I think the problem is the length of the legs relative to the hight of the hip above the ground. Sometimes the walk designer seems to put the hip too low, forcing the legs to buckle at the knee.

You could try this. Reduce the yScale of the thighs to say 98%, or if that does not work even 95%. Apply the walk cycle, then in frame one rescale the thighs to 100%. The feet will probably be going under the ground now, to correct this load a primitive, parent the figure to it, adjust the hight of the primitive (in frame one only) until the feet are back on the ground, then hide the primitive.