fyi, it looks like the current MrInk muscles.ma file has a few issues when using the "Run Mesh Analysis" tool. The quad meshes have a hole/missing face in them.
zMeshCheck -select;
// Error: l_quads_tissue failed checks: -boundaryEdges //
// Error: r_quads_tissue failed checks: -boundaryEdges //
// Result: l_quads_tissue.vtx[0] l_quads_tissue.vtx[32] l_quads_tissue.vtx[70] l_quads_tissue.vtx[755] l_quads_tissue.vtx[892] r_quads_tissue.vtx[0] r_quads_tissue.vtx[32] r_quads_tissue.vtx[70] r_quads_tissue.vtx[755] r_quads_tissue.vtx[892] //