In the first instance, you don't have an offset (translation) for the leg matrix. I suspect Leg.Matrix was originally set up with a translation to its proper position. Then "Leg.Matrix = Matrix.RotationX()" overwrites it and the translation is gone.

In the second instance, you continually add a rotation to the existing matrix.
That is:

Leg.Matrix = Leg.Matrix * Matrix.Rotation;

Next time through the loop, you add a rotation to the previous rotation.

You probably want something like:

// when the leg is initializedoriginalMatrix = Leg.Matrix; // save a copy of the original matrix

and then

// in your loop or OnUpdate or whereever you get TickLeg.Matrix = Matrix.Multiply(Matrix.RotationX(...),originalMatrix);

In the Matrix.Multiply() call, I'm pretty sure you will want to rotate the leg, then translate it. If it rotates about the wrong point, go back to

MatrixMultiply(originalMatrix,Matrix.Rotation(...));