Four weeks of high‐intensity training in moderate, but not mild hypoxia improves performance and running economy more than normoxic training in horses

Abstract

We investigated whether horses trained in moderate and mild hypoxia demonstrate greater improvement in performance and aerobic capacity compared to horses trained in normoxia and whether the acquired training effects are maintained after 2 weeks of post‐hypoxic training in normoxia. Seven untrained Thoroughbred horses completed 4 weeks (3 sessions/week) of three training protocols, consisting of 2‐min cantering at 95% maximal oxygen consumption under two hypoxic conditions ( H16 , F I O 2 = 16%; H18 , F I O 2 = 18%) and in normoxia ( N21 , F I O 2 = 21%), followed by 2 weeks of post‐hypoxic training in normoxia, using a randomized crossover study design with a 3‐month washout period. Incremental treadmill tests (IET) were conducted at week 0, 4, and 6. The effects of time and groups were analyzed using mixed models. Run time at IET increased in H16 and H18 compared to N21 , while speed at was increased significantly only in H16 . in all groups and cardiac output at exhaustion in H16 and H18 increased after 4 weeks of training, but were not significantly different between the three groups. In all groups, run time, , , , and lactate threshold did not decrease after 2 weeks of post‐hypoxic training in normoxia. These results suggest that 4 weeks of training in moderate ( H16 ), but not mild ( H18 ) hypoxia elicits greater improvements in performance and running economy than normoxic training and that these effects are maintained for 2 weeks of post‐hypoxic training in normoxia.

Moderate‐intensity training in hypoxia improves exercise performance and glycolytic capacity of skeletal muscle in horses

“…However, the increase in was similar in HYP (+4.3%) and NOR (+6.4%) and the changes in cardiovascular variables including , SV max, and C a‐v O 2 were not significant and similar in both groups after training. Consistent with previous studies on LLTH training (Mukai et al, 2020 , 2021 ), we did not observe any increase in [ Hb ], and the hypoxic exposure duration (<5 min) in this study appears to be too short to induce hematological changes. These results suggest that the greater improvement in performance in HYP may not be attributed to the changes in aerobic, cardiovascular, or hematological adaptations between HYP and NOR.…”

“…There was no significant difference in run time between NOR and HYP and the increase in run time in HYP was relatively smaller in this study compared to our previous results of high-intensity hypoxic training despite the same/ similar F I O 2 and the same training duration and frequency being used (Mukai et al, 2020(Mukai et al, , 2021. The smaller improvements in performance may be due to the lower training intensity compared to our previous studies, as McLean et al (McLean et al, 2014) stated that improvements in exercise performance at sea level appear most likely after high-intensity and short-term training in hypoxia.…”

“…Collectively, hypoxic training may have a benefit to enhance both aerobic and glycolytic capacity, which is essential for racehorse training. However, we have observed greater weight loss after hypoxic training than after normoxic training (Mukai et al, 2021), and, thus, there is a need for lower intensity, but still effective hypoxic training for horses.…”

“…Our previous studies reported that 4 weeks of high‐intensity training (100% 2 min, 3 sessions/week) in moderate hypoxia (15% O 2 ) and similar hypoxic training (95% 2 min, 3 sessions/week, 4 weeks) in moderate hypoxia (16% O 2 ) increased run time at IET by +28% and +21%, respectively (Mukai et al, 2020 , 2021 ). Consistent with the previous reports in horses, we also observed a significant increase in run time only in HYP (+10%), but not in NOR.…”

“…*Significant changes from pre-training (p < 0.05) F I G U R E 3 Cardiac output ( Qmax ; a), stroke volume (SV max ; b), arterial-mixed venous O 2 difference (C a-v O 2 ; c), and hemoglobin concentration ([Hb], d) at exhaustion in IET at pre-training and post-training in normoxia (NOR) or hypoxia (HYP). Values are mean ± SE 4.1 | Effects on exercise performance and aerobic capacity after hypoxic training Our previous studies reported that 4 weeks of highintensity training (100% V O 2max 2 min, 3 sessions/ week) in moderate hypoxia (15% O 2 ) and similar hypoxic training (95% V O 2max 2 min, 3 sessions/week, 4 weeks) in moderate hypoxia (16% O 2 ) increased run time at IET by +28% and +21%, respectively(Mukai et al, 2020(Mukai et al, , 2021. Consistent with the previous reports in horses, we also observed a significant increase in run time only in HYP (+10%), but not in NOR.…”