Background Eccentric exercise (ECC) is widely recognized as an effective treatment for non-insertional Achilles tendinopathy (NIAT); however, the mechanisms underlying its apparent superiority over concentric exercise (CON) remain poorly understood. This randomized controlled trial aimed to investigate changes in triceps surae motor unit firing properties, pain, function, and AT morpho-mechanical properties following a 6-week intervention involving torque feedback training with isolated ECC and CON contractions in individuals with NIAT.

Methods Twenty-six individuals with NIAT were randomized to ECC or CON training. Motor unit firing properties (mean discharge rate [MDR], recruitment and de-recruitment thresholds, and torque–firing cross-correlation and neuromechanical delay) were assessed in the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SO) muscles using high-density surface electromyography (HD-sEMG) during isometric plantarflexion at 10%, 40%, and 70% of maximal voluntary contraction (MVC). Pain, function (VISA-A), and tendon properties (ultrasound, elastography) were measured at baseline, week 3, and week 6.

Results Both groups showed similar improvements in pain (P < 0.0001) and VISA-A scores (P < 0.001). Tendon stiffness increased in both groups by week 3 but was higher in ECC by week 6 (P = 0.02). Motor unit adaptations differed: CON demonstrated an increase in MG MDR at 40% MVC, while ECC showed a decrease (interaction: P = 0.0008). Only ECC led to increased de-recruitment thresholds in the LG at 70% MVC (P < 0.0001). However, both groups exhibited reduced MDR in the LG at high-force levels (P < 0.05). Additionally, both interventions reduced GL torque–firing relationships (P = 0.025) and decreased SO neuromechanical delay (P = 0.031).

Conclusion A 6-week visuo-motor torque feedback training program involving isolated CON or ECC contractions leads to comparable improvements in clinical outcomes. However, contraction-specific and muscle-specific changes in motor unit function and tendon stiffness suggest distinct neuromechanical adaptations. These differences may underlie the observed effects and warrant further investigation in longer-term studies to determine their impact on long-term clinical outcomes.

HIGHLIGHTS

- Changes in clinical outcomes, tendon morphomechanical properties, and triceps surae motor unit behavior were assessed for the first time following a 6-week rehabilitation protocol for non-insertional Achilles tendinopathy, involving isolated concentric (CON) or eccentric (ECC) contractions.

- Both CON and ECC training led to comparable improvements in pain, self-reported outcomes, and perceived function.

- Despite these similar clinical improvements, CON and ECC protocols produced distinct adaptations in estimated tendon stiffness and motor unit firing characteristics.

These neuromechanical differences may indicate potential long-term divergences between training modalities, which should be explored in studies with extended intervention periods.

The authors have declared no competing interest.

ISRCTN46462385

https://bmjopen.bmj.com/content/12/7/e058683.long

This work was supported by ANID PhD Scholarship awarded by the Government of Chile. Recipient: Ignacio Contreras-Hernandez, Scholarship ID number: 72200295. Francesco Negro was funded by the European Research Council Consolidator Grant INcEPTION (contract no. 101045605). Eduardo Martinez-Valdes is supported by an Orthopaedic Research UK Early Career Research Fellowship (ORUK ref-574).

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This study was approved by the STEM Ethical Review Committee, University of Birmingham, UK (ERN_20-0604A)

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