Effect of training on intrinsic and resting heart rate and plasma volume in young and old horses

Abstract

The chronic bradycardia seen in several species after intense exercise training may be due to autonomic mechanisms, non-autonomic mechanisms, such as increased pre-load, or a combination of the two. Thirteen, healthy, unfit Standardbred mares were split into two groups: young (age 12±1 yr; mean ± standard error, n=8) and old (age 22±1 yr, n=5) to test the hypothesis that there would be age and training related differences in resting heart rate (RHR), intrinsic heart rate (IHR), maximal heart rate (HRmax) and plasma volume (PV). Mares were trained 3 d/wk at 60% HRmax for 20 min and gradually increased to exercising 5 d/wk at 70% HRmax for 30 min and RHR, IHR, HRmax, and PV were measured prior to and after the 8 wk training period. There were no age related differences (P≯0.05) between young and old mares before (41±2 vs. 42±2 beats per minute (bpm); 86±5 vs. 80±4 bpm) or after training (35±1 vs. 34±1 bpm; 81±6 vs. 78±2 bpm) for RHR and IHR respectively. RHR was decreased (P<0.05) following training in both the young (41±2 vs. 35±1 bpm) and old mares (42±2 vs. 34±2 bpm). Training decreased IHR (P<0.05) in the young mares (86±5 vs. 81±6 bpm), but not (P≯0.05) the old mares (80±4 vs. 78±2 bpm). The young horses had a higher HRmax than the old horses (P<0.05) both before (216±5 vs. 200±4 bpm) and after training (218±3 vs. 197±5 bpm). Maximal heart rate was not altered after training (P≯0.05) in either young (216±5 vs. 218±3 bpm) or old (200±4 vs. 197±5 bpm) mares. The PV of the young mares was 15% higher before training and 32% higher after training when compared to the old mares (P<0.05). Training caused an increase in PV in young mares (+9%; P<0.05), but did not alter PV in old mares (-5%; P≯0.05). Training improved RHR in the young but not the old horses. The decrease in measured parameters in the young horses appears to be related to enhanced pre-load associated with a training-induced hypervolemia as well as changes in autonomic function.

Reduced high intensity training distance had no effect on VLa4 but attenuated heart rate response in 2-3-year-old Standardbred horses

“…The present study also showed a clear effect of training on HR at rest. A reduction in HR at rest with increased fitness is a well-known effect in human athletes [ 46 ] but the same response has not until very recently [ 47 ] been documented in horses and never in horses at the same age submitted to different levels of training. For measurement of resting HR the use of HR meters during night time can be recommended since excitement, especially when horses were 1 year old, probably affected the manual measurements with a stethoscope.…”

Electrocardiographic characteristics of trained and untrained standardbred racehorses

“…However, a single study did observe a decreased intrinsic HR caused by training in young adult horses, but not in older horses, suggesting a local alteration within the SAN itself. 13 Bradycardia is strongly associated with training in humans.…”

“…7,[9][10][11] Knowledge on exercise-induced ECG changes in horses is scarce. It has been suggested the trained horse has a slower HR and prolonged PR interval, [12][13][14][15][16] whereas other studies failed to support these findings. [17][18][19][20] The disparity may be explained by small study populations, immature animals and lack of control groups in the different studies.…”

Regulation of sinus node pacemaking and atrioventricular node conduction by HCN channels in health and disease

“…This has been carried out many times in both human athletes as well as animal models (Boyett et al, 2013). The majority of the studies show that a resting bradycardia is still present after complete autonomic blockadefor example, in the horse (Betros et al, 2013). In human athletes, the relative bradycardia is greater after autonomic blockade (Katona et al, 1982;Boyett et al, 2013;D'Souza et al, 2017).…”