Treadmill running has long since become a test of choice when measures
of exercise induced metabolic activation need to be taken [10,11,
26–29]. However the cardiovascular data reported for mice and, to a
lesser extent rats, in this test clearly do not follow the classical pattern
of linear increase of heart rate with increase of physical load, but rather
display a step-wise shift to higher exercise level from a relatively low
resting value [7,10–12]. Indeed, we have also found that HR was roughly
200 bpm higher on the treadmill than at rest in the home cage, while
heart rate increase associated with increase of exercise intensity was approximately
10 times smaller.