True Fitness Treadmill ES9.0 + ES7.0 Owner's Manual page 20

Contact Heart Rate
Chapter One: Basic Operation
A Note on CHR Accuracy
CHR monitoring may be a bit less accurate than a chest
strap, since the heart rate signals are much stronger at
the chest.
About 5% of the population cannot be picked up
by any CHR system. This is because their heart is
positioned in a more up-and-down manner in their
chest, as opposed to leaning over to one side.
ES Treadmills Owner's Guide
20