Table 1.

Force directions for the position protocol

Direction name	Unit force vector [Fx, Fy, Fz]
d1	[+0.577, +0.577, +0.577]
d2	[−0.577, +0.577, +0.577]
d3	[+0.577, −0.577, +0.577]
d4	[+0.577, +0.577, −0.577]
d5	[−0.577, −0.577, +0.577]
d6	[+0.577, −0.577, −0.577]
d7	[−0.577, +0.577, −0.577]
d8	[−0.577, −0.577, −0.577]

Note the global x-, y-, and z-axes paralleled the medio-lateral, backward-forward, and downward-upward directions, respectively (see Fig. 1).