PR Playbook: Understanding Fatigue
It was a 50k race and I was trailing the 1st and 2nd runner. They both had inched out of my sight and assumed a notable lead as I navigated the trails headed back toward Lake Hodges. When I hit mile 27, right after Raptor Ridge, I twisted around a sharp corner and saw the grasslands opening up before me. To my pleasant surprise, I spotted the second-place runner. He was walking and, clearly, dealing with the uncomfortable pain an endurance event can bring to both the body and the mind. As I ran up to him, I asked him if he was okay. He told me he was “done,” “cooked”, he told me not to worry, “I’ll make it off the course, go chase first place!”
If you’ve been racing endurance events for any amount of time, you’re probably well acquainted with fatigue, that sensation of your muscles being done. I remember running the California International Marathon many years prior to the Lake Hodges race — when I felt that same exhaustion as the second-place runner walking on the side of the course. I remember feeling like my legs were lead, as I walked on the side of the course, wearing a Boston Marathon shirt. As I struggled to keep moving, I was passed by so many runners and one runner yelled at me “hey Boston why are you walking!?”. I thoughts about that moment as I passed the second-place runner at Lake Hodges. I knew exactly how he was feeling. I had been there. That day at the California International Marathon was my epiphany. After that day I dedicated my life to developing the PR Playbook.
What Is Muscle Fatigue?
We typically use the term fatigue to describe general sensations of tiredness and the accompanying decreasing muscular performance. In endurance events we also “hit the wall” and cannot run any farther or harder; we can no longer produce the “optimal” output of muscular contractions as we had done at the beginning of the endurance event.
Fatigue is a very complex phenomenon in which multiple sites fail during muscular work. The underlying causes of fatigue fall into one of two categories: central (neuromuscular, the mind/central nervous system) and local (peripheral, the actual muscles themselves).
The central nervous system (CNS) acts much like an automobile engine regulator. Most cars are made with a regulator that causes them to “shut down” when the engine revs too high for too long. This mechanism protects the engine from “over-heating.” In the same way, our brains attempt to protect our muscles from tearing by reducing the rate nerve impulses are sent to our working muscles. In most cases, you’ll experience central fatigue before local fatigue. In other words, when you think you simply can’t do any more work because you’re so fatigued, essentially what’s happening is your mind is telling your body to shut down. But in fact, your muscles are probably able to continue for much longer.
Local fatigue is related to local factors that limit the ability to perform muscular work. These include three the energy systems ATP-CP, anaerobic (glycolysis), and aerobic (with oxygen). The energy systems act much the same way as fuel in a car or a battery in a flashlight but are called upon at different times depending on the demands of your physical activity.
The first energy system is called the ATP-CP system and is called upon during extremely short and intense bouts of exercise, like weight training, sprinting, and jumping. It works by repeatedly breaking down ATP, the basic currency of energy in the body and rebuilding ATP using CP (creatine phosphate). During repeated maximal contractions, fatigue coincides with CP depletion. As an athlete, you need to combat ATP-CP fatigue if you’re going to speed up a hill — or sprint to the finish line to pass that other age grouper you’ve been chasing. This energy system also comes into play during quick changes of direction such as jumping out of the way of an obstacle on a trail or avoiding a collision and potential fall.
The other two energy systems are called into play during exercises that last longer than 30 seconds. Known as anaerobic (or glycolytic) and aerobic (or oxidative), these energy systems are very dependent on the availability of glycogen (the stored form of glucose sugar). As with CP use, the rate of glycogen depletion is controlled by the intensity (i.e., how hard you run, cycle or swim) of the exercise. These are the systems you use through most of the duration of your endurance racing.
Fatigue of the anaerobic system is often caused by the production of metabolic by products such as lactic acid and CO2. As these accumulate in our bodies, our ability to maintain the duration and intensity of exercise diminishes. And, when they finally reach a point of saturation, our muscle capacity comes to a screeching halt. This is often referred to as the “burn,” whereby the muscle feels like it’s on fire signaling you to stop. In contrast, fatigue of the aerobic system is often linked to reduced glycogen. Long-distance runners often speak of “bonking” or “hitting the wall” a few hours into a race or hard training run. This refers to a perceived fatigue usually related to glycogen depletion. At this point, the body begins to use other forms of energy, such as fat and protein, which are not as efficient sources, thus making it harder to sustain energy levels.
Got all that? I realize it sounds like some fairly complex processes, which they are. But, as endurance athletes, when we are aiming to delay fatigue, we need to address fatigue at each of these levels: central fatigue in the CNS, and local fatigue due to poor fueling of our ATP-CP, anaerobic and aerobic systems.
In my next article I will discuss how to delay fatigue. In the meantime, you can catch up on the latest of my coaching tips by joining my Private Group For Endurance Athletes or message me for more information on the PR Playbook.