To be honest sea gull, I wouldn't advise a bag that moves very little or not at all.

With the impact of your body, spear headed by your fist meeting a solid object, something has to give. Usually it is the bag that gives and your fist remains unscathed.
However if the bag refuses to move, objects inside your hand will shift.

A war is impossible to carry out without weapons, a gardener will have difficulty without his tools, a carpenter is useless without his implements. A boxer isn't much good without his tools either.