This example illustrates the co-operation between primary (motor) and secondary (static) muscle work. The localisation of the muscles in secondary action is not always so limited; there are cases where the primary movement does not depend upon co-operation with the secondary, but upon other conditions. In 2 Arm-carrying-forward with resistance (Arm-closing, A5) the movement itself is done by Pectoralis Major and the anterior portion of the Deltoid. If the patient sits on a seat with the back unsupported, in order to maintain the erect position against resistance of the gymnast or apparatus on the front of his arms he must contract the abdominal muscles and Ilio-psoas. The effect then of the movement is not limited to the joint concerned, but is contrary to the principles of Swedish gymnastics in regard to localisation of movement, and should therefore be avoided. This is easily done by supporting the patient's back, when the movement becomes at once localised, making extra muscle work unnecessary.

The objectionable secondary contractions are not always so extensive as the above, and are more easily overlooked. In active flexion of the forearm, if support be not given to the lower portion of the upper arm from behind, Pectoralis Major will contract in order to prevent the upper arm yielding to the resistance pressure on the wrist. This again is a complication which is against all the Swedish gymnastic laws of localisation.

In this is seen the importance of the first principle on which gymnastic apparatus is constructed (see p. 296). The apparatus must be so constructed that in using it the patient takes the most comfortable position suitable both for the body as a whole and for that part of the body which is concerned in the movement, i.e., he must be well supported, while the movement is not hampered. In constructing the apparatus Dr. Zander has not overlooked this important fact, and in this his system differs from nearly all others.

The second condition which governs the construction of the apparatus is that the movement shall take place in the correct anatomical path. The importance of this need not be further explained. I have already pointed out that it is a necessary condition in the construction of the apparatus.

Lastly, we come to the third point, that it must be possible on the one hand to alter the resistance to suit the patient's strength, and on the other hand that it must be varied during the movement, according to anatomical and physiological conditions.

This latter is accomplished by a rather more detailed treatment. The problem of the changes of the strength of the movement during an exercise arises as a gymnastic problem with the mechanical method. This has never before been discussed in gymnastic literature, probably because the gymnast can apportion the resistance according to the patient's strength by his own sensation, and during the movement in the same way he varies his resistance according to the patient's varying power to overcome it. One of the differences between a good and bad gymnast is shown in this power of thus varying resistance, a hard enough matter; on this depends to a large extent whether the exercise is given smoothly and evenly, and consequently beneficially to the patient, or not. Every uneven resistance causes the patient extra innervation to the working muscles, which in its turn causes extra work for the brain in comparison to the almost automatic work when the resistance is even and reliable. This is especially important in the treatment of nervous or heart patients.

Apparatus providing resistance must be constructed with reference to these requirements. Constant resistance, such as that given by friction blocks or weights suspended from pulleys, is not felt to be of the same strength throughout the entire exercise, but is stronger at some parts than at others.

The reason for this is found in several facts, of which I shall only mention two.

The first is of a purely mechanical nature, and depends on the construction of the joints and on the position of the insertions of the muscles on the bone and the existence of trochlear surfaces for the tendons, etc. The bones are the levers on which the muscles work. If the bone is at right angles to the direction of the muscle contraction its whole power takes effect in movement, but the more oblique the angle, whether acute or obtuse, at which the muscle acts between the lever and the tendon, the greater is the loss of muscle power from the movement. The effect of muscle contraction is diminished or increased throughout the movement according to the alteration of this angle.

Another cause of alteration in the effect of muscle work is expressed in Schwann's law, which states that the muscle during contraction continually loses force. When at its longest it can lift a greater weight than when shorter.

In order that the movement produced should be even and agreeable, the resistance appliance in each apparatus must be so constructed that the resistance during the exercise varies along with the power of the muscle to overcome it. In Dr. Zanders apparatus this effect is obtained by the use of levers to which weights are attached. The connecting link between the lever and the patient is made by means of a special shunting mechanism for each apparatus, so that when the patient performs the exercise desired the lever is raised and thus offers a certain definite resistance.

The arrangement of levers serves two purposes.

First, the graduation of the resistance given becomes on the whole very easy, as the weight may be pushed further along the lever or nearer to its axis, according to whether a weak or strong exercise is desired, and it can be fixed by a metal screw. As the lever is graduated there is also, as opposed to the "manual" gymnastics, an objective measurement of the strength of the exercise according to the number on which the weight is placed. The length of the lever and the size of the weight are adapted to the greatest or least amount of resistance for the exercise in question.