KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 114

The negative sign means that the force acting on the foot at the ankle
joint acts downward . This is not surprising because the entire body
weight, less that of the foot, must be acting downward on the ankle
joint.

3. About the COM, M = I 0 α ,

M 2 − R y 1 × 0 . 02 − R y 2 × 0 . 06 = 0

M 2 = 588 × 0 . 02 + ( − 579 . 2 × 0 . 06 ) =− 22 . 99 N · m

The negative sign means that the real direction of the muscle moment
acting on the foot at the ankle joint is clockwise, which means that the
plantarflexorsare active at the ankle joint to maintain the static position.
These muscles have created an action force that resulted in the ground
reaction force that was measured, and whose center of pressure was
4 cm anterior to the ankle joint.

Example 5.2 (see Figure 5.7). From the data collected during the swing of
the foot, calculate the muscle moment and reaction forces at the ankle. The
subject's mass was 80 kg and the ankle-metatarsal length was 20.0 cm. From
Table 4.1, the inertial characteristics of the foot are calculated:

m = 0 . 0145 × 80 = 1 . 16 kg

ρ 0 = 0 . 475 × 0 . 20 = 0 . 095 m

1 . 16 ( 0 . 095 ) 2 m 2 I 0 = = 0 . 0105 kg ·

21 . 69 rad/s 2

α =

Figure 5.7 Free-body diagram of foot during swing showing the linear accelerations
of the center of mass and the angular acceleration of the segment. Distances are in
centimeters. Three unknowns, R x 1 , R y 1 ,and M 1 , are to be calculated assuming a positive
direction as shown.