Just load your model into an animation application and check the vertex assignment of the lower legs and reassign if necessary.
An example is Psionics Female Spy model - the vertices that make up the thighs are assigned to the knee - when you rotate the knee the whole thigh revolves around it. When animating using the stock frames this is not noticed but when you start to create custom animation you need to reassign the vertices first.
An example is Psionics Female Spy model - the vertices that make up the thighs are assigned to the knee - when you rotate the knee the whole thigh revolves around it. When animating using the stock frames this is not noticed but when you start to create custom animation you need to reassign the vertices first.