MSkin
This page describes the MSkin deformer in Cinema 4D. It transfers Muscle object deformation onto character geometry — analogous to Skin for joints — and belongs below the Skin object so muscle effects layer on top of joint deformation.
The MSkin object ties Muscle objects to the geometry of your character, in the same way that joints and the Skin object deform your mesh.
Muscle objects can be placed anywhere in the hierarchy, but it is recommended that you place the MSkin object below any existing Skin object used on a character's geometry.
This is necessary to ensure muscles deformations are applied after the joints deformations.