MUSCLE 5 6 3 



is loaded the greater will this elongation be. So that at the 

 beginning of the actual shortening part of the energy of 

 contraction is already expended without visible effect, and 

 has to be recovered from the elastic reaction during the 

 ascent of the lever. 



Then the inertia of the lever itself and of its load comes 

 into play, and may carry the curve too high during the 

 up-stroke and too low during the down-stroke. This can 

 be minimized by making the lever very light, and attaching 

 the weight close to the fulcrum, so that it has only a small 

 range of movement, and never acquires more than a small 

 velocity. The contraction of a muscle loaded by a weight 

 which is not increased or diminished during the contraction 

 is said to be iso-tonic, for here the tension of the muscle 



FIG. 170. INFLUENCE OF LOAD ON THE FORM OF THE MUSCLE CURVE. 



i, curve taken with unloaded lever ; 2, 3, 4, weight successively increased ; 

 5, abscissa line ; time-trac- ^ sec. (reduced). 



is the same throughout, and its length alters. When the 

 muscle is attached very near the fulcrum of the lever, so 

 that it acts upon a short arm, while the long arm carrying 

 the writing-point is prevented from moving much by a 

 spring, the muscle can only shorten itself very slightly ; but 

 the changes of tension in it will be related to those in the 

 spring, and therefore to the curve traced by the writing- 

 point. Such a curve is called iso-metric, since the length of 

 the muscle remains almost unaltered. 



The maximum of the iso-metric curve (the maximum tension with 

 practically constant length) is sooner reached than that of the iso- 

 tonic (the maximum contraction with constant tension). From this 

 it has been concluded that during contraction the co-efficient of 

 elasticity of the muscle continuously diminishes (Fick), or, what 

 comes to the same thing, its extensibility continuously increases. 



362 



