Forearms. You can strengthen this muscle by putting both hands at chest height parallel to each other and opening you palm completely and closing them tightly. Do this repeatedly.
Flexor digitorum longus and brevis, along with the palmaris longus. Plus some others, like the opponens pollicis and opponens digiti minimi.
The vastus Muscles
The muscles used in gripping strength are those in the forearm and hands. In athletics, gripping strength exercises is measured with a hand grip dynamometer.
Pectorals
Mostly your upper back muscles. Mostly your upper back muscles.
The quadriceps are the muscles used to grip a horse's back when riding on a horse. This muscle is sometimes called the thigh muscles.
To grip a bat, you use the muscles to contract the fingers. To lift it, the preceding muscles and all the arm, shoulder and back muscles.
You have no muscles in your fingers only tendons. The muscles in your lower arm control your hand movements and grip strength
Arm muscles, leg muscles, grip in your palms and fingers, to a certain extent your ankles, and sometimes your abs, it depends on how your climbing it.
Delts (front), chest and mostly your triceps
The muscles that help you move when you want to are called voluntary muscles.
The octopus's "suction cups" are sticky, which help it grip things.
It's about Strength Potential. When your muscles contract and shorten to flex the wrist, they lose the potential to contract more, thus they lose the abililty to create a stronger grip past a certain level. As grip is mostly a function of the muscles that flex, thus the more flexed your are, the less strength potential you have.