Tension changes in the cat soleus muscle following slow stretch or shortening of the contracting muscle

1. The permanent extra tension after a stretch and the deficit of tension a fter a shortening in the soleus muscle of the anaesthetised cat were measur ed using distributed nerve stimulation across five channels. At low rates o f stimulation the optimum length for a contraction was several millimetres longer than that when higher rates of stimulation were used, so that moveme nts applied over the same length range could be on the descending limb of t he full activation curve but on the ascending limb of the submaximal activa tion curve. 2. The extra tension after stretch and the depression after shortening were present only near the peak and on the descending limb of the length-tensio n curve. Effects on final tension of changing the speed and amplitude of st retches or shortenings were found to be small. 3. Statistical analysis showed that variations in the tension excess or def icit due to changing stimulus rate could be entirely attributed to the effe ct of stimulus rate on the length-tension relation, as when length was expr essed relative to optimum for each rate, stimulus rate was no longer a sign ificant determinant of the tension excess or deficit. 4. The extra tension after stretch and the depression after shortening disa ppeared if stimulation was interrupted and tension briefly fell to zero. 5. These effects were explained in terms of a non-uniform distribution of s arcomere length changes at long muscle lengths. During stretch some sarcome res are stretched to beyond overlap while others lengthen hardly at all. Du ring shortening some sarcomeres shorten much further than others. 6. These mechanisms have important implications for exercise physiology and sports medicine.