Use of Body Mass

Note that mass is a co-efficient in the formulae for force, momentum, and energy
transfer alike: all three are directly proportional to mass. Since a human being’s mass
for the time it takes to deliver a strike is constant—a karateka with a body mass of 70
kilograms before a strike will have a body mass of 70 kilograms after the strike—
mass is often and erroneously dismissed as a constant in the equations for force,
momentum, and impulse. What matters is not the karateka’s body mass, but how
much of that mass is involved in the strike. A body mass of 70 kilograms is beyond
the karateka’s immediate control; how many of those 70 kilograms contribute to the
strike is very much within the karateka’s control. It is therefore crucial not to use the
arm alone to extend the weapon and hope for sufficient force and energy to break the
target. The entire body should be used by snapping the hips and pushing with the legs
in the direction of the target. This explains why boxers are seldom knocked
unconscious by jabs, where little more than the mass of the arm contributes to the
punch, but are frequently knocked out by hook punches where the entire mass of the
body is thrown behind the punch. The same principle of using the entire body mass to
deliver a blow applies in breaking techniques as well.