There are better ways to strengthen the legs that will not put unnecessary stress on the knees and other joints of the legs. Note that leg extension and hamstring machines might also be stressful to the knees. Better to go with squats and leg presses.
It is a well know fact that Nadal has had problems with his knees for quite a while now. Is it worth the sacrifice if you aren't going to make the millions that he does? You may or may not see immediate injuries with ankle/wrist weights. However, over time, it can take its toll.
There are too many respectable sources that warn about the
dangers of walking or running with ankle weights to ignore.
WebMd and
Mayo Clinic are two of many that warn about this type of use of ankle weights. The
Harvard Orthopaedic Journal suggests using ankle weights for striaght leg lifts and hamstring curls for rehabilitation but, to my knowledge, does not advocate their use for walking or running.
You should be able to find quite a few sources that discuss the dangers of using wrist & ankle weights improperly. Here a few resources to get you started:
WebMD.com/fitness-exercise/features/get-more-burn-from-your-workout
www.ehow.com/about_5481085_Danger-walking-ankle-weights.html
en.wikipedia.org/wiki/Weighted_clothing#Ankle_weights
www.universalnutrition.com/features/aerobicallyspeaking.html
www.ehow.com/about_5052712_dangers-using-ankle-weights.html
'nuff said!
.