Figure 1.

Three muscle “schematic model” conceptually illustrates the necessity of muscles. a. Muscles can be functionally visualized as force vectors at the endpoint. b. A region of force space, the feasible force set, is achievable given this musculature. A particular target force vector can be decomposed into a target x-force and a target y-force. c. The valid coordination patterns for the x and y targets can also be viewed in muscle activation space as planes; the portion of the intersection of these two planes that is inside the unit cube is the task-specific activation set. Any point on the task-specific activation set will generate the same target force vector. d. The task-specific activation set can be projected onto the muscle coordinate axes, revealing the minimum and maximum activation in each muscle for the given applied force vector. The task-specific activation ranges can be constructed for each muscle, and reveal which muscles are necessary and which are redundant for a given target force vector.