A stiff knee can be caused by tight muscles around the knee, injuries, or even arthritis. Some simple stretches to loosen up the muscles around the knee should help.
Starting off with patella mobs is a great way to get the joint loosened up. Since the kneecap is connected to the quad and patellar tendon, if they are tight, it can push the patella down into the knee joint causing pain and stiffness.
Next are heel slides. Even though these are not technically a stretch, they are a great way to get the knee moving and relaxed before going into the big stretches.
Then using a strap to help stretch, quad stretches, hamstring stretches, IT band stretches, and calf stretches are a great way to get all the muscles that go to and cross the knee joint. These are full stretches, so hold the stretch for 30 seconds, and then do 3 reps on each side.
A butterfly stretch for the groin or adductor muscles, and a lunge stretch for the hip flexors will not only loosen up the knee, but they will also help with the hip. These are full stretches, so hold the stretch for 30 seconds, and then do 3 reps on each side.
Seated heel slides with heel taps are one of my favorite ways to get a stiff knee moving and help increase the bend in the knee.
Finally, a soleus stretch will get the muscle underneath the calf. This is a full stretch, so hold the stretch for 30 seconds, and then do 3 reps on each side.
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