Quote Originally Posted by AdamGB
Its harder to pull your arm back, there is nothing for your bicep to contract against so more effort to bring the punch back.

The bag/pads/kids head/ribs stop your elbow joint from opening 100% so your biceps don't have to work as hard, the same goes for all the other joints and muscle groups used when punching, thats why missing tired you out more.
Oh ok, it feels really awkward throwing punches in thin air coz i seem to get tense when i do it and dont let my arms go.