Correlation between maximal workload as achieved during a physical exercise test (treadmill or bicycle; horizontal axis) and left superficial femoral artery (SFA) collateral flow index (CFI; vertical axis). Correlation between maximal workload as achieved during a physical exercise test (treadmill or bicycle; horizontal axis) and left superficial femoral artery (SFA) collateral flow index (CFI; vertical axis). Michael Stoller et al. Open Heart 2016;3:e000355 ©2016 by British Cardiovascular Society