The objectives of this study were to identify the position where optimal leg muscle force occurs and to analyze the lower extremity muscle activity by comparing different hip and knee joint angle positions using a newly developed closed kinetic chain (CKC) leg press exercise device. Muscle force and electromyogram (EMG) measurements during maximum voluntary isometric leg presses (MVC-CKC) were performed on 6 healthy women in 6 different knee flexion angles (15°, 30°, 45°, 60°, 75°, 90°) and 2 different postures (supine and trunk upright posture). Results showed that the muscle force of MVC-CKC reached its peak at 60° knee flexion angle in both postures. Significant knee angle-dependent differences in EMG activity for both rectus femoris and gastrocnemius muscles were observed, while posture-dependent differences in EMG were found in biceps femoris. These particular joint-dependent EMG activities may be explained by the architectural characteristics of bi-articular muscles and muscle length-tension relationship theory. Co-activation of an antagonist muscle may appear with activation of the gastrocnemius muscle when the knee approaches full extension. Consequently, measurement of isometric CKC leg extension muscle force at 60° knee flexion angle in the trunk upright posture seems to be the optimal position.