About the Author: Tony Webster has a Ph.D. in exercise physiology and currently works within the Pacific Institute for Sport Excellence at Camosun College in Victoria, Canada. He has his Level 1, basic barbell and Olympic lifting certifications through CrossFit. He trains with the crew at CrossFit Taranis, where he enjoys getting his ass kicked by coaches Reed and Dan.

From my interactions with other CrossFitters, I’ve found that muscle soreness seems to be treated like the stereotypical mother-in-law. As CrossFitters we understand the meaning of real muscle soreness. The kind of soreness that makes you hobble out of bed in the morning and clutch at handrails when walking down stairs. It seems as though it is almost a rite of passage in this sport. But have you ever stopped to think about why you are getting sore? Why do some workouts cause severe muscle soreness and others not? What could one do to prevent or reduce it? 

Technically, the kind of soreness we are referring to is called “delayed onset muscle soreness” (DOMS) in the scientific literature to distinguish it from more “immediate” soreness that might be experienced as a result of injury.

For many specialized athletes, muscle soreness tends to be an issue only after prolonged layoffs from their sport or after training sessions that have been unusually tough or substantially different from normal. In CrossFit, of course, there is no “normal,” just constantly varied functional movements performed at high intensity: the perfect recipe, as it turns out, for ongoing muscle damage and soreness.

Mechanisms of muscle damage: Eccentric overload

For something so commonly experienced by us all, you may be surprised to know that the exact mechanisms of muscle damage and the cause of the subsequent soreness are still unclear. It has been, and still is, a source of quite active debate amongst muscle physiologists. There are essentially two hypotheses for what initiates muscle damage in the first place: a “metabolic” hypothesis and a “mechanical” hypothesis.

The “metabolic” hypothesis, predominant for many years, states that muscle damage is caused by metabolic deficiencies or excesses. It was believed that lactic acid (generated during high-intensity exercise) was the cause of muscle soreness. While it is fair to say that muscle burn or discomfort during high-intensity exercise has been linked to lactic acid, there is no evidence to suggest that this is in any way linked to subsequent muscle damage or soreness. Therefore this idea has been disproved.

While there are undoubtedly many metabolic consequences of muscle damage, most in the scientific community now agree that its initial cause is mechanical in nature.

The “mechanical” hypothesis points to eccentric actions of muscles as the main cause of muscle damage. What are eccentric actions? Well, there are essentially three types of muscle action. There are concentric muscle actions where a muscle shortens when it generates tension, a true “contraction.” This usually refers to the actual lifting phase or acceleration phase of any movement: think of the “up” phase in a squat or shoulder press for example—muscles are shortening and the weight is lifted.

There are isometric actions when muscles generate tension but there is no change in length: think about holding a handstand position or attempting a deadlift which is simply too heavy for you to move—lots of muscle tension but no movement.

And there are eccentric muscle actions usually associated with the lowering phase or deceleration phase of any movement. During an eccentric muscle action there is tension in the muscle but it is actively lengthening at the same time. The “down” phases of a squat or shoulder press are examples. It turns out that eccentric actions are structurally much more stressful for muscle fibers than any other form of muscle action. The exact reason why is unclear, but it appears that during eccentric actions the weak links in the microstructure of the muscle fibers are more likely to “pop” or “tear” than during other forms of muscle action.

The old metabolic theory that suggested lactic acid caused muscle damage: The interesting aspect of eccentric muscle actions is that they are metabolically less stressful than concentric muscle actions. In other words, they generate less metabolic by-products such as lactic acid than concentric actions, yet they still manage to cause more damage and soreness.

It is important to understand that eccentric actions are a key ingredient in order for muscles to adapt effectively to a strength or power training program. These are the actions that break down the muscle to a greater degree, thus stimulating greater eventual repair and adaptation. In comparison, concentric actions just do not have the same anabolic effect. Studies have shown that muscles loaded with only concentric contractions fail to respond to a strength training program to the same degree (in both strength and size) as muscles that are exposed to both concentric and eccentric actions.

Reference link:

https://journal.crossfit.com/article/muscle-damage-soreness-overview