  Snakes' bodies are covered in scales that make it easier to slide in some
directions than in others. This frictional anisotropy allows for sliding
locomotion with an undulatory gait, one of the most common for snakes.
Isotropic friction is a simpler situation (that arises with snake robots for
example) but is less understood. In this work we regularize a model for sliding
locomotion to allow for static friction. We then propose a robust iterative
numerical method to study the efficiency of a wide range of motions under
isotropic Coulomb friction. We find that simple undulatory motions give little
net locomotion in the isotropic regime. We compute general time-harmonic
motions of three-link bodies and find three local optima for efficiency. The
top two involve static friction to some extent. We then propose a class of
smooth body motions that have similarities to concertina locomotion (including
the involvement of static friction) and can achieve optimal efficiency for both
isotropic and anisotropic friction.
