An exercise device includes a relatively rigid platform supported by a compressible base. The base has a central flat and then slopes outward and upward from the central flat in a spherical section. The base permits instability in height, angle, and shear strain movement. The user performs postures and/or exercises using the exercise device under each appendage being used for support. The instability triggers the Golgi tendon receptor and the muscle spindle receptor and thereby achieves physiological benefits. The muscular training devices come in different sizes and configurations to make up a muscular training system.