Force Mapping finds the traditional treadmill, and running in general, extremely  beneficial for developing still growing skeletons, so much so that most children should be greatly encouraged, even bribed to use treadmills for tri-weekly cardio exercise, which is almost the exact opposite of what we find for the average adult, however you may be surprised to hear the rest of this story.

 To add to the confusing about running exercises, all adults who can run well, actually need running exercises weekly.  However not as a cardio exercise, but as a mobility conditioning exercise to keep their walking and running ability strong and ready, deep into old age. 

The difference is that the average adult needs to run between 50-100 yards, about once a week. Adults need just enough running exercise that allows joints and discs to completely heal in one week.

These distance of these short runs, which should take far less than one minute a week, will vary based on weight and height, and mobility issues. But generally speaking, running adults only 50-100 yards a week is much closer to the correct amount they need. Training them to assume they need to run 5-10 miles a week is hundreds to thousands of times too much joint and disc abuse, and it has already prematurely hindered the mobility of countless millions of adults who overrun their skeletons. 

Gentle (Almost no compression force travels parallel to extremities or spine) Should not build up adult Joint/disc damage 
light - (Compression force on Joints/discs is lighter than Tension Force pulling against contractions) 
Moderate (similar amounts of joint/disc compression, as muscle contraction opposition
Difficult (most External Motion Resistance is used up during joint and disk compression

*** Motion Range is a estimate of how much of the targeted muscle range is opposed. Very Limited, Limited, Wide, Complete 

****  Opposing Muscles. can the exercise deeply oppose muscle contractions of all motions 

***** Best Use  - One of the best cardio exercise devices for still growing children, as their growing discs and joints develop stronger to handle prevalent stress,  but traditional treadmills are far to abusive to average adult Joints and spinal discs to be used as an adult cardio exercise, (1-10 miles a week).  However using them once a week for 50-100 yards may help the average adult keep their running and walking ability very strong deep into old age 

 ****** Any muscle or cardio exercise method or workout machine that directs all External Motion Resistance Energy, into the body through the hands or feet is Skeletal Loading, as the forces move parallel to the skeleton. 
Any that directs some to all of this energy against contractions first crosses the body or extremities thus allowing muscles to work harder than when joints and discs must first handle all involved forces,  which is better for adults, but not for children. These are called Muscle Based exercises.