Plasticity of muscle energetics in the horse after training.

The research article investigates the impact of high-intensity training on muscle energetics in horses. The study reveals that this training approach enhances the maximal oxygen (O2) deficit in horses, which in turn improves exhaustive running performance at high-intensity levels.

Concept of O2 Deficit

• The concept of O2 Deficit lies at the heart of this research, originating from earlier studies by Krogh, Lindhard, Hill, and others. It suggests that the rate of adenosine triphosphate (ATP) turnover increases instantaneously at the onset of muscle contractions. However, the rate at which O2 is absorbed rises over a finite time course.
• The O2 Deficit is essentially the difference between the ATP supplied oxidatively from pulmonary O2 uptake and the ATP used by exercising muscles.
• Di Prampero and Ferretti (1999) identified three components constituting the O2 deficit: muscle phosphagen store reduction (primarily phosphocreatine, PCr); ‘anaerobic glycolysis,’ leading to lactic acid production; and O2 stores in myoglobin, hemoglobin, and the lung.

Energetic Contribution and Exercise Intensity

• The paper further discusses how exercise intensity and duration affect the contribution each of the mentioned processes makes to the O2 deficit. The greatest energetic contribution is most likely from anaerobic glycolysis, followed by phosphagen stores depletion for heavy/supramaximal exercises exceeding 10–20 seconds in duration.
• Additionally, the time-to-fatigue during high intensity exercise can be predicted closely based on two parameters: the energetic equivalent of the O2 deficit and the maximum rate of energy turnover that can be sustained without drawing upon the O2 deficit.

Training Intensity and O2 Deficit

• Hinchcliff et al. (2002) advocated that for a given supramaximal fatiguing exercise bout, the relative contribution of aerobic and anaerobic energy systems would depend on the ATP turnover rate, maximal O2 deficit, and maximum O2 uptake, among other parameters.
• They further demonstrate that high-intensity training can contribute to an increase in anaerobic capacity and O2 deficit in horses. This establishes a perspective that an even higher increase might be obtained by a more intense training regimen and greater stimulation of anaerobic pathways.

Challenges in O2 Deficit Quantification

• The conclusion, however, is not free from challenges. Quantifying the O2 deficit is difficult for heavy, supramaximal exercises. The presumption that O2 demand remains constant throughout exercise and can be determined by linear extrapolation complicates matters.
• Values of O2 deficit in horses published across different studies show considerable variation. This raises concerns regarding methodologies employed or the presumptions.
• Hinchcliff et al.’s findings open up new questions regarding the mechanisms behind O2 deficit. For instance, understanding how an unchanged level of exercise-induced muscle glycogen depletion before and after training plays out with changes in the O2 deficit remains a challenge.