Achilles tendinopathy is a frequent overuse disorder caused by repetitive loading of the tendon, particularly observed in athletes and middle-aged men. It is typically characterized by mid-portion pain, stiffness in the morning, and functional limitations, with structural alterations such as thickening and disrupted collagen alignment. Chronic progression often compromises performance and quality of life.

Eccentric training protocols, such as the Alfredson regimen, are commonly prescribed but their standardized nature does not fully address individual variability. Alternative approaches including Heavy Slow Resistance training and progressive loading models (e.g., Silbernagel protocol) have demonstrated clinical effectiveness and higher patient adherence, yet they still apply uniform loading across the triceps surae without accounting for architectural differences.

Given the heterogeneous structure of the soleus and gastrocnemii, targeted loading strategies may be required to optimize tendon adaptation. To address this, the present study employs a muscle architecture-based exercise program tailored to the functional and structural properties of each muscle. Ultrasound imaging will be used to evaluate muscle-tendon morphology, while isokinetic dynamometry and functional performance tests will quantify outcomes. Pain (VAS) and functional capacity (VISA-A) will also be assessed.

This randomized controlled trial aims to compare the effectiveness of an architecture-specific program with the Alfredson protocol on tendon remodeling, strength, endurance, functional performance, and symptom reduction in individuals with non-insertional Achilles tendinopathy.

Achilles tendinopathy is an overuse injury that develops as a result of repetitive microtrauma and is commonly observed in athletes as well as in sedentary male individuals. The clinical presentation is typically characterized by localized pain in the mid-portion of the tendon, tenderness, morning stiffness, and symptoms exacerbated by activity. Structurally, the condition is associated with tendon thickening, reduced echogenicity, and disorganization of collagen alignment . As the pathology becomes chronic, a marked decline in functional capacity is observed, which negatively affects both athletic performance and quality of life.

Conservative management is recommended as the first line of treatment, with the eccentric exercise protocol being the most widely used approach. Developed by Alfredson et al. (1998), this protocol consists of performing three sets of fifteen repetitions twice daily, aiming to stimulate collagen synthesis by mechanically loading the tendon. However, studies have shown that this protocol is not effective in every case; in some individuals, symptoms persist, and because the loading scheme is standardized, it fails to account for inter-individual variability. As an alternative, the Heavy Slow Resistance (HSR) exercise protocol, which includes both eccentric and concentric contractions, is applied three times per week. By incorporating controlled, high-resistance, slow-tempo loading, it aims to achieve functional recovery. Studies by Beyer et al. (2015) demonstrated that the HSR protocol yields clinical outcomes comparable to those of the Alfredson protocol, with higher patient satisfaction and compliance. In addition, the protocol developed by Silbernagel et al. (2007) incorporates both eccentric and concentric loading as well as functional activities in a more progressive model. By tailoring exercises to symptom tolerance during the acute and subacute phases and integrating plyometric activities such as hopping and jumping in the later stages, this protocol adopts a function-oriented perspective. Other conservative interventions, including extracorporeal shock wave therapy, cryotherapy, transverse friction massage, and footwear modifications, may provide symptomatic relief; however, their contribution to tendon remodeling is limited, and they do not ensure long-term structural or functional recovery. Although the Alfredson, HSR, and Silbernagel protocols are clinically effective, they carry significant limitations as they are not designed with muscle architecture-specific loading strategies.

The triceps surae muscle group exhibits heterogeneity in terms of architecture and function: the soleus muscle, with relatively short fascicle length, large cross-sectional area, and high pennation angle, contributes primarily to stability and endurance; the medial gastrocnemius is involved in both stability and controlled force generation; and the lateral gastrocnemius, with long fascicles and a narrow pennation angle, plays a key role in explosive force production. Despite these structural differences, both HSR and Silbernagel protocols apply a uniform loading strategy across all triceps surae muscles. For instance, optimal activation of the soleus requires the knee to be flexed, whereas activation of the medial and lateral gastrocnemius requires knee extension. Some existing protocols fail to consider such joint positioning, which may result in deviations from the optimal contraction axis, thereby reducing the effectiveness of muscle activation. This limitation may create uncertainty in the distribution of loading stimuli, hindering adequate tissue adaptation.

The muscle architecture-based exercise program to be implemented in this study is specifically designed in accordance with the architectural characteristics of each muscle, with the aim of providing the most optimal stimulus for each. Muscle and tendon architecture will be assessed using ultrasonography; strength outcomes will be measured with an isokinetic dynamometer; and functional performance will be evaluated through the Single-Leg Balance Test, Single-Leg Forward Hop Test, and Weight-Bearing Lunge Test. In addition, pain will be assessed using the VAS (Visual Analog Scale), and functional status will be measured with the VISA-A questionnaire. By conducting a comparative analysis, this study aims to evaluate the effects of architecture-based loading relative to classical protocols, thereby offering a novel perspective for treatment.

In light of this information, the present randomized controlled experimental trial has been designed to compare the effects of a muscle architecture-based exercise program with the classical Alfredson protocol on muscle-tendon architecture, muscle strength and endurance, functional performance, and symptomatic outcomes in individuals diagnosed with non-insertional Achilles tendinopathy.

Inclusion Criteria:

• Presence of pain and tenderness in the mid-portion of the Achilles tendon (2-6 cm proximal to the insertion) persisting for at least 3 months
• Clinical diagnosis of non-insertional Achilles tendinopathy
• Pain aggravated by physical activity
• Palpable tenderness localized to the mid-portion of the tendon
• Voluntary participation in the study with signed informed consent

Exclusion Criteria:

• Diagnosis of insertional Achilles tendinopathy
• History of surgery or fracture in the affected lower limb within the past 12 months
• Corticosteroid injection to the Achilles tendon within the past month
• Presence of systemic diseases (e.g., diabetes mellitus, rheumatoid arthritis)
• Presence of neurological disorders (e.g., peripheral neuropathy, central nervous system disorders)
• Prior experience with eccentric exercise protocols targeting the Achilles tendon
• Acute onset of symptoms suggestive of a tendon rupture