8 A. V. HILL VOL. 4 (1950) 



3. It has been found by quick stretches appHed to a muscle shortly after a single 

 shock that the full strength of the contraction, defined as the load which a muscle can 

 just bear without lengthening (and equal to the force of a maximal tetanus) is developed 

 abruptly immediately after the end of the latent period. It is maintained for a time and 

 then declines in "relaxation". If stimulation is continued, each successive shock re- 

 stores the strength of contraction to its full height. 



4. Corresponding to (3) there is a "heat of activation" in a twitch, which is inde- 

 pendent of all other factors except the fact of stimulation. The heat of activation starts 

 at its maximum rate before any visible sign of contraction occurs, declining to zero at 

 about the moment when the strength of contraction (see 3 above) begins to fall off, i.e., 

 at the end of the contractile phase. 



5. The "heat of maintenance" in a prolonged contraction is the summated effect 

 of the heat of activation following successive elements of the stimulus. It is greater at 

 first corresponding to the more rapid relaxation after a short tetanus, but after a certain 

 duration of stimulus it becomes constant. It is affected only to a minor extent by the 

 length of the muscle. It is greatly increased by a rise of temperature, corresponding to 

 the more rapid relaxation. 



6. In twitch and tetanus alike, apart from the heat of activation or the heat of 

 maintenance, energy is given out in two discrete forms, (a) as mechanical work and b) as 

 heat of shortening. The heat of shortening is directly proportional to the change of 

 length over the whole range of shortening, and (for a given change of length) is inde- 

 pendent of the work done. 



7. Apart from heat of activation or heat of maintenance, the rate at which total 

 energy, i.e., heat plus work, is given out, is a linear function of the load throughout 



a contraction : / n , \ j / 7* z / d t,\ 



[P + a) ax jilt = o{P^ — P) 



where x is the amount of shortening up to time t, P is the load, dx is the heat of short- 

 ening, Pq is the maximum isometric tension and Z) is a constant related to the maximum 

 velocity of shortening under zero load. 



8. The constant a in (7) can be obtained either from thermal measurements or from 

 the form of the characteristic relation between load and velocity of shortening. The 

 agreement is good. 



9. Relaxation is not an active process. A muscle completely without load or tension 

 does not lengthen again after shortening in response to a stimulus. That its length has 

 really changed and that its fibres or fibrils have not gone into folds is shown by the fact 

 that its latent period is practically the same at a short length as it is at a greater one. 

 If a muscle had to "take up the slack" in fibres or fibrils before its tension could be 

 manifested externally, the latent period would be greatly prolonged. 



10. Simultaneous with the earliest sign of mechanical activity after a shock is a 

 change of opacity. This is due to an alteration of light scattering (D. K. Hill*). The 

 earliest phase has certain characteristics which distinguish it from a later phase which 

 continues into recovery. 



11. If we can assume that excitation occurs at the surface membrane of a muscle 

 fibre, the propagation inwards of the change there started cannot be due to the diffusion 

 inwards of some substance, e.g., Ca ions or acetyl choline, initiating contraction by its 

 arrival at each point. Diffusion is far too slow. Some chain-reaction started at the surface 

 is required. 



References p. 11. 



