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

Abstract

Objectives: It has been proposed that increased tendon elongation after Achilles tendon repair contributes to selective weakness in end-range plantar flexion (Mullaney et al 2006). Excessive tendon elongation during maximum voluntary contraction (MVC) means greater muscle fiber shortening. Since mean frequency (MF) of the electromyogram (EMG) increases with muscle fiber shortening, it was hypothesized that during isometric plantar flexor MVCs MF would be higher on the involved versus non-involved side. Therefore, the purpose of this study was to examine MF during isometric MVCs in patients with Achilles tendon repairs. Methods: Maximum isometric plantar flexion torque was measured at 20° and 10° dorsiflexion, neutral, and 10° and 20° plantar flexion in 17 patients (mean±SD age, 39±9 years; 15 men, 2 women) 43±24 months after surgery (range, 9 months to 8 years). Surface EMG signals were recorded during strength tests. 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. Based on reported plantar flexor MF values it was estimated that with 17 subjects there would be 80% power to detect a 16% difference in MF between involved and noninvolved legs at P<0.05. Results: Patients had marked weakness in 20° plantar flexion (deficit 28±18%, P<0.01; 14 of 17 deficit >20%) but no significant weakness in 20° dorsiflexion (deficit 8±15%, P=0.20; 4 of 17 deficit >20%). MF increased moving from dorsiflexion to plantar flexion (P<0.001) but overall was not different between involved and noninvolved sides (P=0.22). However, differences in MF between the involved and noninvolved sides were apparent in the patients with marked weakness. At 10° plantar flexion 8 of 17 patients had marked weakness (>20% deficit). MF at 10° plantar flexion was significantly higher on involved versus noninvolved side in patients with weakness but this was not apparent in patients with no weakness (side by group P=0.014; Table 1). MF at 10° plantar flexion average across the 3 muscles was 13% higher on the involved versus noninvolved side in patients with weakness (P=0.012) versus 3% lower in patients with no weakness (P=0.47). Conclusion: Higher MF on the involved versus noninvolved side in patients with significant plantar flexion 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 and patients would have had weakness throughout the range of motion. Surgical and rehabilitative strategies are needed to prevent excessive tendon elongation and weakness in end-range plantar flexion after Achilles repair. [Table: see text]