Fig. 6. ADP levels during steady-state (A) and transient (B) exercise determined for mammalian muscle. The line is the normalized rate of oxidative phosphorylation versus [ADP] in isolated mitochondria (Mootha et al., 1997). The symbols in A represent the highest steady-state [ADP] reported in mammalian skeletal muscle (Harkema et al., 1997; Harkema and Meyer, 1997; Jeneson et al., 1996; Kushmerick et al., 1992; Paganini et al., 1997; Walter et al., 1997). The symbols in B represent measurements from the human quadriceps at each step of an exercise stress test (Richardson et al., 1995). These data are normalized to the maximal level and superimposed on the relationship for isolated mitochondria (solid line) (Mootha et al., 1997).