Electromyographic Evidence of Excessive Achilles Tendon Elongation During Isometric Contractions After Achilles Tendon Repair

Abstract

PURPOSE: Increased tendon elongation after Achilles repair is thought to contribute to selective weakness in end-range plantarflexion (PF). Excessive tendon elongation during maximum voluntary contraction (MVC) means greater muscle fiber shortening. Since mean frequency (MF) of the electromyogram (EMG) increases with decreasing fiber length, it was hypothesized that MF would be higher on the involved (Inv) versus non-involved (Non) side during isometric PF MVCs. The purpose of this study was to examine MF during isometric MVCs in patients with Achilles tendon repairs. METHODS: Isometric PF MVC was measured at 20°, 10°, 0° dorsiflexion (DF), and 10°, 20° PF, in 17 patients (age, 39±9 years; 15 men, 2 women) 43±24 months after surgery. Surface EMG signals were recorded during MVCs. MF was calculated from Fast Fourier Transforms of medial gastroc (MG) lateral gastroc (LG) and soleus (S) EMG signals. Effect of weakness on MF was assessed using analysis of variance. RESULTS: Patients had weakness in 20° PF (deficit 28±18%, P<0.01; 14 of 17 deficit >20%) but no weakness in 20° DF (deficit 8±15%, P=0.20; 4 of 17 deficit >20%). MF increased moving from DF to PF (P<0.001) but was not different between Inv and Non (P=0.22). At 10° PF 8 of 17 patients had weakness (>20% deficit). MF was significantly higher on Inv versus Non, across all angles, in patients with weakness versus no weakness at 10° PF (side by group P=0.014; Table 1). MF was 13% higher on Inv versus Non in patients with weakness (P=0.012) versus 3% lower in patients with no weakness (P=0.47). CONCLUSIONS: Higher MF for Inv versus Non in patients with PF weakness is consistent with greater muscle fiber shortening. This indicates that weakness was primarily due to excessive lengthening of the repaired Achilles tendon. If weakness were simply due to atrophy, a lower MF would have been be expected.