You may attribute the burn in your legs and the soreness after exercise to Lactic Acid, but it tends to be more of a friend than a foe in extreme exertion. Contrary to popular belief, Lactic acid is not a waste product. When running at 60% to 80% maximum heart rate, at fairly easy pace, ATP (adenosine triphosphate) is able to be produced as it is used, therefore fueling the muscles efficiently. Once you break approximately (this number goes up as your fitness level increases--one of the primary reasons for intervals) the 80% barrier, you start using ATP faster than you can produce it. Lactic acid allows ATP to be produced without the use of oxygen (anaerobic glycolysis).
How it works: Glycolysis is the breaking down of carbohydrates that occur when hydrogen ions (H+) and pyruvic acid are formed. The buildup of (H+) ions will make the muscles acidic and degrade efficiency, and therefore have to be removed from the muscle. Nicotinamide Adenine Dinucleotide (NAD+) is created as carrier molecules. The (NAD+) are reduced to (NADH) as they deposit the hydrogen at the Electron Transport Gate in the Mitochondria to be combined with oxygen to form water to leave the body in sweat.
If there is not enough oxygen to remove the (H+), the (NADH) build up in the cell. To once again prevent over-acidity, the pyruvic acid accepts the (H+) and forms Lactic Acid. The Pyruvic Acid is like a backup to the (NADH). It then reduces to Lactate and (H+), and some of the Lactate molecules take (H+) with them to help lower the level of acidity in the muscle, whose normal pH is between 7.25 to 7.4*. If the lactate molecules donít round up enough (H+) to take with them into the blood stream, the pH scale drops to about 7.3, where the pain starts. What I get out of this, is basically the (NADH) canít handle the job, so pyruvic acid takes over, combines with a certain amount of (H+), and becomes mighty morphiní Lactate, who is powerful enough to drag more (H+) molecules into the bloodstream. But even then, if the beast is really working, the powerful Lactate molecules are outnumbered and are not able to perform their task.
Lactate can also be brought into the liver to be transformed into glycogen, which is then stored in the muscles for later use, or put into the muscles immediately to be turned into ATP for fast energy. 65% of lactic acid is converted to carbon dioxide and water, 20% into glycogen, 10% into protein and 5% into glucose. Itís a vicious cycle: when you break down glucose and glycogen into lactate to be turned to (ATP), you get waste, (H+), which must be then removed by lactic acid, which turns into lactate, goes to the liver and gets turned into glycogen to be broken down and used by your muscles again. Lactate or Lactic acid is either fuel on its way in or exhaust on its way out, but either way it is beneficial to your muscles.
Training implications: While VO2max is an indicator of performance potential, lactic acid threshold is a much better predictor of an individual's training state. For example, take two athletes with approximately the same VO2max and the athlete with the higher lactic acid threshold will out perform the other. Determine your lactic acid threshold using either a treadmill, track or other controlled location in which you can conduct a sustained bout of exercise. Measure your heart rate using your heart rate monitor and record the last 15-20 minutes of a 5km road race. The average heart rate value should be close to your threshold heart rate value assuming you are rested, highly motivated and push yourself. The above is an example of a field test, as an alternative: many universities and sport medicine facilities offer lactic acid threshold testing such as UIC. Once you have determined your lactic acid threshold (heart rate value), conduct once a week training sessions using your recorded average heart rate value as the lactic acid threshold indicator. Training examples: track or trail intervals near or slightly above your threshold or fartlek (speed play) training. Using a wind trainer is especially useful since you can control load variables (a power hub is a great investment) as well as constant feedback on how your body is responding through a heart rate monitor.
The above paragraphs provide a detailed explanation of why competitive athletes should perform some form of interval training after establishing a solid training base. Establishing the training base prior to interval helps reduce the chance of injury due to increased stress on the joints and muscles during an interval session. Interval sessions can be performed on the track using 400 (quarters), 800 (half miles) meter or on the trail using fartlek technique. An example interval session could consist of a warm-up of 10-15 minutes on the track, followed by 4-6 quarters at 10 seconds per quarter faster than your race pace. If your average pace is 8:00 per mile and 2 minutes per quarter, then you would run 1:50 quarters with either a rest lap or if you have a solid training base use a heart rate monitor and run the next quarter when your heart rate falls below the training threshold. More on heart rate calculations>>. Complete your interval session with a 10-15 cool down and an optional stretch (better to stretch after the workout if you are going to only perform one stretching session). You can complete similar intervals on a bike trainer in which you perform a warm-up and then perform 8-10 intervals of 1/2 mile (or 1/4 mile depending on fitness) and then a cool down with high cadence spin for 15-20 minutes.
The bottom line:
Generally, human pH falls between 7.25 and 7.45, which is slightly alkaline [water is neutral at 7.0]. When blood pH falls near 6.35 (too acidic) hemoglobin in the blood deteriorates and the red blood cells cannot carry adequate oxygen to working muscle tissues. When muscles, and connective tissues are too acidic, acidosis is experienced.
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