Our muscles give us sensations of pain when they are damaged and during their recovery period. Such muscle damage can occur from a number of causes that might be divided into four categories: acute onset muscle soreness (AOMS), delayed onset muscle soreness (DOMS), muscle cramping and muscle trauma (injury/wounding).
Muscles are composed of multiple muscle cells or fibers, the number of fibers being dependent on location and purpose of the muscle. While the number of muscle fibers in a particular muscle varies little between people, within each fiber may be a highly variable number of myofibrils parallel to each other. The more work the muscle cell is put to, the more myofibrils it will construct, as it is within these myofibrils that the contraction and release resulting in muscle function occurs.
Within the muscle – in the connective tissue between muscle fibers, particularly concentrated near small blood vessels, and at the musculotendinous junctions, where muscle connects to tendon are found the free nerve endings of two types of afferent or sensory neurons (nerve cells that carry signals to the spine and brain). The myelinated group III A-delta nerve fibers have faster signal transmission times and therefore signal sharp, localized pain. The unmyelinated group IV C-fibers are slower, they transmit a duller, more generalized ache.
Rubbing and massage are often used in attempts to alleviate or reduce pain. These are perfectly valid practises based on sound physiological principles. The mechano-receptor nerve endings in your skin, those that supply your sense of touch, partially suppress the firing of the above afferent neurons when stimulated, causing the transmission of pain signals to be reduced.
AOMS begins during muscle activity and can last up to four to six hours afterwards. It is primarily due to the build up of metabolic byproducts from energy production within the muscle cell. Energy production requires oxygen to achieve maximum efficiency; when insufficient oxygen is reaching the cell it produces less energy, because it can only perform the initial catabolic step called glycolysis, which is anaerobic (doesn’t require oxygen). The end product of glycolysis is either lactic or pyruvic acid, and these can cause chemical damage within the cell if concentractions get too severe. This is what is being referred to when people speak of “oxygen debt”. During exercise these acids may build-up, as you breath deeply afterwards, the oxygen arriving at the cells is used to complete the catabolism of the acids; producing more energy, metabolic water and carbon dioxide from the harmful acids through repetitions of the Krebs Cycle. The C-fiber nerve cells are polymodal, responding to a variety of chemical, mechanical and thermal triggers. They let you know your muscles are hurting until that oxygen debt has been repaid.
DOMS usually occurs between 12 to 48 hours after exercise that is of an abnormal nature in some regard, with recovery taking from seven to ten days. That “abnormal” is to the specific indiviual’s involved muscle groups; such as being exerted beyond normal usage or for an extensive period of time. Children very rarely suffer from DOMS. The first detailed description of DOMS was given by Theodore Hough in 1902, in which he attributed it to “structural damage due to high tension”, basically overexertion and strain, and this is still the most accepted hypothesis for it. Another possibility is structural damage caused by an increased localized temperature. Rhabdomyolysis, which is the extreme form of DOMS, most commonly occurs in untrained and inexperienced people exercising in the heat. Myofibrillar remodelling may also be a cause of DOMS; the muscle cell is stimulated by the abnormal activity to make modifications to its myofibrils and/or cytoskeleton (cell framework); four main types of changes have been noted. DOMS may occur from any of these individually or in any combination; it is possible such damages could be compounded by the presense of metabolic byproducts as discussed above under AOMS.
While no evidence exists that repeated instances of DOMS may result in long-term muscle damage or reduced function, there is none proving it does not either. Such studies are themselves long-term. As such, exercise programs aimed at muscle development should be approached in a manner that minimizes the instances of DOMS. There was no evidence that asbestos was harmful to people at the time it was being praised as a fire retardant material ideal for installing in homes, schools, hospitals, etc. There was, quite naturally, no evidence it was not either. Couldn’t be, since we now know it is a very harmful and dangerous form of carcinogenic.
Muscle cramps can occur when the muscle group is being exercised, either normally or strenuously, or while it is relaxed. While known to be triggered by hyperexitability of the lower moter neuron (nerve cells that carry signals from the spine and brain), the exact cause is still unclear, but is believed to be associated with low fluid, electrolyte and magnesium levels. The result of the hyperexitability is severe, involuntary, electrically active contractions of the muscle, using a higher percentage of the myofibrils in the muscle per contraction than during normal muscle use. Cramps typically last from a few seconds up to several minutes. They will normally cause AOMS, the longer the period of cramping the more severe the soreness. Continued use of the muscle group after a cramp will almost certainly cause DOMS as well. DOMS can be averted if the muscle is allowed to rest, unused, for two to five hours after the cramp, depending on severity.
Muscle trauma relates to physical damage to the muscle tissue, this may be internal injury such as muscle tearing due to excessive strain, or externally caused from a blow or puncture wound. Pain signals will continue until the damage is healed, essentially the same as for DOMS. Muscle damage can also be caused by deficiencies in required enzymes, for example phosphorylase or phosphofructokinase. These enzymes are built by the cells, if they are in short supply it may be genetically related, but is more often due to an inadequate diet.
author, Perry McCarney.
