290 PEACTICAL PHYSIOLOGY 



force of the muscle is at its maximum, and not later; this is the 

 arrangement in the body. Further, it would be an advantage if the 

 load decreased as the contractile force of the muscle during its con- 

 traction decreased; this is not usually the case in the body, but it 

 does occur in certain movements, as, for instance, in jumping or 

 when, with the upper arm horizontal, a weight in the hand is raised 

 by flexing the forearm on the elbow. 



CHAPTER III. 

 COMPARISON OF ISOTONIC AND ISOMETRIC CONTRACTION. 



WE have already seen (p. 27) that in order to obtain an accurate record 

 of the change in form of a contracting muscle by means of a myograph- 

 lever, it is necessary to use a light lever, and to weight it as near its 

 axis as possible. By this arrangement the inertia of the movable system 

 is reduced to a minimum, and the tension on the muscle throughout r s 

 shortening and relaxation remains nearly constant. This method is 

 therefore called isotonic; it registers the development of that part of the 

 energy liberated by the muscle which appears as the mechanical energy 

 of the change in form. On the other hand, if the muscle is made to pull 

 against a strong spring, the muscle will undergo but slight change in 

 length, and the energy, which would otherwise have appeared as change 

 in form, will now be converted into tension and stored in the spring. 

 This is the so-called isometric method. If the movement of the spring 

 is recorded by a lever attached to it we get a record of this conversion 

 into tension of part of the energy liberated by the muscle when stimu- 

 lated. Further, by allowing the spring to exert tension on the muscle 

 before stimulation, it is possible to investigate the effect of initial 

 tensions on the subsequent liberation of energy. 



Fig. 173 shows a lever which can be used for either the isotonic or 

 isometric method. The only part which needs description is the axis ; 

 it consists of a stiff steel wire. About 2 mm. from one end of the axis 

 the lever is rigidly fixed to it, and the small projecting piece of wire fits 

 loosely into a small socket in the brass support. The other end of the 

 axis is carried by piercing a rigid brass arm, in which it can work 

 loosely for the isotonic method, or to which it can be firmly clamped in 

 any position for the isometric method. In this latter case contraction 

 of the muscle, instead of leading to much shortening, produces torsion 

 of the axis, and the excursion of the lever is proportional to the tension 



