Background: Chronic insertional tendinitis of the Achilles tendon is an overuse injury seen with increasing frequency because of an aging population and an increased interest in sports. We evaluated the change in plantarflexion strength in patients after our surgical technique for chronic insertional Achilles tendinitis. Methods: From our previous clinical series of detachment and reconstruction of the Achilles tendon for the treatment of insertional tendinitis, ten patients were evaluated with an average followup of 32.1 (range 18 to 52) months. The average age was 65.7 years. We developed a mathematical model to predict the difference in plantarflexion strength between a reconstructed ankle and a healthy contralateral one. Isokinetic testing at 60 degrees/second was performed, measuring plantarflexion peak torque, dorsiflexion peak torque, and total work. Results: Our mathematical model predicted a decrease of 4% in plantarflexion torque after the surgery. Isokinetic testing found no significant differences in plantarflexion torque, dorsiflexion torque, or total work between the operated and nonoperated ankles. Conclusions: Complete detachment and reconstruction of the Achilles tendon do not decrease the working capacity of the gastrocsoleus muscle.