That’s a wonderful machine.  I certainly appreciate your service to our country.

People tend to think of impact in a detrimental light, but there is some evidence that compressive loading of the femur may be beneficial for limiting the incidence of fractures. 

Last fall there was an interesting discussion on this correlation in the BIOMCH-L discussion group.  You might look it up, if you are interested.

As I recall several researchers cited a study that indicated that elderly people who engaged in high impact exercises, like running where there is an aerial phase to the gait cycle, have a lower incidence of hip fractures than the control group that followed a walking regiment. 

There was also a NASA study a few years ago that caused them to change their exercise program on the station.  That is why astronauts now exercise on a treadmill with heavy elastic chords pulling them onto the tread.

Way back there was a Russian astronaut who spent more than a year in space who did not follow a similar exercise program.  I think that he had all sorts of bone fractures when he returned to earth.  It may have been a matter of bone strengthening and not bone density.  I don’t remember.

In any event, impact may not always be disadvantageous; it may enhance bone matrix alignment.

Good luck in your program,

Ted