Dynamic Adaptation of Heart Rate and Autonomic Regulation During Training and Recovery Periods in Response to a 12-Week Structured Exercise Programme in Untrained Adult and Geriatric Horses

Changes in resting heart rate (HR) and HR variability (HRV) are utilised to assess training effects in horses. However, limited research has examined how these variables reflect training effects during exercise sessions and recovery periods, particularly in horses of different ages. This study investigated how HR and HRV changed during exercise sessions and recovery periods in untrained adult horses (UAHs; <i>n</i> = 9) and geriatric horses (UGHs; <i>n</i> = 9). HR and HRV were measured before and during a 54 min exercise bout over a 12-week training programme. Mixed-effects model with Greenhouse-Geisser correction was used for statistical analysis. The duration of elevated mean HR, sympathetic nervous system index, and stress index during the exercise sessions gradually decreased over the 12 weeks for both UAHs and UGHs. However, this decrease was faster in UAHs than in UGHs. Mean HR consistently decreased during the exercise sessions for both UAHs and UGHs over the 12 weeks. Furthermore, HRV returned to baseline faster in UAHs than in UGHs during the exercise bouts. These results indicate that the 12-week structured exercise programme positively influenced HR and HRV adaptation during exercise and recovery periods in UAHs and UGHs, with the adaptations differing dynamically between them.