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 after a shortening in the soleus muscle of the anaesthetised cat were measured using distributed nerve stimulation across five channels. At low rates of stimulation the optimum length for a contraction was several millimetres longer than that when higher rates of stimulation were used, so that movements applied over the same length range could be on the descending limb of the full activation curve but on the ascending limb of the submaximal activation 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-tension curve. Effects on final tension of changing the speed and amplitude of stretches or shortenings were found to be small. 3. Statistical analysis showed that variations in the tension excess or deficit due to changing stimulus rate could be entirely attributed to the effect of stimulus rate on the length-tension relation, as when length was expressed relative to optimum for each rate, stimulus rate was no longer a significant determinant of the tension excess or deficit. 4. The extra tension after stretch and the depression after shortening disappeared if stimulation was interrupted and tension briefly fell to zero. 5. These effects were explained in terms of a non-uniform distribution of sarcomere length changes at long muscle lengths. During stretch some sarcomeres are stretched to beyond overlap while others lengthen hardly at all. During shortening some sarcomeres shorten much further than others. 6. These mechanisms have important implications for exercise physiology and sports medicine.