A comparison was made between fluctuations in a length of sarcomere fr
om a myofibril during relaxed and rigor conditions. The fluctuations o
f the length associated with rigor states was due entirely to thermal
fluctuations in the ambient. In contrast, the fluctuations associated
with relaxed states were accompanied by extremely slow and macroscopic
ally modulated thermal fluctuations. A slow, macroscopic modulation of
the thermal fluctuations of the length of sarcomere was found to be u
nique to myofibrils in their relaxed conditions.