Objectives: This study aims to evaluate patients with conservatively treated stable lateral malleolar fractures with isokinetic tests. Patients and methods: Between January 2016 and November 2017, a total of 24 patients (12 males, 12 females; mean age 40.8±15.0 years; range, 18 to 68 years) with an isolated stable lateral malleolar fracture treated conservatively with the circular cast were included in this prospective study. Calf circumferences, ankle range of motion (ROM), pain levels, and functional outcomes were recorded. The muscle strengths and endurance of the injured side were compared with the non-injured side. All patients were evaluated by isokinetic test after removal of the cast, and three and six months after the rehabilitation period. Results: The ROM was found to be lower after removal of the plaster cast, compared to the contralateral ankle. During cast removal, we also found that both dorsiflexor and plantar flexor muscle strength decreased by 25.6% and 44.7%, respectively, and decreased to 10.3% and 3.6% at three months post-rehabilitation. At the end of six months, no statistically significant difference was found between the two sides. In the dorsiflexion-plantar flexion endurance values, 37.8% and 54.1% deficit were detected before the rehabilitation protocol, respectively (p<0.05). At three months, these values decreased to 6.1% and 13.6%, respectively and the endurances of the injured sides surpassed the non-injured sides (p<0.05) at six months. Conclusion: Conservative management of stable isolated lateral malleolar fractures with circular cast causes atrophy and decreases strength-endurance of the calf muscles due to immobilization. These changes are expected to diminish over time and functional outcomes are excellent with a good rehabilitation program.