To evaluate the muscle strength of the lower limbs over time in polio survivors during 5 years of follow-up and to examine the rate of change in their muscle strength, we performed a prospective, longitudinal study of polio survivors (n = 63: 61 with postpolio syndrome) living in the community who participated voluntarily. Their isokinetic knee-extensor and knee-flexor muscle strength (peak torque) at angular velocities of 60 and 120 deg/sec, using a fixed dynamometer (Biodex) were measured over a 5-year period. At 5-year follow-up, approximately 90% of the subjects had decreased knee extensor strength at both angular velocity of 60 and 120 deg/sec: similarly, at both angular velocities, approximately 80% of the subjects had decreased knee flexor strength. The annual average rate of decrease in the peak torque of the knee extensors was significantly greater than that of the flexors at both angular velocities, and the difference in the rates between the extensors and the flexors was marked at the faster angular velocity. The polio survivors had a progressive decrease in lower limb isokinetic muscle strength over time. In addition, the annual rate of decrease of the knee-extensor, the so-called weight bearing muscle, was greater than that of the knee-flexor.