You actually do activate calf muscles when doing curls, and depending on how you point your feet, you activate different parts of your calves. Jeff Nippard (the goat) talks about this in a couple of his videos lol

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I try to keep my feet flexed and pull my toes to my knees to isolate hamstrings

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Your gastroc (a calf muscle) is also an accessory to knee flexion so they are working during ham curls

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