The study covered 89 Polish warm-blooded half-bred mares (51 representing Polish Noble Half-Breds, 23 Wielkopolski and 15 Małopolski horses) which went through a 60-day training (terminated with a performance test) at the Biały Bór training centre. Assessed was motion and jump parameters of individual mares (recorded with a digital video camera) and the heart rate (HR) index – based on ongoing telemetric registration – at rest and during performance test. Heart rate indices were also used to formulate a custom “effort reaction index” (WRW) employed further on in the study. A fairly pronounced inter-breed variation was identified in the mares, mainly in the case of the WRW index. This indicates the usefulness of the proposed index for the breeding selection of performance traits of horses. The index finely differentiated variation between the horses, unidentifiable in the analysis of the heart rate values alone.