Generally speaking the fitter a person is, whether or not he is a martial artist, the heavier punches he can take. A weight lifter or a soccer player can take on heavier punches than a painter or pianist. Proper training in punch shock absorption helps too. Decades ago when Muay Thai fighters first came to Hong Kong, they were pitted against local martial artists in MMA fights. The local guys were shocked by the Thai fighters' endurance in taking heavy punches. I believe had also shocked the local martial artists who had demonstrated their ability to take punches too!
What is the significance of internal martial artist's ability to take punches? Putting aside the mythical belief, which is still prevalent among some martial art circles, that internal martial artists live in a world governed by different physical laws, the ability to take punches through chi conditioning does warrant our special attention. The reason is that seemingly weaker guys can be trained to take on (some) punches only after a couple of months' training - these guys become stronger and healthier too.
In the ancient art of Taoist meditation or Neidan, a practitioner has to visualize his torso as cauldron. Inside this cauldron, fire has to be ignited to cultivate the elixir of life to achieve Immortality as the final training objective. Putting aside the fire, elixir of life and Immortality concepts, let us look at the concept of cauldron. The first level of Neidan is micro-cosmic circulation which essentially means cultivate chi and channel it so that it rises up one's spinal cord to the crown (ren mai) and down the mid-front to the Dantian (du mai). In order that chi can be activated, a practitioner uses his breathing energy to open and move his pelvic joints, in particular the area around his groins. He will concentrate his chi to a point at the perineum, and then up his coccyx. During his "not circulating time", which is ample because one does not do orbiting all the time during his meditative practice, his chi will rise up in a broader area which eventually will cover his whole torso. During such time he will bear witness to the fact that his torso appears to him to have become a cauldron, exactly as the classics have promised. The sensation is caused by chi filling, stretching, and connecting our torso's fascias. A feeling of our torso expanding outward will also be felt too.
The scenario experienced by a Taoist meditator is just like that experienced by an internal martial artist. In cultivating massive chi, a martial artist can use the micro-cosmic method. Most martial artists however choose another route: the practice of advanced zhan zhuang. Standard combat stance and Gold Turtle (of 24 Styles tai chi nei-kung) are good choices. When in stance, a practitioner (with appropriate visualization) can gradually open his pelvic joints and groin areas through subtle movements in three ways: front-back, up-down and open-close; covering the x-axis, y-axis and z-axis. With a focused mind, our joints will be subtly stretched open and made synchronized with our breathing mechanism. Massive chi will be produced. Instead of "cauldron", the martial artists prefer to call his "iron shirt" or "golden shield". A metaphor with substance.
With good instructions and diligent practice, a practice can reach the stage after three to six months' practice. For the martial artists, "friendly punches" can be absorbed (a fitter body is built with torso conditioning as mentioned). Needless to say, the vigor of professional MMA combat demands addition conditioning than chi kung exercise.