With regard to muscles passing over several joints the same rule may not apply, and the results vary with the positions in which the muscle contracts. Rectus femoris always extends the knee joint, and in most cases extends at the same time the hip joint. But when flexion beyond right angles takes place in the knee joint the muscle also bends the hip joint forward. And in the area of movement in which these movements take place there is an area for the knee joint in which rectus femoris has no effect upon the hip joint.

(All this of course applies when both joints are free. If the knee joint is fixed, rectus femoris always flexes the hip joint.) The long head of biceps brachii flexes the elbow joint and extends the shoulder at the same time. But when flexion in the elbow joint has reached an acute angle the long head of biceps brachii also flexes the shoulder joint.

It is obvious that if the origin of a muscle is fixed, on shortening its insertion approaches the place of origin; if the insertion is fixed, only the position of the origin moves, and moves towards the insertion. If one lies free on a plinth and contracts rectus femoris and the other flexors, the trunk is fixed by its own weight and leg-lifting takes place. If a leather strap fixes the knees to the plinth, flexion of the hip joint takes place by raising the trunk.

If a muscle, as is the case in several instances, changes its direction at some part of its course, one must specially study its action on each side of the part where this change of direction takes place, quite independently of the muscular or tendinous structure of the corresponding part - e.g., tibialis anticus above and below the anterior ligament, biceps and brachialis above and below the place where they are bent over the cylindrical joint of the humerus. * * * * *

To understand correctly the whole of our motor life it is important to remember the influence we are always subject to through the force of gravity working towards the earth's mid-point, and the friction against our supporting surface resulting from this force, without which the erect position in particular would be difficult to maintain.

The form and direction of our movements also depend upon both the osseous and soft parts of the joints, i.e., upon the shape of at least two extremities of bone, and the shape of both the capsule and the strengthening ligaments; further, upon the co-operation and direction of the working muscles. Upon all these factors and upon the innervation, with regard to voluntary movement, depends the magnitude of its excursion or range. The power of the movement depends upon many different factors to which I shall return in more detail below. Here I will only point out that the speed of a movement and its power are very often directly connected with each other, but that this is not necessarily always the case. A slow movement may be done much more powerfully than a similar quick one, e.g., in the "self-resisted" movements.

By the expression "measurement of a movement" Swedish gymnasts are accustomed to denote the amount of work which is thereby performed. This work depends primarily upon four points - upon the weight or mass moved, upon the range of movement, upon the direction of the path of movement in respect to gravity, and upon the duration of the movement.

The physiological and therapeutic effects very often appear by degrees and indirectly. But they arise always to a certain extent immediately. What does not take place after or as a result of ten muscular contractions will not follow ten thousand contractions. But the effects depend to a large extent for their appearance upon the degree of muscle work and upon the amount of products of combustion formed. We see in this, as in other cases, that if small doses in general excite and stimulate, large doses often depress and weaken, and are often followed by a quite opposite effect. When the muscles actively contract or thicken, there arise at once certain effects upon the circulation. Similarly, the production of materials which cause fatigue begins immediately. While these "fatigue products" are circulated in quite small quantities in the blood, they do not produce fatigue, and probably are in part the reason why a short spell of physical exercise has a decidedly stimulating effect. The peculiar sensation, painful when intense, and the functional disturbances of which fatigue consists, appear first after more or less excessive muscular work. After moderate exercise for a long time the hypertrophy due to work develops by degrees in the muscles; after continued excessive exercise we sometimes see muscle atrophy as the result of over-strain.

On the whole one may express the opinion with regard to the therapeutic effects of medical gymnastics, as of massage, that they are generally beneficial, but are generally also slowly obtained.

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Active movements are the most important part of medical gymnastics.

Before we enter further into a description of their many physiological and therapeutic effects we may obtain a good epitome of much of their physiology and pathology by specially considering the most important conditions which determine the strength of the movement.

The strength of our voluntary active movements is determined, as already said, to an important degree by the number and strength of the nerve impulses from the motor centres which make up the normal muscle stimulus. Again, it is the will which in this case decides both the strength of the impulses and their number, and which with wonderful precision and certainty deals out its commands for all our movements, from the weakest to those which represent our greatest effort.

With regard to the maximum exertion of force, we are, however, largely dependent on our accidental general condition, our "dispositon," which is an expression of psychic and somatic influences of various kinds (joy, anger, fear, dejection, sleep, waking, mealtimes, etc.) and on the chance functional capacity of our various organs.