Purpose: This study compared the thigh circumference and Lysholm scale of a stable and unstable exercise group of patients who had undergone an anterior cruciate ligament reconstruction (ACL reconstruction). Methods: The subjects were patients more than 4 weeks after their ACL reconstruction and could stand on one leg. The patients were divided into a control group with 9 patients performing closed kinetic chain exercises on a stable floor and an experimental group with 10 patients performing closed kinetic chain exercises en an unstable floor. The degree of muscle atrophy was compared by measuring the circumference of the injured thigh before the exercise program, and 3 weeks and 6 weeks after the exercise program. The Lysholm scale was used to assess the function of the knee joint. Results: There was no significant increase in thigh circumference according to the exercise periods in the two groups. However, there was a statistically significant increase before exercises and 6 weeks after the exercises (p<0.05). There were statistically significant differences in the Lysholm scores between the two groups (p<0.05). Conclusion: There were no statistically significant increase in the two groups, but there was a significant difference between before the exercise program and 6 weeks after the exercise program (p<0.05).