Measurements made at rest and during graded dynamic knee extensor exercise A, concentration of adenosine; B, concentrations of ATP, ADP and AMP in human skeletal muscle interstitial fluid, measured in the dialysate; C, femoral arterial blood flow. Data are mean ±s.e.m., n = 5–7 in each case. *Significant increases in adenosine, ATP and blood flow, other symbols indicate significant increases in AMP or ADP: P < 0.05 in each case. Reproduced from Hellsten et al. (1998) with permission from Lippincott, Williams & Wilkins.