Outside of research, if we think about the way eccentric contraction works, it makes sense that we'd be sore. Concentric contraction finds actin and myosin hooking on to each other like velcro. While we resist force, our contracted muscle fibers are doing their best to hang on. Eventually, however, they can't, and they rip apart. You're literally ripping your muscles (on a very small scale) during eccentrics. Of course, it's going to hurt the next day.
Trying new things is a great way to challenge yourself, but it’s also a recipe for soreness and inflammation. When we do something over and over again, say, distance running - our body adapts. We get used to that stimulus. Our muscles, nerves, and connective tissue have figured out how to respond, and we’re less likely to be sore.
However, if you’ve only ever run for exercise, you’re going to be hurting after that first metcon. Thrusters, burpee box jump-overs, and pull-ups will leave your muscle screaming. The good news, however, is after that first running-based Metcon, you'll be laughing at your friends. It's all relative.
New exercises create discomfort. Your body isn’t prepared, and it essentially scrambles to keep up with the demand. Your immune system responds as if it were dangerous, so inflammation kicks in. And it hurts. After a while, though, your body builds some armor and deals with it much more effectively.
Finally, soreness comes down to some sort of intense stressor. Since we’ve talked about mechanical stress with eccentrics, we’d be remiss to ignore metabolic stress. Your muscle cells are constantly running chemical processes to produce energy. That’s how metabolism works. The harder you work those mitochondrial factories, the more byproducts they yield. Of course, some of those byproducts are energy. Others, however, cause cellular damage.
This is where the whole lactic acid myth was born. While yes, lactic acid does build up in the muscle during intense, anaerobic exercise, it simply slows you down in the moment. Soreness and inflammation, however, stem from reactive oxygen species (ROS). Also known as free radicals, ROS are atoms with unpaired electrons just floating around in your body. If you remember anything about chemistry, electrons don’t like to be without their buddies. Therefore, these ROS roam your tissue in search of something to scavenge. That means they actively detract from your happy, intact cells, damaging them in the process.
Intense metabolic overload happens when we push past our anaerobic limits. Cases include trying to beat your time in a hero WOD, Death by Wall Ball, or a 2000m row time-trial. Feel free to go for broke, but don't be surprised if you're sore afterward.
WHAT ABOUT INFLAMMATION?