While playing shuttle yesterday, I probably went a little overboard to smash it hard on the other side and call it a game. It did result in the expected outcome, but also an unwelcome one, i.e. swelling and pain in my palm. It may be a mild muscle injury, but I wondered, why the body reacts the way it does to such injuries, why swelling occurs and what role does it play.
While I am usually not so self-reflective, probably what got me thinking is the article published in “The New Yorker” titled “Inflamed”. In this article, Groopman talks about inflammation as being a primary mechanism in our body to defend against the invading foreign particles, but how that same mechanism can turn disastrous in case of chronic inflammations.
However, when one has an injury like I did, that is not exposed to the external environment, why does body still trigger inflammation and what role does it serve. Is the swelling and associated pain having any role in healing my injury? Should I let the inflammation simmer for its healing capabilities if any, or pop-up a painkiller that can help reduce it? Is the inflammation, the swelling a good thing or bad? Since inflammation coincides with muscle repair, regeneration, and growth, is there any causative relation between the two?
Lucky me, some researchers had already found this question interesting enough! Several studies that tried to answer this question included methods such as antibody-assisted depletions of inflammatory cells Vis-a-Vis the muscle repair and growth, use of genetically modified animal cells to analyze the relation between inflammatory cells and muscle cell growth. So let’s see what the science has to say on this.
A torn muscle looks something like this.
As soon as a muscle gets injured, torn or stressed, researchers believe that certain signals are generated from the muscle cells itself.  These chemicals are called cytokines and are secreted by muscle after a stressful or traumatic event, which then invites inflammatory cells arrival. These are welcome signals for the white blood cells like neutrophils and their larger cousin, macrophages to arrive at the site.
Neutrophils, though smaller in size, make up for nearly 50% of white blood cells. Within minutes, neutrophils flood the injured area and cause something we know as acute inflammation.
(Image Source: Wikipedia)
Along with them, Macrophages, which are another type of white blood cells, join the party. These are larger cousins of the neutrophils and primarily tasked with eating away the bad guys, which are typically foreign bodies and pathogens. In the case of muscle injury, these same cells are known to eat away the internal debris generated as a result of harm.
(Image Source: https://www.pinterest.com/pin/481603753871981432/ )
A complex picture has emerged of the role inflammation plays in muscle injuries and subsequent repair.
While direct lab evidence has been found, showing damage that inflammatory cells have on healthy muscle tissues, only indirect links have been found between inflammation and muscle growth. Inflammation overall is not found to be wholly beneficial for the muscle tissues, especially when it is severe and remains present for a much longer time. 
To determine the effect Neutrophils have on muscle tissues, studies  found that neutrophils begin invading the injury region soon after the injury. Neutrophils have twin role to play, in the sense that as phagocytes (i.e. cells eating other cells), they release compounds that can help degrade the cell debris left after an injury. In simpler words, you can think of them as an acid based cleanser that can help remove debris from the floor. However, the same molecules are also cytotoxic (i.e. toxic to the cells) and cytolytic (i.e. cause the cells to break) for the healthy muscle tissues as well.
As far as evidence is concerned about whether the presence of neutrophils is beneficial for muscle repair, while correlations exist, no direct evidence has emerged. Studies point that muscle repair is slower in older animals where phagocytosis from the inflammatory cells is weak. One interesting study  found that cell growth was much slower when phagocytes such as neutrophils and macrophages were depleted due to the snake toxin. However, unlike the case of analyzing damage, no study so far has been able to prove beyond doubt that Neutrophils have a role to play in muscle repair.
Similar to Neutrophils, macrophages are also shown to cause muscle tissue damage in experiments. Macrophages play a more active role in removing the debris left after an injury. Do macrophages have a positive role to play in muscle repair? A study found that when macrophages concentration was selectively depleted resulting in decreased macrophage concentration at the site of muscle injury, it affected the muscle regeneration capacity. [4, 5] Macrophages are also believed to carry certain growth factors that can promote cell growth. However, more research needs to be conducted to understand precisely what these growth factors are.
Inflammatory cells have been shown to damage the healthy muscle tissues, but it is still too early to label them as a complete enemy. What studies recommend is that while mild and acute inflammation may have a positive role to play, a severe and longer duration inflammation should be controlled by usage of anti-inflammatory drugs. 
At least, until the researchers can conclusively rule out any growth promoting a role for the inflammatory cells, they remain to us a mixed blessing!
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- Nguyen HX and Tidball JG. Interactions between neutrophils and macrophages promote macrophage killing of muscle cells in vitro. J Physiol 547: 125 – 132, 2003.
- Fielding RA, Manfredi TJ, Ding W, Fiartarone MA, Evans WJ, and Cannon JG. Acute phase response in exercise. Neutrophil and IL-1β accumulation in skeletal muscles. Am J Physiol Regul Integr Comp Physio 265: R166 – R172, 1993
- Honda H, Kimura H, and Rostami A. Demonstration and phenotypic characterization of resident macrophages in rat skeletal muscle. Immunology 70: 272 – 277, 1990
- Bondesen BA, Mills ST, Kegley KM, and Pavlath GK. The COX-2 pathway is essential during early stages of skeletal muscle regeneration. Am J Physiol Cell Physiol 287: C475 – C483, 2004
- Mishra DK, Friden J, Schmitz M, and Liebel RL. Anti-inflammatory medication after muscle injury. J Bone Joint Surg 77: 1410 – 1519, 1995