Runners with chronic mid-portion Achilles tendinopathy have greater Triceps surae intracortical inhibition than healthy controls

      Background: Achilles tendinopathy (AT) is an overload injury, affecting mostly runners. Persistent triceps surae muscle weakness has been reported in AT patients. Muscle force is influenced by inhibitory and excitatory circuits, and imbalances between these (e.g., increased short-interval intracortical inhibition - SICI) may negatively affect strength. Increased SICI has been observed with patella tendinopathy; however, it is unclear if this increased inhibition is present in chronic mid-portion AT patients. Thus, this study aimed to investigate the intracortical inhibitory mechanisms in runners with mid-portion AT.
      Methods: Runners with chronic mid-portion AT (n=11; 44.1±8.4 years) and healthy controls (n=13; 33.9±4.25 years), with a running routine of at least twice weekly for more than 4 months, were recruited. All completed the VISA-A questionnaire AT (AT= 70.7±7.0) and Control (100±0). The most symptomatic leg (8 unilateral AT and 3 bilateral AT) was compared with control group dominant legs. Plantar flexor maximal voluntary isometric contraction (MVIC) torque was measured via isokinetic dynamometer (knee fully extended, ankle at 0°). Triceps surae endurance was measured with standing single leg heel raise to failure (SLHR) test. SICI was assessed using paired-pulse transcranial magnetic stimulation to the motor cortex area associated with the leg, delivering 20 paired-pulses at 0.8x (first pulse) and 1.2x (second pulse) of active motor threshold (AMT) with 3ms interval and 20 single pulse stimulation at 1.2 AMT, all during a 10% plantar flexion contraction. Two-way repeated measures ANOVA was used to compare SICI between muscles (Soleus, and Gastrocnemii) and groups. Independent t-test was used to compare SLHR and MVIC peak torque between groups.
      Results: SICI was higher in AT group (67.4±9.0) than controls (54.0±9.0); (p=0.039,) independent of the tested muscle (no muscle vs group interaction; p =0.828). AT group performed ~28% fewer SLHR repetitions than the controls (AT, 27.5.0±6.8 and control 38.5±5.3 repetitions, p=0.004). There was no difference in MVIC peak torque corrected for body mass (N.m/Kg), (AT=1.4±0.3; Control=1.6±0.4, p =0.093).
      Discussion: The AT group displayed greater intracortical inhibition of the triceps surae muscles and reduced SLHR endurance, without deficit in maximal isometric torque. SICI could be negatively influencing SLHR endurance in AT; thus, rehabilitation aiming to reduce intracortical inhibition should be considered for better outcomes.
      Conflict of interest: The authors declare no conflict of interest related to the present work.