Many terms are used to define or describe exercise conditioning and work rate, also referred to as exercise intensity or work load. The two terms used throughout this chapter for describing how to gauge your work rate will be maximal oxygen uptake and energy expenditure as calories/hour, or kcal/ hr. Other terms to describe work rate and their interrelationships will be discussed at the end of this chapter.
The primary measure or predictors of one's capacity to sustain work performance is maximal oxygen uptake (V02max) or maximal aerobic (cardiorespiratory) capacity. V02max is measured in milliliters per minute (ml/min), Liters/min, or after adjusting for body weight in kilograms, as ml/ kg/min; a higher value indicates a higher level of cardiorespiratory fitness.
Your maximal aerobic capacity or oxygen uptake is the best indicator of how much work you can sustain without fatigue.
Typical V02max values range from 30 ml (of oxygen)/kg/min for an unfit person up to 80 ml/kg/min for an exceptionally fit, endurance athlete. If the unfit and highly fit persons both weighed 70 kg (155 lb) then their respective absolute maximal aerobic capacities would be 2.1 liters (of oxygen)/ min and 5.6 liters (of oxygen)/min.
Using 1 liter of oxygen/min is equivalent to expending 5 kcal/min
As such, the unfit person can only work at a rate of up to 10 kcal/min (2.1 L x 5) whereas the highly fit could work at up to 25 kcal/min (5.6 L x 5 kcal) if needed. If a specific task required 2 L/min, then this would amount to 10 kcal/min of energy. Resting energy expenditure requires less than one kcal/min, or about 0.200 to 0.250 L (of oxygen)/min. In order to account for different body sizes, resting energy expenditure for an individual is usually defined as:
3.5 ml of oxygen/kg body weight/minute.
Thus, for a 70 kg (155 lb) SEAL, resting energy expenditure would be approximately 245 ml/min or 0.245 L/min (3.5 x 70). This is equivalent to expending around 1 to 1.25 kcal/min.
How much strenuous work can you sustain without oxygen? Most people can do very little for more than a couple of minutes. It is very important to realize that most people cannot work for very long at even 90% of their maximal aerobic capacity. This is because everyone has a threshold at which the balance between aerobic and anaerobic energy systems begins to favor the anaerobic; your muscles cannot extract enough oxygen to produce the required energy. This is called your anaerobic threshold; the turning point can be monitored by the accumulation of lactate in your blood. Of course, your body will know when there is too much lactate, because once lactate goes above a certain value, it starts to accumulate and unless you decrease your work rate, you will become too tired to continue working.
This anaerobic threshold, or "break point" varies among individuals, but ranges between 60% and 100% of your V02max; all SEALs should be able to work at 70% of their V02„lax for an extended period, and should have a break point above 70%. Conditioning programs for SEALs should strive to raise the anaerobic threshold or break point to as high as possible, because that means you can work at a higher rate for a longer period of time.
Interval workouts stress the anaerobic energy systems and will increase your anaerobic threshold and power.
Interval and fartlek workouts for running and swimming are described in their respective chapters, and such workouts for other forms of exercise are described later in this chapter.
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