In vivo localized gastrocnemius subtendon representation within the healthy and ruptured human Achilles tendon

Abstract

The Achilles tendon (AT) is composed of three distinct in-series elastic subtendons, arising from different muscles in the triceps surae. Independent activation of any of these muscles is thought to induce sliding between the adjacent AT subtendons. We aimed to investigate displacement patterns during voluntary contraction (VOL) and selective transcutaneous stimulation of medial (MGstim) and lateral (LGstim) gastrocnemius between ruptured and healthy tendons, and to examine the representative areas of AT subtendons. Twenty-eight patients with unilateral AT rupture performed bilateral VOL at 30% of the maximal isometric un-injured plantarflexion torque. AT displacement was analysed from sagittal B-mode ultrasonography images during VOL, MGstim and LGstim. Three-way ANOVA revealed a significant two-way interaction of contraction type*location on the tendon displacement (F(10-815)=3.72, p<0.001). The subsequent two-way analysis revealed a significant contraction type*location interaction for tendon displacement (F(10-410)=3.79, p<0.001) in the un-injured limb only, where LGstim displacement pattern was significantly different from MGstim (p=0.008) and VOL (p=0.005). When comparing contraction types between limbs the there were no difference in the displacement patterns, but displacement amplitudes differed. There was no significant difference in the location of maximum or minimum displacement between limbs. The displacement pattern was not different in non-surgically treated compared to un-injured tendons one-year post rupture. Our results suggest that near the calcaneus, LG subtendon is located in the most anterior region adjacent to medial gastrocnemius. However, free tendon stiffness seems to be lower in the injured AT, leading to more displacement during electrically-induced contractions compared to the un-injured.