What might be causing my heart rate to crash during exercise (walking on a treadmill)? It starts raising gradually during exercise (from around 65 to 110), but all at once it drops from say 110 to 55 immediately. I lose energy to my leg muscles and sweat profusely while feeling clammy. After a few minutes, I can start walking on the treadmill again, and the symptoms may or may not occur again. Lately, in the last couple weeks, these symptoms have been repeating themselves.