A comparison of isometric force, maximum power and isometric heat rate as a function of sarcomere length in mouse skeletal muscle.

The isometric force, maximum power and isometric heat rate have been measured at different sarcomere lengths (SL) between 1.40 and 3.63 microns in two types of mouse muscle, soleus and omohyoideus, at 25 degrees C. The SL force relationship is different in the two muscles. At a SL above optimum filament overlap, 2.44 microns in omohyoideus muscles, maximum power declined while isometric force remained high. In soleus muscles this occurred above a SL of 2.33 microns. In parallel experiments, the isometric heat rate declined linearly with increasing SL above 2.33 microns in soleus muscles, while isometric force remained closer to its maximum. At short SL, between 2.33-1.75 microns in soleus and 2.44-2.15 microns in omohyoideus, maximum power remained at or near its maximum value as did heat rate (soleus) while isometric force fell. In both muscles at SL greater than optimum for force development maximum power output (unlike force) is proportional to filament overlap. The variation in heat rate over this SL range can be described as the sum of constant rate and a rate proportional to filament overlap. These observations are compatible with the idea that maximum power and heat rate are less affected by non-uniformities in SL than is force.