  The three-sphere swimmer by Najafi and Golestanian is composed of three
spheres connected by two arms. The case in which the swimmer can control the
lengths of the two arms has been studied in detail. Here we study a variation
of the model in which the swimmer's arms are constructed according to Hill's
model of muscular contraction. The swimmer is able to control the tension
developed in the active components of the arms. The two shape parameters and
the tensions acting on the two arms are then obtained by solving a system of
ordinary differential equations. We study the qualitative properties of the
solutions, compute analytically their leading order approximation and compare
them with numerical simulations. We also formulate and solve some optimisation
problems, aimed at finding the actuation strategies maximising performance, for
various performance measures. Finally, we discuss the structure of the
governing equations of our microswimmers from the point of view of control
theory. We show that our systems are control affine systems with drift.
