It's no surprise to return from a boxing lesson with sore forearms. Given the impact involved, especially in contrast to the relatively small bones and muscles in your forearms, the surprise is that the pain is not frequent or severe. It is only part of the sport of boxing, although they can take certain measures to relieve their discomfort and know if there are signs of a serious injury.
Simple muscle pain
The pain in the muscles of your forearms from boxing has the same cause as other types of muscle pain. When you repeatedly hit a target with the force involved in a boxing hit, the muscles in your forearms suffer microscopic tears. These tears heal stronger and thicker (this is the basic physiological mechanic behind creating muscles) but they will hurt for a few days after your exercise.
If you put ice on your forearms for 20 or 30 minutes after a particularly strenuous boxing session, this will stop the inflammation that causes most of the discomfort. And over-the-counter painkillers like ibuprofen can reduce swelling and relieve pain.
You can not completely prevent forearm pain from boxing, but you can take steps to minimize your discomfort. The first rule is to not squeeze your fist before hitting. Keep your hand loose even on your way to the target, and relax it immediately afterwards, to minimize the pain in your forearms. In light or moderate sessions, box with your gloves on without the bandages. This forces the muscles of your forearms to work harder to stabilize your wrists, stretching the muscles, in this way they are more resistant to impact. In heavy sessions, wrap your hands and wrists to secure them. Bandages will help protect your forearms from the increased impact of exercise. The contact with the bag or mittens must occur on the first two knuckles of the fist with the hand fully bent.
A dull ache in your forearms is an inevitable part of boxing. However, acute pain may be a sign of a more serious injury; as well as pain and a limited range of movement in your wrist or elbow, or any pain that feels as if it were radiating or shooting through your arm. Numbness is another bad sign. If you experience any of these symptoms, you should go to the doctor as soon as possible. If you do not treat it, these serious injuries can leave you on the bench for weeks or months (or eventually become an injury that ends with your one-time boxing days).