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Why you get tired when you run

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Understanding running fatigue. Dr Simon Sostaric writes.

Can’t quite put your finger on why you have capitulated in the final 10km of your last marathon, or perhaps struggling to drag yourself out of bed for a run? 

Every runner has a story to tell. But everything happens for a reason, and understanding your body’s responses will enlighten your approach to interpreting and acting on your experiences. 

Reading the signs: what to expect with acute fatigue

The magnitude and type of fatigue experienced by runners varies, and is a matter of perspective.

Distance runners are exposed to muscle electrolyte shifts; diminished muscle and nerve cell activity; glycogen depletion; fluid loss and rise in body temperature – to name a few. Tiredness and leg soreness at the end of a run is associated with an accumulative effect of these processes, which is expected. 

Unaccustomed long or intense running may lead to delayed onset of muscle sores (DOMS), usually persisting for several days. This is also normal, and your body will adapt with the right balance of training and recovery. 

Sweat rates also vary considerably amongst runners, and can be deceptive. Early sweating, thus early cooling, is an important characteristic for minimising the rise in body temperature.  After a long run like a marathon, a 2-3% body weight deficit is ideal.  

Accumulative effects of prolonged fatigue

Strategically extending yourself in training is a good way to develop resilience, confidence and understand your limits. Short term performances may suffer for it, but you’ll improve in the long run. 

However, pushing the boundaries too often, with incomplete recovery, may contribute to overtraining syndrome (OTS), which adversely affects health and performance for months or more. OTS symptoms include some or all of the following;
  • Diminished running performance in training and competition for more than a few weeks
  • Lethargy and tiredness
  • Persistent illnesses
  • Irritability
  • Loss of appetite
  • Loss of interest in training and other normally enjoyable pastimes
  • Poor quality sleep
In a clinical setting, OTS is typically characterised by a decrease in VO2max, peak heart rate and peak blood lactate; increased levels of circulating inflammation and proteins specific to muscle damage (detected in a blood test).

The devil is in the detail – 6 tips to better manage of fatigue

  • Take at least one rest day per week
  • Focus carefully on pacing strategies appropriate to the distance and conditions
  • Heart rate variability is a good early warning sign of maladaptation. If your basal heart rate (first thing on waking) is elevated by 4-5 beats for more than 3 consecutive days, lighten your training load or take an unscheduled day or two off
  • Physiology evaluations when you are healthy provides a reference point, and better enables training methods specific to your needs
  • Drink fluids to thirst. Our bodies do not respond well to “fluid loading”, which can cause lethargy, confusion, and dilution of electrolytes; particularly hyponatremia
  • Heat training can help your performance in warm and cool conditions. Seek specialist advice

Dr Simon Sostaric is the founder of Melbourne Sports & Allied Health Clinic  and also consults in Sydney at the Inner West Allied Health & Specialist Centre