT function. The aim of this study was to investigate the extent to which the ISTM facilitates relative sliding between sub-tendons, and serves as a pathway for force transmission between the gastrocnemius (GAS) and soleus (SOL) sub-tendons of the rat AT. In this study, ATs were harvested from Wistar rats, and the mechanical behavior and composition of the ISTM were explored. To determine force transmission between sub-tendons, the proximal and distal ends of the GAS and SOL sub-tendons were secured, and the forces at each of these locations were measured during proximal loading of the GAS. To determine the ISTM mechanical behavior, only the proximal GAS and distal SOL were secured, and the ISTM was loaded in shear. Finally, for compositional analysis, histological examination assessed the distribution of matrix proteins throughout sub-tendons and the ISTM. The results revealed distinct differences between the forces at the proximal and distal ends of both sub-tendons when proximal loading was applied to the GAS, indicating force transmission between GAS and SOL sub-tendons. Inter-subtendon matrix tests demonstrated an extended initial low stiffness toe region to enable some sub-tendon sliding, coupled with high stiffness linear region such that force transmission between sub-tendons is ensured. Histological data demonstrate an enrichment of collagen III, elastin, lubricin and hyaluronic acid in the ISTM. We conclude that ISTM composition and mechanical behavior are specialized to allow some independent sub-tendon movement, whilst still ensuring capacity for force transmission between the sub-tendons of the AT.
Highlights
The Achilles tendon (AT) is the largest and strongest tendon in the human body
The steady state force was significantly greater at the proximal end (p = 0.002; difference = 0.07 N, 31%) than the distal end (Figure 4B)
The SOL sub-tendon demonstrated significant differences between the steady state force at the proximal and distal ends, but in the opposite direction, i.e., greater force was measured at the distal end (p = 0.002; difference = 0.07 N, 173%) than at the proximal end (Figure 4B)
Summary
The Achilles tendon (AT) is the largest and strongest tendon in the human body It forms a fundamental component of the musculoskeletal system, enabling everyday movements by bearing high loads and storing energy to reduce their energetic cost (Alexander, 1984; Komi, 1990; Fukashiro et al, 1995). It is estimated to store up to 35% of the total energy lost and regained during locomotion (Ker et al, 1987; Alexander, 1991). Such high strains mean that the AT functions markedly close to its failure properties (Wren et al, 2001), and is vulnerable to injury. Repetitive overload has been postulated as a major precursor for tendinopathy, but due to insufficient understanding of AT structure-function relationships, the causative mechanisms behind tendinopathy remain poorly understood (Wang et al, 2006; Riley, 2008)
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