What is a muscle system and what does it do?

Muscles are the engine that your body uses to propel itself, they turn energy to motion (Craig Freudenrich, Ph.D.)
The muscles that we are interested in creating are skeletal muscles. They are the muscles that are attached to our skeletal structure. To have accurate muscle simulations is important to have knowledge of the human skeletal structure. These provide an equally important role in creating rigid deformations, in areas that are exposed such as joints themselves i.e the elbow. Knowledge of this is far more common due to the expose of the basic rigging process in which bones are used as influence objects to attach a skin to. Other Muscle types include smooth - found in the internal organs, and the cardiac muscles of the heart.

The muscle layer is the main contributing factor to the surface form. Muscles account for half of the total mass of the body and fill in almost completely the gap between the skeleton and the skin [Richer81].
Made of up of cells fast twitch and slow twitch fibers full of myofibris, proteins that allow the basic action of a muscles, to contract under instruction the nervous system.. Muscle are complex structures that are situated in pairs. one muscle is used to move the skeletal structure in one direction, whilst the other moves it back. Muscle changes shape during contraction, which impacts the shape of the outer skin.

Muscle models can broadly be classified into two categories:
purely geometric models and physically-based ones

Geometric models tend to use the basic ellipsoid blockapproximates representing fusiform muscle volume-preserving multi-belly muscles are used as a set to make up larger muscle i.e the pectorial

muscle flexing and bulging is simulated by binding the rotation scale and translation of each ellipsoid to the degrees of freedom of the underlying skeleton joints. this system cannot
capture most muscle shapes.Scheepers97

cylinders can be used instead. Made up of a certain number of
cross-sections that consist in turn of a fixed number of vertices. Volume variation of the muscle during deformation is reduced by scaling each crosssection so as to preserve its area [Wilhelms97b]

One of the first physically-based models is due to Chadwick et al.
[Chadwick89]. The muscle is embedded in a FFD lattice [Sedeberg86].
Muscle deformation is achieved by simply deforming the embedding space. The FFD control points are moved potential problems is that the FFD box may not approximate the muscle shape very tightly springs termed angular springs that tend to restore the initial curvature of the surface at each vertex.

Upon isotonic contraction, the volume of the belly increases amplifying the influence on the shape of the skin curve-based method for creating wrinkles on reference poses
Using any collection of NURBS objects as collision surface(s), the skin plug-in finds and fills in the empty voids as a Fat
Layer between skin points oint caching Good character animation requires convincing skin deformations (johannason.s)

With the second method Instead, animators typically manipulate an underlying hierarchical skeleton

This technique assigns a set of infuencing joints and blending
weights to each vertex in the character. The skin is computed by
transforming each vertex by a weighted combination of the joints'
local coordinate frames.

There are two parts to skining, process refers to how artists use tool sets to describe the behavior of skin geometry as the skeleton moves.
Skin computation refers to the method by which the deformed mesh geometry is evaluated for display.
For high-end applications, the authoring methods drive skin creation while for interactive systems, computation methods dominate