Author(s): CONTRERAS, I., ARVANITIDIS, M., FALLA, D., NEGRO, F., MARTINEZ-VALDES, E. , Institution: UNIVERSITY OF BIRMINGHAM, Country: UNITED KINGDOM, Abstract-ID: 2295

INTRODUCTION:
Non-insertional Achilles tendinopathy (NIAT) is a common debilitating tendon disorder characterised by an insidious onset of pain accompanied by swelling, stiffness and thickening in the mid-portion of the Achilles tendon (AT). The aetiology of the NIAT remains debated; however, excessive tendon loading is considered a fundamental causative factor. Recent evidence suggests that the triceps surae muscle may influence the magnitude and distribution of the AT load, stress and strain. Consequently, it has been proposed that asymmetrical load transmission to the AT may play an essential role in the development of this condition. This study investigates the triceps surae motor unit firing parameters in individuals with NIAT compared to controls at low, intermediate, and high force levels.


METHODS:
Twenty-five healthy (17 males, 8 females, 28.60 ± 3.92 years) and twenty-six participants with NIAT (14 males, 12 females, 29.04 ± 8.46 years) participated in the study. Motor unit firing parameters of the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SO) muscles were assessed using high-density surface electromyography (HD-sEMG) during isometric plantarflexion contractions at 10%, 40%, and 70% of the maximal voluntary contraction (MVC). EMG signals were decomposed into individual motor unit spike trains by convolutive blind source separation. Motor unit firing rate parameters were compared using a linear mixed-effect model with fixed effect group (control, NIAT), muscle (MG, LG, and SO), torque (10%, 40%, and 70% MVC), and random effect subjects.
RESULTS:
Linear mixed-effect model analysis showed that the DR of the LG increased (P=0.002), and the derecruitment threshold of the LG decreased (P=0.04) in the NIAT group compared to the control group at 70% MVC. Additionally, the cross-correlation coefficient (between the cumulative spike train and torque) of the LG decreased at 10% MVC (P<0.0001) and increased at 70% MVC (P=0.013) in the NIAT group compared to the control group.
CONCLUSION:
Our findings demonstrate that individuals with NIAT have load-dependent changes in the LG motor unit firing rate parameters, affecting the contribution of the LG to the net plantarflexion force. These results provide additional evidence of the central role of asymmetrical load transmission on the pathophysiology of NIAT.