If the pelvis is firmly fixed, e.g., in High-ride-sitting starting position, rotations of the trunk take place, as already described, chiefly in the joints between the tenth, eleventh, and twelfth dorsal vertebra, and are brought about by the contraction of the oblique muscles of the trunk.

In describing "Turn-standing" position these muscles are specially spoken of, and it need only be remembered here that we can class them in two groups, "Left and Right Rotators." The former on the front of the body go from below on the left obliquely upward and to the right; on the back they go from below on the right obliquely upward and to the left. The right rotators naturally go in the opposite direction.

But rotations of the trunk are also performed in starting positions which do not at all, or only slightly, fix the pelvis, e.g., Close-st., Knee-std.-st., etc., and then the movement takes place more or less also in the joints of the legs, and especially in the hip joints, so that the rotators of the hips must work. In describing "Turn-standing" position it is also mentioned more in detail how these muscles work, but it must be remembered that by the "left rotators of the pelvis" we mean the inward rotators of the left hip and the outward rotators of the right.

Fig. 104.

With the exception of the free trunk-rotations, where the muscles only work concentrically, trunk-rotations may be done in several different ways, each of which necessitates certain modifications in muscle action.

(A) The patient turns the trunk alternately to the right and left as far as possible. In this, resistance is given by the gymnast, who, standing in front of or behind the patient, places his hands one at the back and one in front of the shoulders or elbows, or grasps the wrists, according to the starting position. (N.B. - The rotation must be pure, without side-bending.)

Over-stretching at the end of, or more correctly immediately before, each rotation must not be forgotten, nor that the patient must take a deep inspiration between each rotation.

This form of the movement necessitates concentric work for the muscles, and the whole range of movement is traversed, i.e., the muscles contract as completely as possible.

It is less used, partly because it is more uncomfortable and more difficult to give than the following, partly because respiration is impeded during the movement.

(B) The gymnast turns the patient to turn position against the resistance of the patient, who then turns back to the starting position against the resistance of the gymnast, who slightly raises the patient's shoulders. Given alternately to each side three to five times and in time with respiration. The movement is so arranged that forward-turning is done at the same time as inspiration, which then is facilitated by the lifting of the patient's shoulders spoken of above. Expiration is performed partly while the gymnast changes the grasp, partly while turning sideways.

When the movement is done in this way the muscles work eccentrically and concentrically in lengthening (i.e., in the outer part of the range of movement).

It is most used of all the trunk-rotations, and is the one used, unless some other form of the exercise is indicated, because it is comfortable to give, feels pleasant to the patient, and helps rather than hinders respiration.

The forms of movement described under (A) and (B) are called "AIternate-Trunk-turning."

(C) Starting from turn position the patient turns the trunk forward against the resistance of the gymnast, who then turns him back to turn position against the resistance of the patient. Repeated three to five times - first from one side, then from the other. Inspiration during forward-turning.

The muscle action is almost the same as in the preceding modification of the exercise, but it is rather more tiring because the muscles belonging to the same group of rotators are made to work continuously for three to five rotations following each other, while in the previous exercises they work alternately and so have a short rest between each contraction.

It is used in those starting positions and movements in which the gymnast cannot change his grasp easily and quickly, e.g., in Wing-high-ride-fall-turn-sitting position.

When the movement is performed in the above way it is called "Forward-turning."

(D) Finally, the movement may be done so that the patient performs the turning against the resistance of the gymnast, and then in his turn resists while the gymnast turns him back to the starting position.

The muscles work then concentrically and eccentrically in shortening (i.e., in the inner half of the range of movement).

It is only used as one-sided hip-rotation in Heave-grasp-close-standing position to correct faulty position of the pelvis in certain scolioses.

General Effects of Trunk Rotation.

(a) Exercise of muscles and joints.

(b) Because the oblique muscles play a very important part in the performance of these exercises, they all aid peristalsis by associated movement between them and the non-striated muscles in the alimentary canal.

(c) By alternate lengthening and shortening of the abdominal vessels and by alternations of pressure in the abdomen, the circulation is assisted, especially in the portal system.

(d) Given in arch position they are repleting to, and in stoop position depleting from, the abdominal and pelvic organs. The depleting effect can be considerably increased if the gymnast resists so that the posterior shoulder muscles work especially (resistance only, or more strongly, on the shoulder or arm which is being carried back by the patient).

The most usual Starting Positions for Trunk-rotations and their effects upon the movement.

Wing-Neck-rest-

Stretch-Yardstanding Alternate-Trunk-turning.