The concept of the “tight-gut” was a point of contention to Mr. Jacobs. Muscles of the body operate in two ways: they contract and they relax. For example, the biceps can contract into a shorter position, affecting the position of the forearm. Then the biceps can relax, letting go of the forearm, and gravity will cause it to limply fall back into place. But the biceps has no power on its own to push the forearm in the opposite direction. There are antagonistic muscle groups to accomplish that: primarily the triceps, which contract, shorten, and thus pull the forearm into another position.
Systems of antagonistic muscles are found throughout the body, including the respiratory system. For inhalation we have one set of muscles that will contract, and an entirely separate set of muscles for exhalation. When you bring multiple antagonistic muscles into contraction states at the same time, you are in a state of isometric contraction. You experience stability, but also a great cancellation of function in regard to movement. If you contract to “show off your arm muscles,” you will see the bulges of the biceps, triceps, and possibly the deltoid, and you will feel the hardness of muscle all over, but if you then try to move your arm freely in this state you will find it difficult. (Jacobs, 1984) This phenomenon exists in the abdominal and thoracic region: if you flex for the sake of support, you will achieve a great deal of muscle flexion and hardness in general, but then you have cancelled out the ability of the body to achieve free movement, which means a cancellation of quality air flow as well.
There are always contractions in the act of exhalation, but these contractionsoccur as a result of the individual blowing, and should never be thought of as causative. Trying to contract muscles involved in exhalation is one of the greatest hazards in brass playing, because the air or wind is the motor force to the vibrating surface of the lips, which feed the “buzz” to the source of resonance, “the instrument.” Unless you have sufficient motor force, great struggle and failure ensue. (Heath, 1996)