Legs going "bad"

[broodymomma]

Don't know if there is a ryme or reason for this, but find it unusual that I have had two of my chickens a little under a year old lose the use of one of their legs. They were at separate times. Seems that they have both had a time of limping a couple of days, then the leg is "just useless." Then they hop about. I had a hen this happened to and she has been hopping around for quite some time, (months) and now I just had one of my roos been limping. We looked him over and there are no injuries. It appears like his leg is just not working. Is there something too this? Just wondering if anyone has experienced anything like this before. Thanks.