By now, most of us know what it feels like after going through a really tough, high intensity workout. If you train right, you should be quite familiar with experiencing fatigue. Fatigue is the failure to maintain the required or expected force due to muscular activity.
Many of us know this as failure during a set. However, the majority of people don't really know the causes of fatigue and why it occurs. The answer is not limited to one reason or one cause. There are many reasons for fatigue, and many causes as well.
Causes Of Fatigue
The causes of fatigue depend on what exactly you are doing. If it is something like sprinting, fatigue will primarily involve bioenergetics (phosphocreatine shuttle, glycolysis, or oxidative metabolism), the rate at which you can metabolize carbohydrates or the rate of phosphocreatine breakdown in your body. There is a point here to take into consideration when speaking of fatigue.Fatigue results due to metabolic reasons, not structural alterations (referring to muscle structure). If the exercise is long duration (such as weight training), fatigue may be more heavily dependent on how strong you are mentally, and less dependent on physical strength.
That brings me to my next point. Fatigue is not limited to mechanics or bioenergetics; it also includes the psyche (or brain). The psyche refers to things such as motivation or dedication (ie: central drive).
I think that the majority of fatigue can be contributed to the central drive of a person when doing work. Also, I think that with weightlifting, this is especially true.I have seen so many people in the gym quit a set before reaching their actual point of failure many more times than once. The same person will get up from a set and claim that they had nothing else left for that set, even when their spotter wasn't helping them out.
Nine Possible Sites Of Fatigue
Recently in exercise physiology, nine possible sites of fatigue have been determined. These sites include the psyche, spinal cord, peripheral nerve, muscle sarcolemma, transverse tubular system, calcium release, actin-myosin interaction, cross-bridge tension and heat and force/power output.
Of these nine sites, there are really only a couple of sites that are the main focus. As I said before, the psyche is one of the main sites of fatigue.
Another major site is in the actin-myosin cross-bridges.
Actin is a thin muscle contractile protein filament to which myosin attaches. Those filaments provide mechanical support for the cell, determine the cell shape, enable cell movements, and participate in certain cell junctions.
ATP, Lactic Acid and Energy
Explaining this can get rather complicated, so pay attention! ATP is needed for both the activation of cross-bridges to cause muscular contraction, and the dissociation of actin and myosin to cause relaxation. Calcium is needed in order to expose the active sites on actin so that myosin can bind to actin (for contraction).
When you workout, you produce lactic acid. Since this is an acid, the more lactic acid you produce, the more hydrogen ion is released. This increased hydrogen ion level can cause a reduction in the force per cross-bridge, a reduction in the force generated at a given calcium concentration, and block in the release of calcium.
Also, when you contract muscles in a workout, you use up more energy than you can produce. As a result, inorganic phosphate levels begin to rise. This high level of inorganic phosphate acts directly on the cross-bridges to reduce its binding to actin.
Actual scientific studies have also shown that high levels of inorganic phosphate inhibit maximal force and the more inorganic phosphate, the lower the force produced during recovery from fatigue! From this information, it seems that the most important sites of fatigue concern the psyche, actin, myosin, ATP and calcium.
Type II Muscle Fibers
As far as lactic acid is concerned, the reason that you produce more of it with higher intensity is because of a term called the "principle of orderly recruitment." Most people have heard the term fast-twitch and slow-twitch fibers. There are two types of fast-twitch muscle (Type IIa and Type IIb) and one type of slow-twitch muscle (Type I).
Type II fibers are predominately used in anaerobic exercises (weightlifting), and Type I fibers are used in aerobic exercises (marathons). When you begin any muscle contraction, whether it is in an anaerobic or aerobic exercise, you always recruit fibers for use in the same pattern. You always recruit Type I fibers first and then, as you increase the intensity or workload, you recruit Type II fibers.
As a result of increasing intensity, you increase Type II fibers, and this causes an increase in lactic acid levels because these fibers are used in anaerobic exercises. So, there is your explanation for increased lactic acid levels, the effects of which can be found in the previous paragraph.
All of this scientific jargon can become dense, but if you are a personal trainer talking about fatigue to a fellow colleague or client, you better know what you are talking about!
Now, knowing all of this, you're probably asking, "OK, how can I prevent fatigue so I can go longer and harder?" For most people, this can be answered by telling the person to try a little harder, and deal with the pain a little better. However, for those who have it together upstairs, it is important to not prevent the chemical and metabolic aspect of fatigue.
This is because fatigue acts as a warning to your body that if it goes any harder, muscle injury will occur. So, fatigue prevents muscle injury. This is why people who take pain killers before a workout are at a higher risk of muscle injury. If you can't feel the pain, or your perception of the pain is dampened, you are at a higher risk for muscle injury.
However, you can slightly speed up some aspects of the recovery time from fatigue. You recover from fatigue by the restoration of phosphocreatine levels, removal of hydrogen ion and lactic acid, and the breakdown of calcium bound to phosphate. The first way happens quickly, so you can't do much to speed up that process.
The second way is pretty much an absolute and is dependent mostly upon the rates of bioenergetic pathways in your body. The last aspect of recovery can take up to 6-10 hours. However, the better trained you are, the faster you will recover.
Remember, hardcore lifters, fatigue is necessary for preventing injury, so don't try to prevent it with painkillers. However, if you are a half-ass lifter, this point does not apply to you!