Fatigue alters the way we run. Transition from mile one to mile 26 of a marathon and there’s no hiding the stark contrast in how our movement transforms. The presence of pain (injury or lactic acid) and fatigue causes us to limp, drag, and claw our way to the finish line. The clawing may not be the most economical movement from point A to B, but when systems begin to fail the body finds a way. Sure, that may be fine for race day, after all you’re throwing all your eggs into one race basket. But what about training? What happens when those of us grinding through our 9-5, fueled off coffee and will power, begin to log countless miles on legs that are simply fatigued?
Our muscles intend to control for the forces associated with running. Our neuromuscular system catches us as we enter the ground, store landing forces, and redistribute said forces back into the ground. No matter the level at which you compete, fatigue is always a limiting factor—it’s true for all sports.
Again, fatigue is inevitable during racing, but the chronic fatigue of training (or racing too often) is what I see all too often in the clinic. Missed PR’s result in desperate attempts for another shot. “I’ve always 60 mile weeks for 10+ years,” litters the mouths of master runners. Or simply, “I like to race, it’s social for me and I love it.” And although you may get away with it for a season or two, it all comes full circle. I’m not asking you to not run when you’re legs feel tired, but simply to become more aware.
A study in 2001 found that upon fatigue of the dorsiflexors (front of shin muscle), the rate of impact loading significantly decreased.1 You may see the word impact paired with decreased and think, “oh, good!” but don’t be fooled. The rate of loading decreased. Ideally, we want a longer loading rate as it gives our body time to shock absorb. Shorter loading rates mean the same force, but in a smaller amount of time. Similarly, a 1998 used an accelerometer to find increasing acceleration of the lower leg as fatigue began to increase.2 Increasing acceleration translates to decreased control, likely increasing your risk for injury. This pertains specifically to lower body injuries. A retrospective study found that 50% of stress fractures are found in the bottom third of the tibia3 (shin)—the same area where the study conducted in 1998 placed their accelerometer.
You don’t have to ask too many runners before you find one with injury woes below the knee. The leg is really left out to dry, and as we see, particularly in the presence of fatigue. It all comes full circle to a few key principles: strengthen what’s weak, stretch what’s tight, and watch your training. If the fatigue increases loading rates, then don’t chalk a few bad runs up to “being tired”—rest. In a perfect world, we could all run, improve, and stay healthy. Unfortunately, our planet is far from perfect (pointing at the inhabitants—not the planet).
Remember, your muscles control your landing forces. So what happens when the muscles are tired and weak? The force of landing doesn’t change–just your ability to control it. Loads are transferred from muscle to tendons, ligaments, cartilage, and bone. Sore joints chronic tendon injuries, and inflamed soft tissue injures are just the start. Recovering from these injuries can be a nightmare. These chronic fatigue injuries can accrue over years, while taking months to heal. Be smart and take a cross training day when needed. It’s completely impossible to avoid all running injuries, but you can certainly steer their frequency.
1. Christina K, White S, Gilchrist L. Effect of localized muscle fatigue on vertical ground reaction forces and ankle joint motion during running. Hum Movement Sci. 2001;20:257-276.
2. Verbitsky, O., Mizrahi, J., Voloshin, A., Treiger, U., & Isakov, E. Shock transmission and fatigue in human running. J APPL BIOMECH. 1998;14:300-311.
3. Monteleone, G. P. Stress fractures in the athlete. Sports Med. 1995;26:423–432.