  We discuss the dynamics of a generalized three-sphere microswimmer in which
the spheres are connected by two elastic springs. The natural length of each
spring is assumed to undergo a prescribed cyclic change. We analytically obtain
the average swimming velocity as a function of the frequency of the cyclic
change in the natural length. In the low-frequency region, the swimming
velocity increases with the frequency and its expression reduces to that of the
original three-sphere model by Najafi and Golestanian. In the high-frequency
region, conversely, the average velocity decreases with increasing the
frequency. Such a behavior originates from the intrinsic spring relaxation
dynamics of an elastic swimmer moving in a viscous fluid.
