This is a great video. This is a video we found on the internet. There is another one showing her with a much more pathologic steppage gait, we may bring this one to your attention another time. She may have had a cerebral event or mild stroke from what the other video showed. But we like this one, because it is subtle and much more likely what you are going to see day to day in your world.
We see here on the video a left sided circumducting gait. What this means is that the left limb is not swinging purely through the sagittal plane from heel lift-toe off phase to repeat heel strike again. Instead we see the foot sweep laterally. The foot moves immediately into an increased progression angle (turned out), and it sweeps around a half circular motion to repeat the contact phase again at heel strike.
In this case here it is clearly evident, especially on the lateral side view, that she is blocked out of left ankle rocker (dorsiflexion) for some reason. She does not even get to 90 degrees dorsiflexion range at the ankle mortise. She can no longer allow the treadmill to draw her left foot backwards because she hits this limitation. As the limitation is met, the foot must come forward now (because it cannot go anywhere else, certainly not backwards). But, she does not display enough right gluteus medius use to create hiking of the left hip to create the clearance necessary for this premature forward left leg swing. Thus, she has to spill the left leg out to the side in a circular-circumducting fashion to clear the limb from hitting the ground. She is also likely doing it because if you are not able to dorsiflex the ankle, you will be relatively plantarflexed at the ankle. And if you are plantarflexed, you are much more likely to catch your toe on the treadmill-ground during the swing through phase. This case is a problem of not creating sufficient clearance for the left foot.
Also take note of the subtle foot drop on the left. The foot cannot maintain adequate dorsiflexion, due to weakness of the anterior compartment (possibly from a cerebral event?). Did you see the lack of inversion of the left foot as well. This is usually accomplished by the tibialis anterior and long extensors, especially EHL, neither of which is seen (take a look at the amount of hallux extension on the R on the lateral view and compare it with the left). How about that subtle midfoot collapse as well?
There are other reasons for a circumducting gait. This is just one. Some are functional limitations, some are ablative orthopedic type limitations such as post surgical, some are neurological, and some are a combination. Taking a good history, knowing your gait parameters and assimilating the information will bring you to an answer.
Then you have to figure out what to do. In this case, we do not know, because we do not know what parameter is causing the compensation you see here. This is a good case of “what you see is not what is wrong, you are just seeing what they are doing while working around limitations and finding parts to use and cheat with” to still be able to more forward.
We are The Gait Guys, —- Ivo and Shawn