The original Stretch Armstrong toy by Kenner was a big figure doll filled with corn syrup in the shape of a well-muscled blond man wearing a pair of swimming trunks. The figure was introduced in 1976 but the original idea came from James O. Kuhn in 1974. It stands out from other vintage action figure toys because its arms and legs could stretch outwards (presumably without breaking).
The corn syrup was condensed to the right viscosity to make the stretch return slowly to its original position, as the latex skin reacted against this highly viscosic goo. From 1976 to 1979 Kenner produced 40,000 Stretch Armstrong toys making over 50 million USD, and was the largest buyer of corn syrup over those same years!
In 1981 Kenner sued Mego for copying the Stretch Figure concept and design with their Mego Elastic Heroes.
Given the solidness and weight of the Stretch Armstrong toy, it didn't fit in with other action figures as far as being able to stand on its own, bent into different poses, or to be used in 'fights' between different figures, as boys will do.
Ah.... kids! As many children found out, the doll was filled with a thick, dark-colored gel that would ooze out of any damaged area, quickly turning hard and making the Stretch Armstrong toy useless. The doll's skin was made from soft material allowing it to be damaged easily, and of course many kids would stretch the toy beyond its normal capacity.
Stretch Armstrong came back, though shortly, in the early 1990s. This new version of the doll looked more cartoonish and had longer, blonder hair. It also featured a big, white smile with a lot of teeth visible. It looked more like a wrestler from WWF than the original doll. The more modern and modest Stretch also covered his manly torso with a muscle t-shirt with the text 'Stretch Armstrong' and wears boxers.
What's it worth? Take a look at this Stretch Armstrong price guide: sold listings for a value indication.