The human body is so dependent on movement that total immobility results in a steep decline in function.
Immobilisation is the beginning of the end.
This is the situation I found myself in after being struck by a truck while doing the cycling portion of my triathlon training. Because I was extremely fit and the greatest threat was the loss of my fitness level, I had to devise ways to maintain as much of my fitness levels as possible. The treatment of the fractures would have required six to eight weeks of bedrest. Besides the above declines in bone, muscle, etc. Without the ability to do any weight-bearing on either side, the prognosis was grim.