Load and muscle-dependent changes in triceps surae motor unit firing properties in individuals with non-insertional Achilles tendinopathy.

Non-insertional Achilles tendinopathy (NIAT) induces morpho-mechanical changes to the Achilles tendon (AT). Evidence on how triceps surae motor unit firing properties are influenced by altered tendon mechanics in NIAT is limited. This study investigated motor unit firing properties (mean discharge rate (DR), recruitment and de-recruitment thresholds, and discharge rate variability (COVisi)), motor unit firing-torque relationships (cross-correlation coefficient between cumulative spike train (CST) and torque, and neuromechanical delay (NMD)) and neural drive distribution (connectivity strength and functional networks) of the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SO) muscles during isometric plantarflexion contractions at 10%, 40%, and 70% maximal voluntary contraction (MVC) using high-density surface electromyography (HD-sEMG) on 26 individuals with NIAT and 25 healthy controls. AT's morpho-mechanical properties (thickness, cross-sectional area, length, and stiffness) were assessed via ultrasonography. Motor unit properties changed in a load and muscle-dependent manner. LG DR increased (p = 0.002) and de-recruitment threshold decreased (p = 0.039) at 70% MVC in the NIAT group compared to controls. The CST-torque cross-correlation coefficient of the LG decreased at 10% MVC (p < 0.0001) and increased at 70% MVC (p = 0.013) in the NIAT group. Connectivity strength for the 0-5 Hz and 5-15 Hz frequency bands decreased (p < 0.01) in the NIAT group at 10% MVC. Furthermore, NIAT individuals showed reduced tendon stiffness and increased thickness (p < 0.01). This study shows that individuals with NIAT exhibit load- and muscle-dependent changes in motor unit firing properties, motor unit-torque relationships, and neural drive distribution to the triceps surae. These alterations may be due to muscle-specific compensations for AT's modified mechanical properties. KEY POINTS: Individuals with non-insertional Achilles tendinopathy (NIAT) have changes in the neural drive to the lateral gastrocnemius (LG) muscle and altered contribution of the LG to the net plantarflexion torque. Individuals with NIAT show a more uneven distribution of neural drive to the triceps surae muscle at low force levels, characterised by reduced intermuscular coherence between the medial and lateral gastrocnemius in the 0-5 Hz and 5-15 Hz bands compared to the control group. Our findings support the idea that the LG may have a central role in the pathophysiology of this condition, possibly affecting the load transmission to the Achilles tendon (AT).