A free body diagram of the shank and foot is drawn as shown in Figure 1, and
only the forces which traverse the free body boundary are considered. The force
conditions have been simplified to the patellar tendon force, the tibio-femoral
joint reaction force, the weight of the limb, and the weight of the exercise boot.
Other forces across the joint are not accounted for in this analysis: however,
inclusion of other forces (e.g. gastrocnemius) will increase the value calculated
for the patellar tendon force and therefore the patello-femoral joint reaction