I hear "use them legs" a lot at the sport practice. I doesn't sound like proper English to me. What makes it valid?
It is non-standard. So you are right, it isn't "proper" English.
It has some dialect uses (in certain regional variants of British English, in Appalachian English, and Caribbean, possibly more) and has examples going over 200 years (so it isn't a modern mistake).
Learners should accept such non-standard dialects, but generally learners should not try to emulate. It would appear to be a simple mistake if used by a non-native.
As others have noted, "them" functions as an adjectival (or determiner) in some dialects. M-W specifically notes that this usage is "nonstandard":
: THOSE —> used chiefly in nonstandard speech and for humorous effect
Here is an example from dialogue that contains plenty of apparently dialectal speech:
“Mr. Linden,” she said as he passed, “them children of yours want more care than they get. . . . I says it’s a shame, Silas Linden, the way them children is treated.” (Arthur Conan Doyle, The Land of Mist, A. L. Burt, 1926, chapter xi)
Note that this is not the only object-form pronoun that is sometimes used adjectivally. For example, "me" can be used instead of "my" in various dialects.