Spring-mass models have been widely accepted to explain the basic dynamics of human gait. Researchers found that the leg stiffness increased with gait speed to increase energy efficiency. However, the difference of leg stiffness change with gait speed between the young and the elderly has not been verified yet. In this study, we calculated the lower limb stiffness of the elderly using walking model with an axial spring. Vertical stiffness was defined as the ratio of the vertical force change to the vertical displacement change. Seven young and eight elderly subjects participated to the test. The subjects walked on a 12 meter long, 1 meter wide walkway at randomly ordered four different gait speeds, ranging from their self-selected speed to maximum speed. Kinetic and kinematic data were collected using three force plates and motion capture cameras, respectively. The vertical stiffness of the both groups increased as a function of walking speed. Average walking speed of the elderly was significantly slower than that of the young, but the walking speed correlated well with the optimal stiffness that maximizes propulsion energy in both groups. The results may imply that human may use apparent limb stiffness to optimize energy based on spring-like mechanics.