For my thesis i made a study about Heart Rate Variability (HRV) in the horse before and after an osteopathic treatment.
Through HRV we measure the input of the autonomic nervous system to the heart. Using the HRV parameters we can measure the balance of the autonomic nervous system (ortho/parasympathicus) and the relaxation rate of the horse.
With this study we have proven that an osteopathic treatment has an influence on the HRV and so on the internal environment of the horse.

Link Thesis

Further on HRV is also a very important touchstone to measure the physical recuperation during and after training. This will be discussed in the part "Training".