Rest, Ice, Compress, Elevate (RICE) for sure, but I'd also use a knee support which you'll typically find at a pharmacy.
The ligaments you've stretched (sprained) are responsible for the stability of the joints and the knee is a pretty complex joint and under a lot of load.
The stiffness suggests you went down on it or pulled a muscle or tendon at the same time (a strain).
Not a great combo, though a common one.
To control the swelling and pain, do also consider our Atrosan® and Atrogel®. We get some wonderful testimonials from these products.
Best wishes.
Do you have questions? Please feel free to ask.
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