Why do some people breath more deeply and more rapid than others during and after exercise performance? Why does the heart rate increase? The answers to these questions lie in how much oxygen you owe your muscles and how much oxygen you possess. If the balance of your debt is greater than the balance of your reserves, the muscles must first demand these reserves as a down payment. It is only through increased respiration and accelerated heart rate that your oxygen payment in full is transacted. Let’s take a closer look at the variables that dictate supply and demand of oxygen to working, or in some cases ‘already worked’, muscles.
Oxygen Debt & Resistance Activity…
It is quite widely understood how energy is produced in the muscle and the role oxygen plays. At first exertion, there is enough ATP present in a cell to fuel the cell for only a few seconds. Then Creatine Phosphate takes over for a few more seconds. At the point of creatine phosphate depletion the muscle has sustained a maximum contraction for about 25-30 seconds, give or take a few seconds. It is at this point that oxygen desirably becomes involved (metabolic oxidation.) This is when the internal sugar Pyruvate, derived from the muscle’s Glycogen stores, is used along with oxygen for ATP production. Energy production via metabolic oxidation will only occur for as long as there is sufficient oxygen present. The muscle does not store oxygen, therefore it must be taken up from outside the cell.
In a best case scenario, for the muscle cell, there have been intermittent periods of momentary relaxation. It is only during these periods when the cell’s walls become permeable and allow for the inward movement of nutrients to include oxygen, and the outward movement of wastes to include carbon-dioxide. If a maximum contraction is sustained, no oxygen can be introduced and the contraction will fail upon depletion of internal creatine phosphate. This explains, at least in part, why maximum contractions cannot be sustained for longer than this 25-30 second time period. Try it for yourself, and you will find this to be correct. To the contrary, when intermittent periods, even extremely short periods, of relaxation are practiced, the contraction can be sustained for longer periods.
Likewise, if the contractions are not maximal, there is some degree of permeability in the cell wall allowing intermittent oxygen transport into the cell also prolonging the contractile period. These concepts can be easily proven in the gym. Perform a maximum sustained contraction in a particular movement and measure the time period to failure. Then, after sufficient recovery, maintain a less than maximum contraction in this same movement and measure the contractile period, and you will find it to last longer. Lastly, perform a maximum contraction with intermittent periods of relaxation. In this case as well, you will experience longer contractile periods than while maintaining maximum sustained contractions.
Oxygen Debt Repayment & Aerobic Activity…
On the other side of the exercise coin, aerobic effort allows for the presence of oxygen from the onset of work since contractions are both less than maximal and intermittent. It is simply a matter of when the system switches from ATP, to creatine phosphate, to metabolic oxidation to aerobic metabolism (when oxygen and fatty acids from extracellular sources dominate energy production.) In the beginning of all muscular effort, both anaerobic and aerobic, the above energy provision sequence remains the same, there is always an initial oxygen debt. During these periods when the output of energy exceeds the ability of the body to provide needed oxygen, there is an oxygen ‘debt’ and the participant experiences more rapid breathing and stronger heart muscle contraction. The cardiorespiratory system is trying to ‘come up with the down payment’ to this debt by more labored functioning.
The point at which the oxygen debt is repaid during aerobic activity is commonly referred to as, ‘catching your second wind’. At this time the payments are being made on time, breathing is less labored, the vascular tissue has dilated and the heart’s stroke volume is optimal. Plenty of oxygen is being provided to the working muscles. As intensity of aerobic activity increases, the oxygen payment is higher and the cardiorespiratory system will once again become more labored.
In short, if at the point of creatine phosphate depletion during sustained contractions, there has been no movement of oxygen into the muscles, an ‘oxygen debt’ will quickly develop that must be repaid by the body. Generally speaking, in the case of resistance training, increased duration of between set recovery is recommended to counter the labored cardiorespiratory activity. And, during aerobic activity, initial labored cardiorespiratory activity will ultimately lead to the more tolerable second wind of aerobic metabolism. There you have it. Now you not only know how you get into oxygen debt in the first place, but how to get out of it as well!