Fig. 4. Time course of ³¹P NMR spectra of elephant seal muscle acquired in vivo during an apneic episode. The ³¹P spectra were recorded continuously, each scan requiring 70 s of signal averaging. The spectra correspond to (a,b) eupneic state, (c–f) apneic state and (g,h) eupneic state. The ³¹P signals exhibit no change during the eupnea to apnea transition. Peak 1 denotes P_i; peak 2, PCr; and peaks 3, 4 and 5, the , and β signals of ATP, respectively.