Like in every contact sport, you can get injuries due to hard contact. Every karate student must train himself through various body conditioning exercises to be able to absorb all but the hardest contact to the body.
This not only improves the physical condition but also removes the mental fear of injury. Another advantage is that injuries are less common. While training with a partner, you also get to learn where and how to focus your attack.
Condition yourself gradually and regularly and inform your partners if their pounding is the wrong strength. Alwaysfocus or lean into contact; this helps you condition deeply and preventthe blow from going painfully to the bone." Always strike deeply, never whacking the surface or bouncing the blow off the target; hit in and let it sink in before withdrawing.
Arm Pounding: Partners bow, face each other in mirror image fighting stances. Partner #1 steps through, "punching" with the (previously) rear hand; #2 steps back, so that throughout the drill they remain in mirror image stance. There are four motions for #2: first, strike sideways, with the back of the mirror image forearm; second, strike from underneath, with the back of the opposite side forearm; third, with the opposite hand, wauke your partner’s arm across your body; fourth, hammerhand with the mirror image arm. Then #2 "punches" and the drill is reversed. Each cycle, the three strikes are directed either closer to the wrist, the middle, or the elbow end of the forearm, and the target is switched each cycle to work the whole forearm and prevent over pounding of certain sites. Always hit muscle to muscle, maintain a strong stance, keep tension in your arm whether striking or being struck, and in the wauke motion, keep your shoulder down and elbow in. This is a conditioning , not sparring drill, so don’t try to hit your partner and don’t hurriedly "block" with the first motion. Instead, take your time and deliver it accurately. Lastly, maintain mushin; see your whole partner and strike accurately without staring right at the target.
Other drills are less formal and come in many forms. This is not a complete list and it does not mandate a certain style of conditioning.
Stomachs: Push stomachs out and down, then trade blows with alternating arms, to the abdomen—not the ribs. Favor the solar plexus because it is weakest, but work the whole abdomen (this also gives the solar plexus time to rest). Hit with the whole fist, not with two knuckles, and hit in (do not "slap") as to make your partner take a small step back. Other methods include facing opposite directions and trading roundhouse kicks.
Hands: Knuckle pushups, makiwara, striking into sand or beans with fists or wrist. There are a number of ways to increase strength, from wrist curls to pushing exercises with partners to squeezing devices.
Backs: strike from behing with simultaneous forearm blows to a partner who has extended arms to the front; or simply punch the latissimus dorsi area.
Legs: ball of foot kicks or rolling pins for shins, or trade roundhouse kicks to crossblocks and work arms and legs simultaneously. Trade just roundhouse kicks to thighs, or work the whole leg by alternating instep or shin blows to inner and outer lower and upper leg.
Arms: Try rubbing by facing a partner thumbside wrist to thumbside wrist and pulling back to rub the side of the arm missed ; face a partner in horsestances and swing opposite arms together, striking the inner arm, then hit backs of the arms together and switch to the other arm.