The biceps is the ball-shaped muscle group located at the front of the humerus, or upper arm bone. Two heads make up the biceps: an outer long head and an inner short head. Both heads function to flex the elbow, thus pulling the forearm upwards. The two heads also function to supinate the forearm, turning the forearm so the palm of the hand is facing forward. A third movement that the biceps take part in is shoulder flexion, which raises the arm upward. When training the biceps, especially with heavy weights, the forearm muscles are also worked. However, by modifying the exercises, you can take the forearms out of the movement to a certain degree.
Use an Underhand Supinated Grip
When performing a biceps curl exercise, you should use an underhand supinated grip if you want to maximize biceps involvement and minimize the use of other muscles, such as the brachioradialis. This muscle is considered a forearm muscle, and it assists during elbow flexion. When the forearm is in a neutral position, so the palm of the hand is facing towards the side of the body, the brachioradialis is the primary mover during a biceps curl. However, when the forearm is in a supinated position, the actions of the brachioradialis are minimized, and the biceps act as the primary mover.
Maintain a Supinated Forearm Position
It is all too common when performing a biceps curl to alter the forearm supination to pronation during the eccentric, or downward, part of the curl. This pronation takes some of the workload off the biceps and places it on the brachioradialis of the forearm. To avoid this, you must keep the forearm in a supinated position throughout the biceps curl, whether it's the upward or downward phase.
Use an Open-Hand Grip
Another technique you can use to minimize forearm involvement through grip alteration is by utilizing an open-hand grip. Doing so relaxes the forearm muscles, so they are not involved while performing a biceps curl exercise. This technique is only recommended for biceps curl exercises that are done using a machine, due to safety reasons. If you try using an open-hand grip while using a free weight such as a dumbbell, it is highly likely that the dumbbell will roll off your hand.
Use Lifting Straps
When executing a biceps curl movement with heavy weights, your forearm muscles are more involved than when using lighter weights. This is due to the added difficulty of gripping the equipment being used. To lessen this, and thus lessen forearm involvement, you can utilize lifting straps.