Originally Posted by Boxmaster
Muscular arms are definitely bad because the smaller your arms the faster your punches. The speed punches are thrown using the stomach muscles, not the chest or the arms. The power punches come from torque and from the leg muscles. You can generate more power by keeping your legs spread more widely apart and twisting your hips with your punches.
So the muscles you'd want to train are abs and legs. Abs are for speed and legs for power.
This is good, for the most part. A proper punch usually develops power from the abs and legs. However, you may find in practice that you need arm muscles (particularly a good grip) for stability of the entire fist/arm connection.
Overall, it's pretty important to be strong throughout your body.