Fig. 2. Representative ³¹P-NMR spectra collected from perchloric acid extracts of large dark levator muscle fibers that demonstrate the changes in relative concentrations of AP and P_i during a contraction-recovery cycle. Spectra were collected from crabs at rest, and after 0, 30 and 60 min of recovery from burst exercise. The same pattern of recovery was observed in the small dark fibers, except that complete AP resynthesis occurred in only 15 min. Chemical shifts are in units of parts per million.