The morpho-mechanical and neuromuscular properties of the Triceps surae muscle-tendon unit in individuals with Achilles tendinopathy and the effects of exercise

Contreras, Ignacio (2024). The morpho-mechanical and neuromuscular properties of the Triceps surae muscle-tendon unit in individuals with Achilles tendinopathy and the effects of exercise. University of Birmingham. Ph.D.

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Abstract

Achilles tendinopathy (AT), whether non-insertional (NIAT) or insertional (IAT), is a debilitating overuse injury causing significant functional impairments in both athletic and general populations. However, these are two distinct disorders with different underlying pathophysiology and treatments. NIAT is the most prevalent tendinopathy in the lower limb. Nevertheless, its aetiology remains unclear, and it is likely to be multifactorial. This thesis consists of 6 Chapters. Chapter 1 provides an overview of tendon structure and function, with a particular emphasis on the Achilles tendon. Subsequently, the chapter explores relevant topics about Achilles tendinopathy and the assessment of the morpho-mechanical and neuromuscular properties of the muscle-tendon unit. Chapter 2 presents the results of a systematic review showing that individuals with NIAT and IAT exhibit neuromechanical changes indicated by differences in the triceps surae muscle’s neuromuscular properties and the Achilles tendon’s morpho-mechanical properties. Additionally, exercise-induced mechanical tendon loading showed inconsistent results in modifying the morpho-mechanical properties of the Achilles tendon in individuals with NIAT. Chapter 3 revealed the results of an observational study showing a contraction-intensity dependent relationship between motor unit firing rate properties of the triceps surae muscles and the morpho-mechanical parameters of the Achilles tendon in asymptomatic individuals. One of the most relevant findings is that individuals with increased tendon stiffness showed a lower triceps surae muscle discharge rate at low target forces. Chapter 4 exhibited the outcomes of an observational study demonstrating that individuals with NIAT have load-dependent changes in triceps surae motor unit firing rate properties. Specifically, an increase in the discharge rate and a decrease in the derecruitment threshold of the lateral gastrocnemius muscle were observed in individuals with NIAT compared to controls during isometric plantarflexion contractions at high forces. Furthermore, it was observed that the contribution of the lateral gastrocnemius to the net plantarflexion force is altered in individuals with NIAT. Chapter 5 presents the results of a randomised controlled trial showing that a 6-week torque feedback intervention based on eccentric or concentric exercises induces load-dependent changes in the triceps surae motor unit firing properties in individuals with NIAT. Additionally, these exercise interventions effectively decreased pain, improved function, and increased tendon stiffness in individuals with NIAT. Moreover, our findings indicate that both exercise interventions produced similar triceps surae motor unit firing rate adaptations compared to controls, suggesting a normalisation of these properties after the intervention in individuals with NIAT. Finally, Chapter 6 highlights the most relevant findings of this thesis, explores the limitations of the studies and suggests possible future investigations.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):