<P> Most muscle cells store enough ATP for only a small number of muscle contractions . While muscle cells also store glycogen, most of the energy required for contraction is derived from phosphagens . One such phosphagen creatine phosphate, is used to provide ADP with a phosphate group for ATP synthesis in vertebrates . </P> <P> The structure of the sarcomere affects its function in several ways . The overlap of actin and myosin gives rise to the length - tension curve, which shows how sarcomere force output decreases if the muscle is stretched so that fewer cross-bridges can form or compressed until actin filaments interfere with each other . Length of the actin and myosin filaments (taken together as sarcomere length) affects force and velocity--longer sarcomeres have more cross-bridges and thus more force, but have a reduced range of shortening . Vertebrates display a very limited range of sarcomere lengths, with roughly the same optimal length (length at peak length - tension) in all muscles of an individual as well as between species . Arthropods, however, show tremendous variation (over seven-fold) in sarcomere length, both between species and between muscles in a single individual . The reasons for the lack of substantial sarcomere variability in vertebrates is not fully known . </P>

What links the thick filaments to the z line