The sign for "strong" has a couple of variations. A good general version is to form fists and then pull them out away from your body a bit as if flexing your biceps.
Notes:
The sign STRONG is related to the concepts of: brave, courageous, heal, healthy, robust, fit, and well.
The difference between all these concepts and the concept of "strong" is that STRONG starts with "fist" handshapes whereas the other concepts tend to start with "relaxed five" handshapes.
HEALTHY / BRAVE / ...
Sample sentence: "Are most farmers strong?"
