Supertraining

I remember reading about trainees who could only train one arm or one leg due 
to injury experiencing a training effect on the untrained limb. For example a 
skier broke his right leg and could only train his left leg. But when the cast 
was removed there was less atrophy in his right leg than was expected.

Is this effect a myth or is there substance to the story?

Yehoshua Zohar
Karmiel, Israel