A 6-week-old female infant with congenital talipes equinovarus (CTEV) has undergone 4 weeks of serial casting using the Ponseti method. On review, the adductus and varus have been corrected, but residual equinus persists despite appropriate casting. The foot cannot be dorsiflexed beyond 5° plantarflexion. The treating orthopaedist plans a percutaneous tenotomy. Which tendon is being sectioned?