I had a stroke in June 2018 which causes difficulty with walking.
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Answer transcribed from Brightway's interview with Dr. Cristen Gordon (pt.1):
Hyperextension of the knee is probably one of the more frustrating things to work on as particularly after recovering from a stroke. The first thing to do is figure out what the cause is. When we walk without having a stroke, our knees tend to stay straight. When we put our weight on it, it holds our body weight, keeps us up, and then it bends when we bring our leg through the swing pattern, which is the part where your foot's not touching the ground. When you put your foot down on the ground and you want your knee to stay straight, the way that it can stay straight is because your quadriceps, which are in the front of your knees, and your hamstrings, which are in the back of the knees, are balanced as far as strength. Knee hyperextension happens when you lose the muscle balance. Either the quad is way too strong, which would be in the case if you have spasticity in your quadriceps, or your hamstring is way too weak and it can't counterbalance the action of the quadriceps. The quadriceps job is to straighten your knee, so it just keeps straightening until something stops it. When your knee snaps back and it goes into hyperextension, the hamstrings are not stopping it for whatever reason. Again, it may be that it's too weak or that the quads are too strong. The only thing left to keep it from snapping completely is the ligaments in the back of your knee. That becomes a problem over time because those ligaments can't hold out forever. They're just tiny little pieces of tissue that are keeping the two bones of your knee together.
The way to work on that is to figure out which one if not both of those need to be worked on. If your quad is too strong, nine times out of ten you'll need to stretch it. I haven't mentioned the gastroc muscle, which is your calf muscle in the back of your ankle. If it's very tight, it can also contribute to a hyperextending knee. So the first thing is to stretch the calf muscle and the quad muscle. Then, you've got to strengthen the hamstrings to counterbalance the strength of those two muscles as well. This is another chance or an opportunity where you may see your therapist give you a brace called an AFO. The AFO is an l-shaped brace that goes into your shoe. It's usually or oftentimes used if you can't lift your feet when you're walking. It’s where you can't pick your toes up off the ground but because of the way the calf muscle works and how it connects to both the ankle and the knee. The AFO can also restrict the knee locking back. I've had plenty of patients who were using an AFO not because they couldn't pick up their feet, but because they can't control their knee locking back. It's one of those things we want to work on and it gets better. You can also use electric stimulation to work on that, but you don't want to practice that at the chance of damaging the ligaments in the back of your knee. Your therapist may have you use a brace just to protect the ligaments in the back of your knee so you can maintain some knee stability.