Think about you first strength workout. Your muscles were probably sore for days (a.k.a. DOMS - Delayed onset muscle soreness). Years ago we thought that muscle soreness was due to lactic acid. We now know that this is not true. In fact the lactic acid that is formed during exercise is rapidly removed and metabolized by your liver, your heart and by slow twitch muscles. The soreness is actually due to muscle damage. When we examine muscles under an electron microscope following heavy resistance exercise we see that the normally orderly arrangement of the myofibers (muscle fibers) is completely disrupted. This damage is referred to as “micro injury” and typically occurs in a small percentage of fibers in a muscle. This muscle damage is common following exercises that have a strong force component (like weight training).
Muscle damage proceeds in two phases, the autogenic phase and the phagocytic phase. The autogenic phase begins immediately following the exercise. Mechanical tension during contractions (especially lengthening contractions) opens stretch activated Calcium channels. Calcium then floods the interior of the muscle cell. The increased Calcium concentration activates lysosomes (lysosomes are specialized structures that can digest proteins). These lysosomes may then begin destroying structural components of the muscle. Calcium also activates Phospholipase A2. Phospholipase A2 then begins to punch holes in the muscle cell membrane by converting phospholipids to arachidonic acid. There is also an increase in free radicals. In other words, entry of Calcium (and the subsequent inability to remove the calcium) into the muscle cell initiates a self destruct mechanism in muscle. Approximately 3-4 hours following the damage phagocytes begin to invade the area to clear away the remaining debris. These phagocytes can then release heparin and other inflammatory chemicals and induce swelling. This is the phagocytic phase of muscle damage. In fact the phenomena referred to as the “second day lag” (the increase in soreness two days post-workout) is due to greater phagocyte activity 48 hours post workout leading to more swelling.
“How does this relate to muscle growth? When the cell membrane is disrupted (has holes punched in it) several substances that are normally inside of the cell can leak out. Among these are growth factors and prostaglandins. These growth factors may affect protein synthesis in neighboring muscle cells and may induce the migration, differentiation, and proliferation of satellite cells. A satellite cell is an undifferentiated muscle cell that is basically just a nucleus. Remember that the nucleus of a cell is like a protein synthesis factory. Growth factors can cause the satellite cell to fuse with a damaged muscle cell and beef up its protein synthesis ability. Alternatively, satellite cells can fuse together and create a new muscle cell. This fusion of satellite cells to create a new muscle cell is referred to as hyperplasia and has been demonstrated in an animal model. In summary, the muscle cell is damaged by exercise, growth factors are released and satellite cells fuse with existing muscle fibers and increase protein synthesis capabilities or fuse with one another and create new muscle cells.
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