no PHYSIOLOGICAL TRIGGERS 



to the metal strip was recorded with another mechano-transducer. The output 

 of this transducer was proportional to length, as the tension in a spring is 

 determined by its length. A typical result of stimulating the loaded muscle is 

 diagrammatically shown in figure 2. The load raised the passive tension to T', 

 equal to the weight. On stimulation the muscle began to shorten to a (fig. 2) 

 normally at first, but soon went into vibration, the vibrations increasing in 

 amplitude with further shortening until a steady state {bb') was achieved. The 

 beam moved counterclockwise around the loop, and work, equal to the area 

 of the loop, was done against the damping forces. When damping was in- 

 creased the area usually became larger. 



COMPARISON OF FIBRILLAR MUSCLE WITH THE MODEL SYSTEM 



There was considerable variability in the shape of the loops experimentally 

 obtained. Apparently the phase relations between tension and length can vary 

 during a single cycle. Symmetrical loops were, however, found, as illustrated 

 in figure iC, the dotted line. This loop corresponds quite well with the one 

 drawn for the model system, having a constant phase angle of 30° and sinus- 

 oidal motion. The characteristics of the mechanical system to which the muscle 

 was attached determined in large part the shape of the loops. 



Position of the Loop in the Tension-Length Area. The position of the loop 

 may be discussed in terms of the equilibrium point, T'L', of the vibration 

 (fig. iC). The tension, T', is the weight load. If the oscillations are damped 

 out by an external agent, the muscle shortens until it attains the length at 

 which, under static conditions, it can just exert a force equal to the weight. 

 In figure 2 the steady state condition is the loop bb' \ the tension and length 

 of the completely damped muscle is represented by c. When the damping 

 agent is removed, the vibrations reappear and the equilibrium length of the 

 muscle increases to the steady state value, L' (fig. 2). While in vibration the 

 muscle has an average velocity which determines the average force the muscle 

 can exert, as defined by Hill's characteristic relation. As the amplitude or 

 frequency increases, the average velocity also increases. The tension the muscle 

 is able to exert at higher velocity is less at each length. Consequently, if the 

 tension, T', is constant, the muscle must elongate as the velocity is increased. 

 By this device the operating loop is forced over into the region of the tension- 

 length area in which the greatest amplitude and tension changes are possible. 

 L' depends upon amplitude and frequency, T' on the weight load. As tension 

 drops below the isometric value when the muscle shortens, and as the loop 

 moves to the right in the tension-length area with increasing velocity, the 

 characteristic force-velocity relation of Hill is present in fibrillar muscle. 



Work Cycle of Fibrillar Muscle. Any theory explaining the physiology of 

 fibrillar muscle must account for the large energy output. If the muscle behaves 

 as the model in its tension-length relations, the work done can be calculated 



