By: Barbell Logic Team

When you train hard enough—when the stress of training is high enough—you are signaling the need for change to your body. “Feel that” you are saying as you grind out the fifteenth rep of a hard three-by-five, sets-by-reps, of squats, “that’s our new normal.” In this article, we talk about how to recognize fatigue from productive stress outside of a lab, in your gym or resting at home after a hard workout.

How Much is Enough? Identifying Fatigue and Positive Indicators of Training Stress

We love the feeling you get from finishing a DIY project. That moment when you can sit back and admire your handiwork, there’s a deep-down kind of satisfaction that comes from knowing you just did something worthwhile and lasting. It’s no accident that a good workout can help you feel the same way. There is something satisfying about a workout that leaves you fatigued but not crushed, broken, or excessively sore, when you know you did something very hard and that you will come out better for it.

That satisfaction is the emotional side of a physical principle. What makes the training session worthwhile is that it moves you closer to your goal. That means your training has to be sufficiently stressful to cause an adaptive response.

The idea is that when you train hard enough—when the stress of training is high enough—you are signaling the need for change to your body. “Feel that” you are saying as you grind out the fifteenth rep of a hard three-by-five, sets-by-reps, of squats, “that’s our new normal.” Your body gets the message through numerous sensory receptors that this nature and magnitude of stress is something that it needs to adapt to and plan to encounter in the future. You haven’t exceeded the abilities of your bodily systems, but you’ve tested them and pushed them beyond their normal levels of function. When this happens a lot of internal processes kick off that will make you stronger when they run their course.

So, you need to plan to recover from training, but how can you tell whether training stress was great enough? There is such a thing as not enough stress. By the stress-recovery-adaptation framework, if you trained at levels of insufficient stress, you would be wasting your time. Indeed, the correct magnitude and specific nature of stress is the dividing requirement between training and exercise. In a laboratory setting, with blood tests, muscle biopsies, and neuromuscular measuring equipment, you could test for indicators of sufficient stress, acute fatigue from the training that indicates an increased response from your system. But outside of a lab, in your gym or resting at home after a hard workout, you want to know whether you trained hard enough or whether you were sandbagging or wasting your time. That comes down to recognizing fatigue from productive stress.

Fatigue

When you lift weights you are causing stress to your body. There are several possible outcomes from that stress depending on its nature and magnitude. If we think about these outcomes on a spectrum, you have, on the shallow end, non-productive stress. Anything you do that does not contribute to a physiological change, particularly of the form that you desire, is non-productive stress. If, for example, you walk 10,000 steps every day, continuing to do so may maintain the fitness necessary to do that task, but the repeated task will not cause improvement. Similarly, if you train at the same intensity and volume over and over again, then you will adapt to that level of stress but not more. To cause further change, you will need some new kind of stress.

This stress-response spectrum plots change. When you introduce a new stress, then change follows even if the new stress is low-level and not specific to the change. For continued change, however, the stress from training will need to increase. What starts as relatively low- non-specific stress gradually increases and becomes more specific. Said another way, novices adapt to lower levels of stress and to less-specific stresses than advanced lifters do.

At the far end of the spectrum, you have the possibility of overtraining. Overtraining is the accumulation of fatigue without the chance or resources for recovery to the point at which the physiological response is more of a breakdown than a building up. “The primary diagnostic indicator is a reduction in performance capacity that doesn’t improve with an amount of rest that would normally result in recovery.” (Practical Programming for Strength Training 3d ed). Overtraining requires both the capacity to induce this high level of stress and sufficient accumulation of fatigue that overwhelms the recovery capacity of the system. The result is a state of fatigue that you cannot recover from in the usual amount of time that you be able to otherwise.

Somewhere between non-productive stress and overtraining lies the Goldilocks zone of training stress, beyond the minimum threshold of stress and lower than the risk of overtraining. This target zone of training we can think of as productive stress.

By definition, productive stress causes fatigue. Fatigue is a non-specific indicator that we often recognize simply as “tiredness.” Tiredness may be the result of acute fatigue, related to a single training session, or it may be residual fatigue that comes from multiple sessions. Either way, you get weaker as a result of productive training, at least for a little bit.

More than tiredness, fatigue is a physiological decrease in performance, a short-term drop in your ability to produce through voluntary muscular contractions. Fatigue can occur anywhere along the chain of events that are involved in muscular contractions. This chain of events, what we often call a “motor pathway,” starts with the central nervous system (CNS), includes the transmission of electrical impulses, chemical reactions, and mechanical contraction of muscle fibers.

When we talk about fatigue, we break this chain of events down into two pieces. Fatigue related motor impairment that originates in the CNS is known as central fatigue. Fatigue originating at the junction between a motor neuron and a muscle fiber or farther down the motor pathway is peripheral fatigue.

Peripheral fatigue may include depleted energy stores or muscle impairment due to damage to the fibers. When you lift, you drain your glycogen, phosphocreatine, and ATP stores. A reduction in available energy means that your muscles cannot perform at the same pre-workout levels—depleted energy results in the reduced function of your skeletal muscles. Peripheral fatigue may also be a muscular response to accumulated byproducts from muscle activities, impairment of the muscle contractile mechanism due to exercise-induced muscle damage or immunological or genetic responses to the training stress.

Studies tend to focus on peripheral fatigue because it is more easily measured in a laboratory setting. However, not all measures account for all of the loss of force production. This grey zone is where central fatigue comes in.

Central fatigue gets the blame for fatigue that is not easily measured. Your central nervous system integrates input from various sources of the body. It acts like a governor on a car engine, limiting muscle contraction and force production. One hypothesis views this as a protective mechanism that prevents premature or excessive peripheral fatigue—the muscle wisdom hypothesis. The existence of peripheral fatigue, combined with the sensory input transmitted to your CNS, signals the need for a down-regulation of your ability to produce force as a way to prevent excessive damage to your muscle cells. You experience a decrease in performance in the same way you would if as a result of depleted energy stores.

That’s what is going on internally—or at least a simplified picture of some complicated processes. But, other than tiredness there are some things you can look for in your training life that can help you tell when you’ve provoked enough fatigue in the gym to be satisfied with your hard day’s work.

Indicators of Training Stress

We want to induce fatigue. Acute fatigue is the natural result of simple, hard, effective training, and residual fatigue is increasingly a part of training—more advanced lifters rely on accumulated training stress rather than workout-to-workout fatigue. First, there is no easy, bright-line indicator that you have trained hard enough today. The best indicators of adequate training stress come from your training log. Consistent tracking of data is the best way to know how much will be enough and what kinds of training will fall below your level of current adaptation.

Even if there is no simple indicator (just feeling tired might be an indicator of training stress or it might just be that you had a tough day at work), there are several factors that you can track that can help you know whether you are in a fatigued state.

Coaches and athletes will sometimes track subjective markers of fatigue as indicators of training stress. (Bompa and Haff “Periodization: Theory and Methodology of Training” 5th ed.) Some of the most common measures are the quality and duration of your sleep, your sensation of tiredness, willingness to train, appetite, and muscle soreness.

Sleep. Good training and recovery practices will usually lead to an increased sensation of tiredness but also an improvement in the quality and duration of your sleep. Too much training and you can descend toward “painful tiredness” and poor sleep. Not enough training stress and these do not move much past your baseline levels.

For an informative and entertaining look at sleep hygiene listen to this recent episode of the Barbell Logic Podcast.

Willingness to Train. A short-term drop in your willingness to train often accompanies fatigue. This should immediately precede a return to your normal enthusiasm for training, however. As we’ve talked about with signs of overtraining, apathy toward training is very often an indicator of too much stress (whether in the gym or outside of it).

Appetite. Productive training stress tends to make you hungry. A “good” or baseline appetite should exist prior to training. Productive training stress will then typically kick your energy homeostasis into overdrive and you will begin food-seeking behavior. So, an increase in appetite is a typical sign of productive training stress. Too much stress will usually have the opposite effect. A loss of appetite is a potential indicator of residual fatigue that is not being dissipated sufficiently.

Muscle Soreness. Soreness comes from exercise-induced muscle damage. Eccentric or lengthening contractions tend to produce the greatest amounts of muscle damage and, as a result, soreness. Muscle damage and soreness, however, decrease dramatically after just a single training session. When you take time off from training you might get sore after your first workout back, but after that, soreness should not be a regular part of your recovery process. This is known as the “repeated bout effect.” “The repeated bout effect refers to the protection of attenuation in muscle damage markers observed following a second bout of exercise. It is well known that a damaging bout of exercise through eccentric actions will result in a protective effect in subsequent repeated bouts.” (Exercise-Induced Muscle Damage).

Soreness due to muscle damage is an indicator of eccentric loading and exercise not subject to the repeated bout effect—usually some stimulus that is new to you. It is not an indicator of productive training stress. Acute fatigue from training may bring with it mild muscle soreness, but for someone who is training consistently, you should not experience much soreness from normal levels of training. Soreness tends to occur after a layoff. Extreme muscle soreness in a trained individual as the result of training may be cause for concern. And, as Rip has argued, chronic soreness from exercise is not only a poor indicator of productive stress it is a poor practice in some exercise circles. (Read more here.)

As with so many other training issues, pay attention to how you respond to training. Take note of any changes in your habits or your responses to training. And rest and recover with the satisfaction that every time you are breaking yourself down a little bit with weight training, you are building something greater: A better, stronger you.


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