The relation between applied force and speed of isotonic shortening was studied in frog and tortoise muscle. The observed relation is satisfactorily described by Hill's hyperbolic equation (P + a) (v + b) = const. The constant b (per length of muscle) is about 14 to 40 times, the constant a (per isometric tension) about 2-5 times greater in frog than in tortoise muscle.
The development of isometric tension can be analysed as a gradual internal shortening of the contractile substance against an elastic, passively extending portion. The isometric tension records were compared with Hill's theoretical relation. A satisfactory agreement between observed and calculated tension records is found in the frog's sartorius. In tortoise muscle large divergences are observed, the reasons of which are discussed.
The relation between a force greater than isometric and the resulting speed of "reversible" lengthening was determined. The observed veloci- ties are considerably smaller than calculated from theory. If the applied force exceeds the isometric tension by 70-100%, the muscle "relaxes" rapidly.
A shortened muscle " gives" and later "reshortens" if its tension is suddenly increased from small to isometric strength.
Rapid stretching of an active muscle, beyond its optimum length, is apt to break or weaken permanently parts of the contractile substance.