Hi Chris .... some advices ....
- Try to animate each part of the model independently... I notice clearly that the hand´s keys ( right/left ) are in the same frame.. and the same occurs with the feet
- In this context there are two situations in which the knee bends :
-------1) to generate energy to take of ...or...
-------2) to absorb the impact in the land
note that your character comes down until the frame 38, then it goes up a little , stabilizes the y-axis for 10 frames, then lands flexing legs.
if we consider the second item mentioned above, your character should not bend his legs, because, as you have stabilized the Y axis, there isn't impact to absorbing.
To solve this problem, I suggest you to keep the curve of the Y axis always descending until the absorption of the weight through the legs.
Pay close attention to the curves of the axes in the timeline. analyze and understand it .. this is a key to a good animation