The limits of human endurance
We all have that one friend that likes to do triathlons or other crazy feats for fun. They seem superhuman, able to achieve the impossible. How are our bodies able to sustain situations of such high endurance?
The Physical Limits
On a physiological level, our body has certain limits that constrain performance. One of these is VO2 max, which is the limit of aerobic (oxygen requiring) exercise. When we reach this point, the mitochondria can’t convert glucose and oxygen to ATP fast enough to meet demand, and we won’t last much longer. There is a biological, physical limit. Elite marathoners can remain at the very edge of hitting their VO2 max for over two hours: if you increase the speed by just 5%, the time they can maintain it becomes halved.
Our body also switches the type of fuel it’s using depending on intensity. When resting, we burn fat because it takes longer to process, which isn’t a problem when energy expenditure is low. But as we do more work, we need fuel faster, so we start breaking down glucose from our bloodstream and glycogen stores, because it’s quicker to burn than fat. This is why before big races, loading up on carbs can be common practice.
But as endurance durations start lasting days, weeks, or even months, there is also a very clear metabolic limit. The longer the endurance event, the lower our energy expenditure becomes, because our bodies adapt to the increase in intensity by decreasing energy spent on nonessential processes. Based on duration, it one end of the spectrum could be an ironman triathlon, taking less than a day; at the other, pregnancy, lasting nine months.
In both cases, and everything in between, the body can only absorb between 4,000 to 5,000 kcal per day. Studies have shown that regardless of the circumstance, energy intake is at maximum around 2.5 times BMR (basal metabolic rate, or the calories needed to perform basic functions for survival). This is true for overeating studies as well as athletic endurance events, where any energy expenditure above 2.5 times BMR level pulls from fat stores. For sustainable, durable endurance, maintaining body weight is vital, so daily energy expenditure has to be kept below the limit.
The Mental Limits
That being said, the point at which we stop is not only a case of running out of fuel. Our brains are processing and coordinating information about muscle fatigue, perceived time remaining, task difficulty and more. It’s no secret that the harder you work, the less time you can do it. But when you finally can’t do it anymore, it’s your brain that shuts you down.
When a runner collapses after a long race, it’s not uncommon for them to get right back up a few minutes later and do a victory jog. During a marathon, runners still find the energy to go even faster for the last mile. This shows that even if we’re exhausted, and feel like we’ve reached our absolute limit, we haven’t completely spent our energy reserves. There’s still plenty of ATP in our muscles, and glucose and fatty acids floating around our bloodstream, that we can use for fuel.
Athletes and coaches know the importance of being in the right mindset to win — desperation and determination unlock new levels of ability. So just by plainly refusing to stop, you can achieve much higher levels of endurance than you believed possible.