Ramsey, if I am not mistaken, has an EKG kit which might be a place to start. I've found others who have described their work on heart monitors or EKG (also called ECG) by using Google.
Search on biofeedback and you'll likely find that they measure skin conductivity. Pressure sensors are used in footwear to indicate, measure and record the process of walking (or running). Strain gages can be applied to a platform that someone might stand on to indicate reactive forces - applied to an exercise machine these gages can reveal a lot.
Whatever you do - electrical safety (and other safety issues) must be addressed. The EKG, for example, has low resistance leads connected to the body - voltage/current that in other situations might be harmless, can be quite deadly.