The force-length relationship of the cat soleus muscle.

According to the force-length relationship, cat soleus optimal sarcomere length should lie between 2.3-2.5µm. Rack and Westbury (1969) found optimal sarcomere length around 2.8- 3.0µm. The purpose of this study was to repeat their study to check for these discrepancies between the expected and the measured optimal length. The soleus muscle of both hindlimbs of three cats was supra-maximally stimulated. Isometric forces were measured for lengths ranging from -20 to +20mm relative to the optimal length. Mean sarcomere lengths were obtained by laser diffraction. Fibre length was obtained post-mortem by video analysis and in situ with sonomicrometry crystals. Sarcomere number was determined and in situ sarcomere lengths were calculated. The sarcomere force-length relationship showed an ascending and descending part with a plateau between 2.0-2.4µm. Peak forces were obtained at smaller average sarcomere lengths than reported by Rack and Westbury and closer to the optimal sarcomere length based on sliding filament considerations.