Special thanks to Linda Wanek, PhD, PT, for her help in preparing this article.
A muscle is made up of thousands of muscle fibers. Muscle fibers are composed of microscopic proteins and other molecules that allow a muscle to contract. These fibers are what are injured and then repaired in a muscle strain. In addition, another kind of tissue is present in muscle which does not contract but helps support the overall structure of the muscle. This is connective tissue and, as the most abundant of the primary tissues of the human body, is present in many forms and has a wide variety of functions. As we will see later, damaged connective tissue has been implicated as one of the causes of delayed onset muscle soreness.
Muscle fibers make up the bulk of a muscle. In order to attach to bones, they often blend into a different type of tissue which makes up tendons. Tendons generally cross a joint and help facilitate movement. For example, the quadriceps muscle on the front of the thigh has a large tendon that attaches to the shin bone. When this muscle contracts, the knee is straightened. A tendon is different from a ligament, which generally attaches from one bone to another bone and mainly serves to stabilize a joint. For example, the knee joint has several strong ligaments that help hold it together but they do not cause movement in and of themselves. Later, we will see how the junction between a muscle and its tendon is a frequent site of damage.
Concentric contractions occur when a muscle shortens in order to exert force. For example, the biceps muscle does a concentric contraction in order to do an arm curl.
Isometric contractions are when the muscle exerts a force but there is no movement in the limb. An example would be carrying a bag of groceries; the arm muscles are contracting in order to hold the bag, but the arm itself is not moving.
Eccentric contractions are the contractions we rely on the most. These are contractions that occur in a muscle while it is lengthening. For example, as a person lowers the curl bar in a biceps curl, the biceps are performing eccentric contractions in order to slow the descent of the bar. The muscle is getting longer, but it is still exerting force.
Surprisingly, the precise physiologic mechanisms that characterize muscle strains have not been extensively studied (11, 17). It is thought that strain injuries most frequently happen near the muscle-tendon junction and that the muscle fibers themselves are partially torn (2, 11, 17). Tears can occur from a single, violent excessive force (acute strain) or from prolonged overuse or one episode of over activity (chronic strain) (2).
Skeletal muscle has an impressive ability to regenerate itself, which it does on a daily basis as well as in response to injury. When muscle fibers are damaged, several steps occur before regeneration can take place (12). First, the dead muscle fibers are removed by special cells called macrophages. These cells migrate to the damaged area through the bloodstream, so if the blood supply to the injured area has been significantly damaged, regeneration cannot take place until new blood vessels penetrate the area (7). Regeneration of single muscle fibers or entire muscles can then occur only when certain cells are activated. These special cells are called satellite cells (4, 18). When a muscle is damaged, these cells are activated within 18 hours of the injury (18). It also appears that these cells are able to migrate, and may move from healthy areas of the muscle to the injured area in response to a chemical stimulus (4, 18). These cells then fuse together to form myoblasts, which in turn fuse to form myotubes (3, 18). The myotubes then mature to form muscle fibers (3, 18). This may sound very straightforward, but many factors determine the success of the regeneration process. The extent of injury to nerves and blood vessels has a clear effect on how quickly the damaged area can obtain nutrients and be under normal control by the nervous system. Furthermore, all muscle is not the same. Not only can both the final shapes and functions of muscles be different, but the molecules within the muscle may not be alike (6). Finally, the properties of muscle may be different than they were before injury. Fewer numbers of muscle fibers are present, with a subsequent reduction in muscle mass (18). Regenerated muscle, however, does regain normal contraction and relaxation times (13, 18). This essentially means that the repaired muscle will not be as "strong" as it was before it was injured, although it may be able to contract just as quickly.
Muscle may also heal with scar tissue instead of with new muscle fibers (18). Scar tissue is a normal part of the healing process for many injuries, but may obstruct further muscle regeneration if present in muscle tissue (18). It also interferes with the normal contraction and elasticity of skeletal muscle. In addition, it is also composed of a different type of tissue than muscle and grows much more quickly (18).
This damage to muscle fibers or connective tissue would be repaired in the same way as the more severe damage found in muscle strain. Regeneration of muscle fibers would occur and the repair of connective tissue would proceed according to its own repair processes.
Improper warm-up before exercise, fatigue, previous injury, and other environmental conditions have been shown to increase the chance for injury, so reducing these factors is important for prevention (3). Gradually training muscles specifically in eccentric contractions is helpful in reducing muscle strain injuries and DOMS. One study has found that a single bout of eccentric exercise had a protective effect which decreased muscle soreness in subsequent episodes of eccentric exercise (7). A warm-up or conditioning period has been shown to alter the properties of muscle and make it less susceptible to injury (10). The warm-up period should consist of a mild to moderate level of activity that should prepare the muscles for more intense work. This does not necessarily include heavy stretching, which is probably best done after intense exercise as a cool-down. For jujitsuka, this means gradually introducing muscles to more activity in any one training session and over several sessions. Experienced jujitsuka and judoka can begin with some Nage Te after general warm-ups, making sure to do the throws more slowly. This will prepare the body for more dynamic or complicated techniques such as speed uchi kome or techniques from Oku No Te.
It is hoped that this discussion has cleared up some misconceptions about muscle injuries as well as offered some helpful suggestions for jujitsuka to maximize their training time and minimize injuries. After all, most of us would rather spend our time on the mat that off of it with an injury!
Special thanks to Linda Wanek, PhD, PT, for her help in preparing this article.
Linda Wanek, PhD (who helped edit the article), is a member of the faculty in Nerissa's physical therapy program who has a special interest in research on skeletal muscle injury and healing.
This information can be found in any exercise physiology text. You can reference Exercise Physiology: Energy, Nutrition and Human Performance, by W.D. McArdle, F.I. Katch, and V.L. Katch.
Thanks for the questions!
Nerissa Freeman, CMT, SPT, MPT. December 8, 2001.
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