The purpose of this study was to evaluate the validity of the simple measurements of the muscle performance of the lower extremity, one legged hop testing for distance and the measurement of the circumference of the thigh 15 cm proximal to the joint line, by comparing them to the isokinetic strength testing 5 to 9 years after an ACL reconstruction with a bone-patellar tendon-bone (BTB) autograft. The measurements were performed on 86 patients on average 7 years after the surgery. The clinical evaluation was performed using the standard knee ligament evaluation form of the International Knee Documentation Committee (IKDC) and the Lysholm and the Marshall knee scores. At seven years, the isokinetic mean strength deficit of knee extension at the 60 degrees per second was 10 % in the operated limb as compared to the contralateral limb (NS). In the knee flexion, the differences were even smaller and statistically not significant either. The strength deficit of the knee extension at all knee angle velocities (p < 0.005), and flexion at 60 degrees per second (p < 0.05), correlated to the one legged hop testing, so that the patients with the greatest strength deficit also had the worst outcome in the one legged hop test. Also, the correlation between thigh atrophy and the deficit in the isokinetic strength test was significant in knee extension at all knee angle velocities (p < 0.001), and in knee flexion at 180 degrees per second (p < 0.005). In the final evaluation of the IKDC, 21 patients were rated as "normal", 51 as "nearly normal", 13 as "abnormal", and one as "severely abnormal". The mean of the Lysholm score was 83, classified as good, and that of Marshall score 43, classified as good. In conclusion, the one legged hop testing, as a functional muscle strength testing after an anterior cruciate ligament reconstruction, seems to correlate well with the isokinetic strength testing of the knee, especially in the knee extension. Because the hop testing can be easily performed and without extra equipment, we recommend its use for the evaluation of the functional muscle performance after an anterior cruciate ligament reconstruction. The measurement of the thigh atrophy is also easy to perform, and should be used beside the one legged hop testing, especially if the isokinetic strength testing is not available.