Good luck exercising your forearms without using your hands!
I assume you're interested in increasing strength for the purpose of playing guitar. Is that correct?
Although you're correct that there are no muscles in the fingers, and also that there are larger muscle clusters in the forearms that connect to the fingers, there are also a large number of important muscles in the HAND that control the fingers. These are referred to as the intrinsic muscles of the hand, where the forearm muscles are referred to as extrinsic. Fine control of hand and finger motion is dominated by the function of the intrinsics, and there are overlapping and cooperative roles between muscle groups, and even between muscles and their opponents. It's very complicated.
Furthermore there are 2 different types of muscle fiber, generally referred to as fast and slow twitch that are distributed differently in different muscles. Slow twitch muscle tends to have more power and endurance, while fast twitch has more speed. Forearm/extrinsic muscle tends to be slow twitch predominant and the intrinsics have a higher proportion of fast twitch fibers. My understanding of the physiologic differences is relatively superficial, but in general what I gather is that fast twitch fibers are generally harder to build up endurance in, and when muscles gain bulk, they tend to gain slow twitch fibers. If so, building strength and endurance may interfere with speed and agility.
It may be that an important part of the experience of gaining "strength" and "endurance" in the hand really just involves learning how to economize the duration of intrinsic muscle usage, and how to relax unnecessary opposing tension.
"If I have seen further it is by standing on the shoulders of giants." -Sir Isaac Newton
Armin Hanika 56PF