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TL;DR

I'm wondering if there is any difference in the types of receptors that exist for muscles vs connective tissue. I ask because I compete in sports where I definitely strain/injure muscle, and I also occasionally strain/injure tendons, and the pain feels completely different.

Muscle pain is sharp, immediate and activates when I move the injured muscle, and makes the area sore to the touch (e.g. I try to massage it). As soon as I stop moving or touching the muscle the pain is gone.

Tendon pain is radiating, burning "toothache-like" throbbing pain that sets in and becomes inflammed about 2 or 3 minutes after I move the area that is injured and sustains for several minutes thereafter.


I'm an armwrestler (yes its a real thing :-) ). Before that I played racquetball very competitively, and before that I was a gym rat/wannabe powerlifter, and in high school I was an alpine ski racer. I've also played soccer, baseball, football and for a short while wrestled in junior high. I've been in the gym pretty much my whole life.

I can say that no sport inflicts as much unique pain on the body as armwrestling. With armwrestling, after a single practice, your arms are throbbing; we call it the "toothache pain" because it feels just like the stabbing, throbbing pain you get from a bad toothache. Except instead of just a single, localized area (like a single tooth), its a deep, burning throbbing that starts in your shoulders and goes all the way down to your wrists. On both arms. For about 2 hours. Then it takes about 7 - 10 days to heal and not flare up again, and then you can go back to practice again.

This is a much different type of pain than the typical muscle soreness you get from gym lifting or even intense powerlifting. With gym lifts, even if you go extremely hard (1 rep maxes or just a few reps at very heavy weight) you'll get DOMS (delayed onset muscle soreness) about 48 hours later, and you'll be very sore, but no throbbing burning aching pain. It's very different.

I'm looking to understand why.


After talking to some more educated armwrestlers in the community, the predominant theory is this:

  • Gym lifting, even lifting heavy, is still a positive exercise for your muscles. They contract, you lift the weight (quickly if its easy, slowly if its difficult), but your muscles are working and doing all the heavy lifting/work the entire time, throughout the entire range of motion for the exercise
  • Armwrestling, on the other hand, is mostly either a static or a negative exercise. If you're armwrestling someone who's near your level or capability, it may take up to a minute or longer to ping them. That means your arm and wrist aren't really moving all that much: they're static/isometric and holding on for dear life, all the while your muscles are under 100% maximum strain. Watch an armwrestling video like this one and you'll understand why! Or, if you're going up against someone who's better than you, its a "negative" movement: your arm and wrist will fail against them as they pull your arm towards the pad, despite your muscles again working at 100% maximum effort.
    • In all cases, with armwrestling, your muscles are at 100%. They can't do any more work. They cannot contract, they're simply either holding in a locked position (static) or they are slowly failing as you begin to lose (negative).
  • The theory is that this is when your connective tissue (primarily tendons) takes over. Tendons hold muscle to bone. If a force is being exerted on the bone and the muscle is already at 100%, the force will start being applied moreso to the tendons that are no desperately trying to hold that muscle onto the bone and to not detach. This places enormous stress on the tendons, causing them to stretch and tear, just like muscles do when you lift weights at the gym like a normal human being.
  • As such the theory is that armwrestling aggravates both muscle and tendons. Since no other sport is really like this, none of us walk into the sport with tendons that are well-conditioned for it.
  • Tendons take longer to health then muscles, which is why most armwrestlers can take up to 2 weeks to fully recover from a single practice.

So first, before I get onto the real core of my questions, I ask for either clarification or confirmation that my assumptions/theories are either correct, greatly misled, or somewhere in between!

But assuming I'm more or less correct so far:

My Question

Again, I'm looking to understand the fundamental difference in the type of pain between armwrestling and normal gym lifting. When you exercise in a gym, your muscles get sore for a few days, heal and then you're good to go. While they're sore, it hurts to move them or exert them a lot, they can be sore or tender to the touch, etc.

With armwrestling, its literally a burning, radiating, throbbing toothache pain up and down your whole arm for several hours after practice. And then, for the next 7 - 10 days, if you try to exert your arms by any real amount (you have to shovel snow, mow your lawn, go swimming, etc.) the throbbing comes right back.

My theory is that there might be different types of pain receptors ("nociceptor"?) in the tendons and ligaments in your arms that are activated when injuries to your connective tissue occurs, and that perhaps the brain is just trained to interpret these receptors firing as a throbbing pain. Whatever pain you feel when you get a bad toothache might also be activated by similar receptors.

Is there any medical credence to this theory? If so, can someone explain it more in depth or point me to some decent reading? And if my theories are total junk, can you offer anything to replace it with? Thanks in advance!

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  • +1 for the research, but I‘m afraid it will probably take some time until someone can read through the entire question. I admit that I skimmed over it a bit as well. I think your chances of getting an answer are higher if you somehow find a way to condense the post. If I find one after I‘ve given it another read, I‘ll try to do the same. – Narusan Jul 18 '18 at 21:08
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    I'm somewhat doubtful this specific issue has been studied thoroughly enough (i.e. in the context of arm wrestling), but I'm not sure you are correct in focusing on nociception. Your description of the process makes it sound like actual injury is happening. In that case, it's more an issue of how much damage is done versus how much pain is felt. Pain, after all, is almost always a good thing (there are exceptions for some chronic forms of pain that aren't associated with a real injury of course): people who fail to sense pain have serious serious problems. – Bryan Krause Jul 18 '18 at 21:14
  • Frankly, I think with as much research as you've already put into this question, you're probably best suited to answer it. – Carey Gregory Jul 19 '18 at 1:11
  • Thanks @BryanKrause (+1) - again my totally unqualified theory is that the static forces cause the connective tissue to stretch and tear (break down; like muscle does during a good workout that results with you being sore the next few days). Is this the kind of injury you're talking about, or are you implicating a different type of injury? Thanks again! – hotmeatballsoup Jul 19 '18 at 1:29
  • Yeah, I'm not an exercise/muscle physiologist so I'd be hesitant to make any specific statements, but I was thinking of more damage than what causes the common "DOMS" experience with exercise, which usually happens most when you first start exercise or push your limits extra far. – Bryan Krause Jul 20 '18 at 20:44
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Your question focuses on the location of the receptors (the part of the nociceptors that is reacting to mechanical, thermal, or chemical stimuli).

An equally important part of a nociceptor is the axon, or nerve fibre, that conducts the action potential from the muscle or tendon (to go with your example) all the way to the spinal cord from where it makes its way to the brain. The slowest nerve fibres (type C) are unmyelinated and have a small diameter (around 1 micrometre) whereas larger diameter myelinated nerve fibres (type A) have a higher conduction velocity.

The brain interprets information from all nociceptors in the area to generate the sensation of pain. For that reason, there is no strict one to one mapping between a specific nociceptor firing and a specific sensation of pain. However, experiments show that when excited in isolation, nociceptors with A-type axons produced an acute, sharp, pricking pain. On the other hand, nociceptors with unmyelinated axons (C-type) produced a burning, dull, long-latency pain. ("Nociceptors and the perception of pain", Alan Fein, 2014.)

Another factor to consider is nociceptor sensitization. This is an adaptation to protect already injured tissue from further damage. It causes nociceptors in injured or inflamed tissue to fire even though the stimulus would normally be painless (allodynia) and/or to react much stronger to normally painful stimuli (hyperalgesia).

From what you are describing above it seems that arm-wrestling may cause sufficient strain to the muscle-bone-tendon structures of the arm to induce nociceptor sensitization during the recovery period.

This article might be of interest: "Where is the pain coming from in tendinopathy? It may be biochemical, not only structural, in origin", British Journal of Sports Medicine, August 2000

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