(a) We assume that the human body is a uniform cylinder with the radius R and the height H. It is supported on a rigid substrate (a representation of human feet). Suppose that the human body cylinder is tilted forward as shown in (b) by a small angle θ. To achieve the stability of the tilted posture, two competitive torques, one from the skeletal muscle force F
and the other from the gravitational force F
, must be balanced. The gravitational torque is written as T
(x/2) with x/2 being the shift of the horizontal position of the center of mass (CM). We assume that F
depends on the extension y of muscles as shown in (b). For the tilting angle θ, we get \(x=H\,\sin \,\theta \) and \(y=R\,\sin \,\theta =xR/H\).