Thursday, August 9, 2012

Lactic Acid: What's the Deal?

Lactic Acid: Urban Legend?

Imagine your running a 400m dash, you hit the home stretch with a lead, but then the awful happens. 

You tighten up, your legs feel like bricks, and you can't exert any force into the track. 

What's happened? 

It's the boogyman of sport, Lactic Acid! 

Everybody and their mother is quick to blaim lactic acid accumulation for muscle fatigue, loss of muscular force/power, and that hitting the wall feeling. 

You hear this from coaches, parents, and just about every couch coach there is. 

I was watching the Olympics the other day when Ryan Lochte finished a race and hurried to the "warm-down" pool.  The announcer then proceeded to explain the reason for doing so was because of lactic acid build up.

Gotta hit the the warm-down pool, hop in an ice bath, do a cool down, or stretch so that lactic acid doesn't build up and stay in your body long and ruin another workout.  But the reality is this could not be the case, and this whole lactic acid being the ultimate bad guy is false.

Must Be Lactic Acid, Right???



Intracellular acidosis due mainly to lactic acid accumulation has been regarded as the most important cause of skeletal muscle fatigue (Allen DG, Lännergren J, Westerblad H).

Let's say you partake in intense anaerobic exercise, this will lead to an intracellular accumulation of lactic acid.  Since lactic acid is a strong acid, it breaks down into lactate and H+.  The increase in H+ leads to reduced pH or acidosis and is the classic cause of skeletal muscle fatigue (Håkan Westerblad, David G. Allen, and Jan Lännergren). 

Decreased pH levels and acidosis caused by lactic acid have been found in fatigued muscles by source of skinned muscle fibers (muscle cells where the surface membrane has been chemically or physically removed).  Studies like skinned muscle fiber are on single muscle fibers and this provides the most direct way to address cellular mechanisms of fatigue  (Håkan Westerblad, David G. Allen, and Jan Lännergren).

The presence of lowered pH and acidosis are the main reasons lactic acid is believed to cause muscle fatigue, pain, and "heaviness" during intense exercise. Another substance often found in fatigued muscle samples in high concentrations is K+ (Ole B. Nielsen, Frank de Paoli, Kristian Overgaard).

Recent studies on mammalian muscle, however, show little direct effect of acidosis on muscle function at physiological temperatures (Håkan Westerblad, David G. Allen, and Jan Lännergren).

In fact, it has been shown when muscle forces are depressed by high concentrations K+, acidification by lactic acid actually produced a recovery of force.

As stated before, intense exercise is associated with an increase of K+, acidosis, and decreased pH, but this might indicates that acidosis may protect against fatigue rather than being a cause of fatigue (Juel, C., Pilegaard, H., Nielsen, J. J. & Bangsbo, J.). 

This relationship of acidosis actually improving muscular fatigue and power is getting more and more recognition of being the true case in muscles.  So, now the focus has been shifted to more on lowered pH levels and high K+ concentrations, as multiple studies have recently looked into this. (Links at the Bottom)
Don't Let This Happen To You

     
Recently the role of reduced pH as an important cause of fatigue is now being challenged, and several recent studies (Posterino GS, Dutka TL, and Lamb GD) show that reduced pH may have little effect on contraction in mammalian muscle at physiological temperatures.

Phsysiological temperatures mean temperatures similar to that which would be seen in muscles during exercise.  Many previous studies have based the idea of pH causing fatigue on studies of muscles at colder temperatures (less than 15 degrees C), however recent studies looking at muscles at around 30 degrees C and more (physiological temperatures of muscle), have found that the muscle fatigue and reduced muscle force are not present at these temperatures despite lowered pH's (Renaud JM and Light P). 

Another key finding has been when looking at muscle fatigue and recovery, it has been found that muscle force and fatigue recovers faster than pH levels after a strenuous workout (Sahlin, K and Ren, JM). This means that non-fatigued muscles have at times reduced pH levels, which obviously contradicts the idea of pH causing muscle fatigue.
  
This leaves the accumulation of K+ ions in the interstitial fluid as being a key cause of fatigue, by inhibiting the propagation of action potentials along the cell membrane.

This K+ accumulation depolarizes the fiber, preventing Na+ recovery, which causes a reduction of action potentials and reduction in force production (Bandschapp, Soule, Iaizzo). 

In a recent study done by Bandschapp, Soule, & Iaizzo, they wanted to test findings to see if they found similar results.  So they did a test on muscle biopsy on live pigs (muscle very similar to humans).  They first added K+ to these muscle biopsy's and then added lactic acid to test the findings.

The researchers found that when K+ was added to the muscle solution, muscle force/power decreased, but when lactic acid was then added, muscle force/power was restored.  The exact reason for this is not completely known, but a possibility explanation is that lactic acid helps in decreasing the permeability of chloride ions, which restores the excitability of the cell membrane (Bandschapp, Soule and Iaizzo).
    
The exact cause of muscular fatigue and loss of power due to intense exercise is unknown.  There are many factors that contribute, like metabolic factors, genetics, nutrition, peripheral muscular factors, central nervous system, temperature, oxidative factors, elevation, sleep, etc.

But it is becoming more and more evident that lactic acid is not a major cause of fatigue, loss of force, and might actually have the opposite effect on muscles (Pedersen TH, Nielsen OB, Lamb GD, and Stephenson DG). 

More research has to be done, but it looks like K+ is a big cause of fatigue, loss of power, and we might need to shift focus on how to reduce the accumulation of K+ and how to recover from this accumulation. 

Exercise Science is always shifting and what was thought as being a truth yesterday could be proven wrong tomorrow.  I don't know if this research will hold up, or if future studies will confirm or deny some of the work done by these scientist.  But I do know that the further we look at lactic acid, the less and less we are seeing it have the effects that most think of. 

I can't say for certain what goes on in a fatigued muscle, and really no knows for exact certain what causes that fatigued, hit the wall feeling.  But I think these studies need to be held up to brighter light, as lactic acid is about as big a "myth" out there as anything. 

And as always hope you enjoyed and Go Get 'Em!


References

            Sahlin K and Ren JM. Relationship of contraction capacity to metabolic changes during recovery from a fatiguing contraction. J Appl Physiolgy 67:648-654, 1989.
            Allen DG, Lännergren J, Westerblad H. Muscle cell function during prolonged activity: cellular mechanisms of fatigue. Exp Physiol 80: 497–527, 1995
             Posterino GS, Dutka TL, and Lamb GD. L(+)-lactate does not affect twitch and tetanic responses in mechanically skinned mammalian muscle fibres. Pflügers Arch 442: 197–203, 2001
           Bandschapp, Soule and Iaizzo.  Lactic acid restores skeletal muscle force in an in vitro fatigue model: are voltage--‐gated chloride channels involved? American Journal of Physiology –Cell Physiology, 2012
            Nielsen OB, de Paoli F, and Overgaard K. Protective effects of lactic acid on force production in rat skeletal muscle. J Physiol 536: 161–166, 2001.
            Pedersen TH, Nielsen OB, Lamb GD, and Stephenson DG. Intracellular acidosis enhances the excitability of working muscle. Science 305: 1144–1147, 2004.
            Posterino GS, Dutka TL, and Lamb GD. L(+)-lactate does not affect twitch and tetanic responses in mechanically skinned mammalian muscle fibres. Pflügers Arch 442: 197–203, 2001.
            Spangenburg EE, Ward CW, and Williams JH. Effects of lactate on force production by mouse EDL muscle: implications for the development of fatigue. Can J Physiol Pharmacol 76: 642–648, 1998.
               Juel, C., Pilegaard, H., Nielsen, J.J. & Bandsbo, J. (2000). Interstitial K+ in human skeletal muscle during and after dynamic graded exercise determined by microdialysis. American Journal of Phusiology 278, R400-406, PubMed         



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