For a 70-kg subject, the weight on the barycenter corresponds to about
50 kg on the shoulders (due to the larger lever arm), i.e. about 25 kg on each shoulder.
During the pushup, the ratio between the lever arms changes
from about 4 to about ten.
Thus, to compensate the increasing torque, the muscle must develop a force
more than twice as large.
In addition to this: (a) the change in muscle length may require a slightly larger
or smaller activation to develop the same force (Frank-Starling law), and
(b) the distance of the tendon from the rotation center of the elbow joint decreases
durinf flection and this will further decrease the torque and require more force
