Anyone who has ever competed knows the feeling of muscle fatigue. You want to keep going at the same pace but can’t. The mind is willing but the body is weak. Power output cannot be maintained. Even inactive people know the feeling after some hasty yard work. While a certain level of muscle fatigue is normal with physical activity, too much can impair training, reduce health and ruin your race.
When a single muscle is evaluated, fatigue can be measured as a reduction of power. In the case of the whole body, it is a slowing down of pace during running, cycling, swimming or other activity, all while the heart rate remains the same, or is elevating. Managing muscle fatigue is an important aspect of effective training, one that contributes to improved performance. The process begins by understanding the normal, abnormal and the origins of fatigue.
Normal and Abnormal
In the natural state, mild and moderate muscle fatigue is normal and even necessary. Proper training means working the body and its muscles a bit harder than usual—a state called overreaching—then allowing adequate recovery. When properly done, the body learns to accomplish more work with the same effort. In other words, we get faster at the same heart rate. (When this does not happen, it means there is some imbalance preventing progress.)
While normal, healthy fatigue encourages muscles to better adapt and prepare for more power, there are other benefits. Included is improved heart, circulation, and lung function, enhanced regulation of oxygen and carbon dioxide, better utilization of muscle energy, and optimal metabolism of lactate. The most important end result is a better brain, one that can best manage all the body systems leading to greater athletic performance.
When the delicate balance of work and rest is disturbed, normal fatigue turns abnormal and problems arise. Too much intensity or higher volume training can cause undue, excessive muscle fatigue because we go outside overreaching, into that zone where muscles are damaged. While this occurs during and after a race, making proper recovery a necessity, it can happen more regularly with imprecise workouts, or following a program not specifically designed for our particular needs.
Working out this way presents several problems. We build a “debt” of muscle fatigue, whereby training exceeds the body’s ability to properly recover. It can eventually lead to injury or ill health, although just one fatiguing session that is damaging enough, especially without appropriate recovery, can impair health. As the problem escalates we may pay the penalty of poor performance on race day because muscle fatigue still exists. The same dilemma occurs when racing is too frequent, there is inadequate taper, recovery is too short, or a combination of factors.
These two types of muscle fatigue—normal/healthy, and abnormal/harmful—are best exemplified in strength training. When performing most weight programs of several sets of high repetitions, muscles are isolated and worked to the point of failure, which means significant fatigue. The muscle can no longer lift the weight. This is traumatic, and the body’s response is to produce stress hormones. The need for recovery is significantly increased—something many people don’t obtain. The next workout, whether easy or hard, typically starts before one is properly recovered. What is the alternative?
A healthy option is to perform less repetitions and sets, and avoid excess fatigue. A single set of strength training, for example, is as effective as three sets for increasing strength. Because muscles are not significantly damaged, added benefits include faster recovery, the strengthening of bone, stronger muscles without bulking up, more fat burning, and no impairment of aerobic function.
Excess fatigue is often glorified as part of the “no pain, no gain” work ethic in today’s social sports world. While some athletes, such as those in track and field, power lifting and football, get closer to the line of overtraining and are injured more easily, endurance activities are much different. Running, biking, swimming and multisport events rely mostly on aerobic training, which, when properly done, does not produce significant muscle fatigue unless the volume of training increases too much. Despite this, improvements in performance should regularly occur—in other words, swimming, biking, and running at faster paces at the same heart rate during training and racing.
Muscle fatigue is also associated with other problems:
- Fatigue generated by an intense workout can increase stress hormones and potentially impair endurance.
- A muscle that is fatigued will not contract as many fibers, reducing power and increasing weakness.
- Muscle fatigue can impair associated joint movements (e.g., quadriceps and knee joint), and contribute to such chronic conditions as osteoarthritis.
- Poor posture and gait irregularity result from muscle fatigue. This problem can last many hours following training or racing.
- Working a fatigued muscle can lead to muscular imbalance and result in further damage to ligaments, tendons, joints, fascia or bone.
- A fatigue-producing anaerobic interval track workout, like a popular weight lifting session, requires significant recovery—at least 48 hours, often more, before working out again.
- Muscle fatigue can significantly reduce training and racing performance.
- During competition, varying ones speed can cause more muscle fatigue compared to racing at a steady-state pace.
- Respiratory muscles can often fatigue, sometimes more than leg muscles, and limit maximum exercise intensity and duration. (This is associated with the relationships between inhalation and exhalation, and gait and foot-strike.)
- Muscle fatigue can result in metabolic disturbances, including the over-accumulation of ammonia from protein breakdown. (Taking branched-chained amino acid—BCAA—supplements can worsen this condition.)
- Equilibrium can become disturbed with muscle fatigue. Poor balance can lead to inefficient movements and increase the risk of trauma (such as bike crash or twisted ankle).
- Muscle fatigue can worsen symptoms in those with chronic fatigue syndrome, fibromyalgia, and chronic pain.
- Anaerobic muscle fibers fatigue much more easily and quickly, while aerobic fibers are relatively fatigue-resistant.
Causes of Muscle Fatigue
While there may be many reasons for muscle fatigue, through the years a variety of trends have pinned the blame on the “usual suspects,” as if the answer to the age-old question has been found. These include cardiovascular limitations, oxygen deficits, lactate levels that change pH, diminished muscle energy, and others. Any or all of these common issues may contribute to muscle fatigue, but not as a primary factor.
Muscle fatigue is usually countered with energy issues. Our muscles use both fat and sugar (glucose) to create ATP, the ultimate source of energy for muscle contraction. Currently, the popular notion is that when ATP falls below the muscle’s ability to contract, we become exhausted. If local energy levels were the primary reason for muscle fatigue, it would just be a matter of gulping down enough sugar during a race to keep going. But as studies show, muscle energy usually remains more than adequate even after a long, intense effort. This is the reason most athletes have a “kick” at the end of a grueling race. Despite the fatigue that occurs during the event, there is almost always a seemingly newfound ability for the muscles to work much harder and quicken the pace in the final segment of an event.
Other popular notions about muscle fatigue have included limitations of the cardiovascular system—the lungs to bring in oxygen and the heart to circulate blood to the working muscles. Associated with this is the obsession of VO2max—a number athletes love to toss around despite serving no practical purpose.
Another culprit is said to be lactic acid, where changes in muscle and blood chemistry, a rise in pH, impair muscle function leading to fatigue. For decades, products proclaiming to “neutralize” these exercise-impairing metabolites are found in pop magazines and store shelves—they don’t work. As we now know, lactate is an important metabolite and, in fact, an important energy source.
These theories proclaiming the cause of muscle fatigue do not make sense if we just consider that, if the levels of oxygen or ATP falls below the level necessary for muscle contraction, or lactate levels rise too high, an athlete would not just slow down but stop performing, and even collapse since muscles would be unable to maintain a standing posture.
Fatigue can also be discussed from a standpoint of peripheral versus central nervous systems. In other words, what happens in an individual muscle versus the big picture? The latter refers to the brain’s control of muscles, a topic discussed in Part 2.
Avoiding excess muscle fatigue should be a key goal of training and being healthy. Recovering from it before a race can result in greater performances. But cunning public relations campaigns have brought us lines of nutritional supplements that supposedly gives us what a great diet won’t, pharmacologic compounds that cover symptoms of fatigue, and physical items such as bad shoes that supposed to give us energy, all aimed at an athlete’s feelings and emotions. And they are successful as millions of people will go to the extreme to improve performance. But there is a better, easier and cheaper way to control muscle fatigue as will be discussed in Part 2.