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If you haven't thought much about the concept of pain, you might assume it's a simple matter of an impact at the skin level (e.g. stubbing your toe) causing a pain signal to be sent to the brain, resulting in a negative experience.
But a few examples quickly indicate that pain is much more complex than that.
Phantom limb pain is a common condition felt by those who've lost a limb but still feel pain where the limb used to be.
The placebo effect can cause reduced pain in someone given a pill, even if the pill contains no pain-relieving chemicals.
An athlete might get a kick to the leg and barely feel it in a match, where it would have had him hobbling to work if it happened on his daily commute.
These examples, among many others, indicate that the brain not only is a signal-receiving machine for pain, but also plays a contributing role in how the pain is experienced.
Nociceptors are cells that sense tissue damage (at the skin for example), which send an electrical signal to cells in the spinal cord, which then pass an electrical signal onto the brain, alerting it of the tissue damage at the first region. So far, this sounds similar to the simple explanation outlined previously.
However, at that junction where the nociceptor cell passes the message to the spinal cord, other cells, such as those coming down from the brain itself, have the opportunity to give their input, either increasing the electrical signal to be passed on to the brain (increasing the perceived pain) or decreasing the signal (decreasing the perceived pain).
Put simply, the brain can have an effect on the pain signal, before the signal gets the chance to reach the brain.
The nature of that effect is partly dependant on your current external circumstances, what you're body is currently doing, your perception of, and attitude towards, what is happening, and even your mood.
This is illustrated by the fact that people who are depressed or even just high in negative emotion tend to perceive more pain from a given stimulus than other people do.
This may be useful information in the context of intense exercise, where occasionally battling through the pain of a burning chest or heavy legs during endurance exercise may be necessary to elicit the physical fitness adaptation you're aiming for.
Could developing a positive attitude, or smiling, or positive self-talk cause the brain to send signals to intercept that pain signal somewhat, decreasing your perception of pain, and helping you run for longer?
A study was done where runners experienced less physical exertion and lower heart rate on a run where they were asked to use "cognition reappraisal" (not necessarily viewing the experience more positively, but aiming to view it more dispassionately, almost as if you were a scientist observing your experience from the outside) vs. a run at the same speed without such instructions.
Anecdotal experience backs this up for me, with the addition of some positive self-talk also helping to decrease the physical exertion I've felt during the run.
I'm not aware of any extensive research showing this to be a consistent outcome, but based on what I've outlined above, it wouldn't be surprising.
Either way, it's probably worth trying. With a more positive approach, even if your performance doesn't improve in that session, you're likely to enjoy it more (or at least hate it less), potentially leading to increased engagement in upcoming training sessions, and the cumulative improvements that come with that.