ISOMETRIC COXTRICTIOXS 169 



These two faetors, nuiiihcT of (ibres (thickness) and length of 

 fibres (resting), determine the strength of the nuiscle. The 

 former gives a measure of the tension dexeloped (ef. volts and 

 number of cells), while the latter bears an obvious relationship to 

 the work which can be done. 



Tension (dynes) X length (in cms.) 

 ^ ' Work (ergs) ~ ^ 



Isometric and Isotonic Contractions. 



It is usual (and simpler) to make the muscle develop tension 

 against a spring so adjusted that the muscle cannot shorten. In 

 an experiment of this sort, the tension developed bears a simple 

 relationship to the heat produced. Such an experiment is called 

 isometric because the length of the muscle is kept equal to its 

 resting length. In a maximal isometric contraction Tl/H is a 

 constant ratio (= 5 approx.) applicable practically to all muscles 

 of all animals and over a very wide range of temperatures. Now 

 H, which may be expressed as microcalories, or in work units as 

 ergs, bears a quantitative relationship to one of the metabolites of 

 muscle action, viz., lactic acid. Meyerhof expressed this relation- 

 ship by equating what he called the isometric coefficient for lactic 

 acid {K^) with Tl in this way : 



^ _ Tension (in kgms.) x /(ems.) 



lactic acid liberated (mgrms.) 

 or in c.g.s, units : 



i^A K = T X 1-02 X 10~^ X / 



lactic acid (grams) 



From these two formulae one finds that : 

 (from 1) Tl = 5H, and 



(from 2) T/ = ? -^ grams of lactic acid. 



1-02 X 10-9 s 



H (ergs) K 



2 



lactic acid (grams) 5 x 1-02 X 10""^ 

 Substituting the value for K^ = 77-5, we find 

 H 77-5 1 



lactic acid (grams) 5 X 1-02 X 10"^ 65 X lO^^'^' 



That is, for every erg develojjed there should be liberated 65 X lO^^^ 

 grams of lactic acid, and, conversely, for every gram of lactic acid 

 appearing one should have 1-5 X 10^" ergs. 



