Neuromechanical Modulation of the Achilles Tendon During Bilateral Hopping in Patients with Unilateral Achilles Tendon Rupture, Over 1 Year After Surgical Repair

Abstract

Background: Patients who have had an Achilles tendon (AT) rupture repaired are potentially at higher risk for re-rupture than those without previous rupture. Little attention has been given to the neuromechanical modulation of muscle-tendon interaction and muscle activation profiles during human dynamic movements after AT rupture repair.

Objective: The purpose of this study was to examine muscle-tendon behavior and muscle activation during bilateral hopping.

Methods: We enrolled nine subjects who had undergone surgical repair of unilateral AT rupture within the past 1-2 years. Subjects performed bilateral hopping while we took ultrasound, kinematic, and electromyogram recordings and measurements. AT behaviors were also recorded. We then compared responses between values obtained from the ruptured AT leg (LEG_ATR) and non-ruptured AT leg (LEG_NOR).

Results: During hopping, the AT stretching amplitudes were greater in the LEG_ATR than in the LEG_NOR, although the peak AT force and stiffness were smaller in the LEG_ATR than in the LEG_NOR. The AT negative mechanical work did not show any significant differences between both legs. However, positive works were significantly lower in the LEG_ATR than in the LEG_NOR. Electromyogram patterns in both soleus and tibialis anterior muscles clearly differed after ground contact for the LEG_ATR and the LEG_NOR.

Conclusions: These results suggest that the repaired ruptured AT can be compliant and have insufficient Young's modulus, which can influence mechanical responses in muscle activities. The modulation of agonist-antagonist muscle activities corresponding to the different levels of stiffness between the LEG_ATR and the LEG_NOR may not be fully functioning during the pre-activation phase.