The dorsal interossei are small muscles in your hand that sit between the long bones of the palm (the metacarpals). You have four of them in each hand, and they run from one metacarpal to the next, then attach into the base of the fingers and the connective tissue that helps control finger movement. Their main job is to spread your fingers apart, meaning they move your fingers away from the middle finger. If you place your hand flat on a table and try to separate your fingers as wide as possible, you are using the dorsal interossei.
They also help with finer control of the knuckles and finger joints. When you grip something, type, hold a dumbbell, or do a pull up, your fingers need to stay stable while still being able to adjust pressure and position. The dorsal interossei assist the larger forearm muscles by giving your hand precision and stability, especially at the knuckle joints. They work closely with other small hand muscles to keep your fingers aligned, help you maintain a strong grip without cramping, and control subtle movements like keeping one finger steady while another moves.
For someone chasing visible abs, these hand muscles matter more than you might think because many effective ab exercises require hanging, gripping, or holding weight. If your hands fatigue early, your core work often ends early too. Improving hand strength and endurance can let you perform more quality sets of hanging leg raises, heavy carries, pull ups, and rows, all of which support better overall training. Simple ways to build them include controlled finger spreading against a rubber band, squeezing and releasing with a soft ball, and gradually building time holding heavier weights with good form.