The Tweet: “Human muscles are limited by our brain – we actually have the strength to move cars and boulders.”

Stories of mothers who were able to lift cars to save their trapped children, passersby summoning super-human strength to come to the rescue of strangers and climbers releasing limbs caught under huge boulders sound like stories from superhero films. But anecdotes of incredible feats of human strength can be supported by research.

Under stressful conditions, our bodies are able to perform at a higher intensity as our brains signal the release of chemicals that give us a competitive edge. This is what makes normal people momentarily very strong, and how weightlifters are able to set new records.

Having studied the biomechanics of weightlifting, Vladimir Zatsiorsky, a professor of kinesiology at Penn State University, highlights what he calls “absolute strength” and “maximal strength.” Absolute strength is the theoretical force that our muscles are able to apply, and maximal strength is the maximum force that our muscles can generate through conscious exertion. So the limit on how much we can lift is dictated not solely by the strength of our muscles, but by our psychology and chemistry. Therefore, our maximal strength is always less than our absolute strength. However, athletes can exploit a greater amount of their maximal strength through training, and competitive conditions can increase this effect even further.

The average person, according to Zatsiorsky, is able to use about 65% of their absolute strength, compared to a trained weightlifter who can exceed 80%. Under competition conditions, athletes can push this limit even further – up to 92%. Zatsiorsky calls this “competitive maximum strength.” The more intense the feeling of competition, the greater the effect.

The added effect of competition can be explained by the body’s response to stress. In stressful situations our body prepares us for fight or flight by pumping cortisol and adrenaline from the adrenal gland into the blood stream. Cortisol is a steroid that increases blood glucose production and when combined with adrenaline they act to raise blood pressure and heart rate to deliver more oxygen and energy to the muscles.

The effect of competitive maximum strength goes some way to explain why so many records continue to be broken at major athletic events. For women, 11 of the 21 Olympic records in weightlifting were broken at the London 2012 Games, 8 of the 24 Olympic records were broken for men, although modern records only go back as far as 2000 when the weight categories were last adjusted.

Our brains limit how much weight we are able to lift on a daily basis in order to protect us. Bodybuilders who use steroids to achieve their muscly physique do so because the drug inhibits their reception of pain, allowing them to train harder and for longer, and getting closer to their maximum limit. However, steroid users also report higher incidences of injuries, particularly ligament damage. Ligaments anchor muscle to bone, and they themselves have a maximum load. So, by preventing us from trying to lift anything beyond our capabilities we protect ourselves from injury, reserving superhuman feats of strength for fight or flight moments.

Moving cars to save trapped children is therefore not impossible and can be explained by the biological survival technique we evolved to fight competitors and run away from predators. In many of the stories it is also worth noting that the hero only needed to move the car a fraction, a few inches at one end, certainly much less than the total weight of the vehicle and well within the realms of possibility for a reasonably strong and healthy adult.

Image: Mads Klokov Thøgersø/ Flickr