Bones
While a hug chew bone will keep a great dane occupied for a few minutes human bones are not suggested. It is said that a vehicle collision at 10mph generates a similar impact to a vehicle falling off a one store building, one store for every 10 mph. Based on this fact I would jump to the conclusion that a 150lbs four legged beast moving at a speed of approximately 20mph, which it reaching in approximately .04 seconds can generate a fairly similar force. This acceleration can pretty easily be reached in the distance of a normal leash and fairly easily triggered by a startled feline. Needless to say an unexpecting leash holder attached to an unexpecting arm does not fare well. Bone snaps at about 7lbs of pressure. Four legs + low center of gravity + 120lbs and up + major acceleration = easily equals the necessary force needed to cause long term physical therapy. Retractable leashes are even more dangerous. Great Danes have the capacity to reach a full gallop before reaching the end of one of these bone crushing leashes. And for added kicks other extremities are pretty easy targets for a loose leash to snare and be included in the bone crushing fun.
If you walk a Dane, which you should if you have one because they respond very well to exercise and it greatly extends their life and aids in weight management, you should be very careful. Pay close attention, take the time to train the Dane, keep the leash taught, use a choke collar with (the kind that looks like a medieval torture device with spikes that point inward, Danes feel very little pain), know where problem areas may exist, think twice before walking with kids or other dogs.
The bone crushing principle applies to play as well as walking. If you must play rough realize that you will sooner or later get hurt. Playing rough with a great dane is similar to wrestling with Mike Tyson when he was 10 years old, they are strong as hell, they feel no pain and they are out of control. Keep your mouth closed to avoid losing teeth and watch out for head butts and when you get tramped don’t said I didn’t tell you. One of the four feet will most certainly be applied with great force to the family jewels.
On a personal note. My wife learned this the hard way and has been recovering from a broken scapula for over a year now.