We now try to calculate how much weight a human being
can carry. Just as strings muscles can only pull. In the human arm
the biceps lifts the arm and the triceps lowers the arm. The humerus
bone pushes down on the forearm. Muscles contract when
subject to an electric stimulus from the brain. The max pulling force
is approximately
where A is the cross sectional area of the muscle.
We ask how much weight a weight lifter can hold in a position where
the forearm is horizontal. The torque equation with the end of the
humerus bone as reference point is
Where d is the distance to the tendon attachment point and L
is the distance to the point of contact with the weight being held and
is the pull by the biceps muscle. Solving for m we
have
We consider a biceps with radius 4 cm and thus area 
cm
and use d=4 cm and L=35 cm. Putting in numbers we find
Remembering that we have two arms we conclude that the max
mass which can be supported is 80 kg. Note that as is the case for most muscles in humans they are at a severe mechanical ``disadvantage''. In this case for example the muscle exerts
a force of 3500 N for lifting a weight of 392 N. The benefit
is that motion of the arm is faster by the ratio of these forces
which evidently was more important for our ancestors than
brute force.