Resistance movements may be of two kinds. They may be performed so that the patient performing the movements overcomes the external resistance (of a gymnast, machine, or weight), so that the patient's working muscles become shorter. Such resistance exercises are called concentric or shortening movements; in them "the ends of the working muscles come nearer to their middle-points." While the muscles working concentrically become shorter, they also become thicker through the thickening of the muscle fibres and the dilatation of the blood vessels, and harder owing to the stronger stretching of the muscle between its origin and insertion (see later). But resistance movements may also be performed in such a way that the patient's muscle work is overcome by the external force, so that his muscles while working become longer. Such movements arc called eccentric or lengthening movements; "the ends of the corresponding muscles become more distant from their mid-point." The muscle working eccentrically lengthens, but as in concentric work it becomes harder. As to the thickness of a muscle working eccentrically, I would here point out that in lengthening it becomes thinner, but that while performing eccentric work it is as thick as it is for the same length while doing equally strong concentric work, and thicker than it is when it is passively stretched to the same length. This is a natural consequence of the fact that the blood vessels and fibres of a muscle working eccentrically are for the same work and the same length of the same thickness as those of a muscle working concentrically. In concentric flexion the patient's flexors work and the resistance to the movement is given by the gymnast, but is overcome by the patient. In eccentric flexion the resistance is given by the patient, but the gymnast's strength overcomes that resistance and the movement is called eccentric flexion.

* Besides the apparatus which is used in almost all systems of medical gymnastics and also in Ling's system (see Arvedson's chapter), portable apparatus is also used, as in "stretch-stoop-stride-sitting back-raising with a rod " in Swedish medical gymnastics, or in Frenkel's gymnastics, when ninepins arc put up for the patient to knock down, or when he must stick pins into a board furnished with holes.

+ The classification of movements in Swedish medical gymnastics is ancient, Etius spoke of resistance movements. Of these concentric movements are sometimes called active-passive, eccentric passive-active, but these names seem to me less suitable.

The importance of resistance movements is that they allow an accurate determination of the working muscles and their work. But it is obvious that in character and effects they do not differ from the analogous free active movements. If I with arms hanging down flex them at the elbow joint, this is called a free active movement; the flexors here do so much work as to overcome the resistance exercised by the weight of the forearm (plus some slight resistance which I leave out of consideration). If, instead, I take a weight in my hands or let some one resist the flexion, it becomes a resisted concentric movement, in regard to muscle work differing only from the flexion above described in that a greater amount of mechanical work is performed. If when the arm is flexed I suddenly cease all muscle innervation, the arm becomes suddenly extended by the weight of the forearm and the latter "falls" - a purely passive movement without any muscle work. But if instead of suddenly ceasing innervation of my flexors I consciously allow them to perform a little mechanical work, which is overcome by the weight of the forearm, so that the latter slowly "sinks" (the extensors not being innervated at all), it is obvious that this movement, in which the flexors are active but become lengthened, does not differ in character from a resisted eccentric movement. If I increase the extending force by grasping a weight, the movement is called a resisted eccentric movement.

In active movements, even to some extent in passive movements, it is apparent that for the maintenance of a suitable position other muscles than those which are shortening (or lengthening) come into play, although only statically. Even this "secondary muscle work" may have therapeutic value. In the treatment of scoliosis, for example, resistance movements in the cervical region are given (concentric head-bending backward = "concentric neck-raisings," and eccentric head-bending forward), because in these the extensors of the spine work, and to strengthen these by static as well as by dynamic work is part of the aim of scoliosis treatment.

With regard to static, concentric, and eccentric muscle work, see the next chapter.

Another of Ling's methods of classification is of value to a certain extent, because it clearly shows us the relation between educational and medical gymnastics. Practically there is no system of medical gymnastics which does not contain positions and movements which also belong to educational gymnastics and the positions and movements of daily life. As is well known, these also have hygienic value. As examples I may point out that the fundamental positions of Swedish gymnastics, often only slightly, if at all changed, are found in daily life, and both in educa -tional and medical gymnastics, and that walking is of considerable importance both in "terrain-cures' and in Frenkel's medical gymnastic treatment for ataxia.*

The classification in question is based partly on anatomical, partly on physiological grounds, and is therefore better than the purely anatomical division into trunk, head, arm, and leg movements (so-called elementary movements). It contains various classes of movements, of which each class contains different groups.

The simplest, least complicated, kinds of movements are called fundamental movements.

As the most important classes of these I may mention, along with Dr. Arvedson (though the following is in slightly different order) -