Exercise physiology: muscle contractions research paper
Muscles are the very important parts of our bodies that enable us to move. Muscles are unique tissues that have the ability to contract and thus, they are able to produce force and cause motion. Our heart is a muscle that is continuously pumping in order to produce force that allows blood to circulate in our bodies. When we decide to move a part of our body, say our arms or our legs, the muscles contract and produce force which in turn is converted to motion. This is how we are able to move the different parts of our bodies, be it our eyes, our mouths, or our finger. Movement occurs because of muscle contraction. Muscle contractions are also called muscles twitches and they occur when a tension is created in the muscle fibers. The muscles can either be lengthened, shortened, or they stay the same. Even though the word ‘contraction’ means that something is being condensed or reduced, in biological and muscular terms, it means the creation of tension in the muscle fibers. When we see an athlete run a hundred meter dash in record-breaking time, he or she is running so fast because of repeated contraction of many muscles at the properly timed intervals, which is achieved by constant practice and vigorous training. There are three main types of muscle contractions: concentric, eccentric, and isometric, and these shall be discussed herein.
Concentric contractions are when muscles are consciously shortened in order to perform an action such as lifting a weight. For example, when you pick up a book bag or a grocery bag, your muscles are undergoing concentric contractions. “In concentric contractions, the force generated by the muscle is always less than the muscle’s maximum. As the load the muscle is required to lift decreases, contraction velocity increases. This occurs until the muscle finally reaches its maximum contraction velocity. By performing a series of constant velocity shortening contractions, a force-velocity relationship can be determined” (NSMRC, 2006). Many scientists have referred to this contraction of the muscles as a sliding filament mechanism. The action occurs across the whole length of the muscle and it generates force at the junctions where the muscles and the tendons meet. This allows shortening of the muscles and the angles of the joints are also altered. Thus, for an arm, a concentric contraction of the bicep would all the arm to bend at the elbow and move up to the shoulders. Concentric contractions are also called isotonic contractions and some other examples of this type of contraction include “lifting objects above the head - front shoulder (anterior deltoid) shortens, lifting object up from lying position - chest muscle shortens, lifting body up from squat position - quadriceps muscle shortens as legs extend, doing a sit up, throwing a ball, and swinging a bat” (Weightlossforall, 2003).
The other type of muscle contraction is eccentric contraction. This is basically the lengthening of the muscle which is cause by a force that is greater than that which is created by the muscle itself. This means that the muscle does not pull the joint towards itself for motion but it acts as a decelerator because some other load is being strained on the joint. This contraction occurs when someone is trying to lift a weight that is too heavy for one to move. Other examples of eccentric contraction is walking, when the knee extensors become active right after the heel hits the ground and just as the knee flexes. Also, when one is trying to gently put something down, eccentric contraction is taking place in the muscles. “There are two main features to note regarding eccentric contractions. First, the absolute tensions achieved are very high relative to the muscle’s maximum tetanic tension generating capacity (you can set down a much heavier object than you can lift). Second, the absolute tension is relatively independent of lengthening velocity. This suggests that skeletal muscles are very resistant to lengthening. The basic mechanics of eccentric contractions are still a source of debate since the cross-bridge theory that so nicely describes concentric contractions is not as successful in describing eccentric contractions” (NSMRC, 2006). Some more examples of eccentric contraction include “running downhill, walking downstairs, and landing on the ground from a jump” (Weightlossforall, 2003).
The third type of contraction is called isometric contraction and this occurs when a muscle generates force without changing its length, that is without shortening or lengthening. This is like when you put pressure on something through your fingers, such as holding a something and using your grip. Let’s say you were holding an apple, your joints do not move but your fingers exert enough pressure so that the apple does not drop. The muscle is held at a constant length and the “force generated during an isometric contraction is wholly dependant on the length of the muscle while contracting. Maximal isometric tension is produced at the muscle’s optimum length, where the length of the muscle’s sarcomeres are on the plateau of the length-tension curve” (NSMRC, 2006). Some other examples of times when isometric contraction occurs include attempting to lift an immoveable object, holding a weight at arm’s length, and some wrestling movements (Weightlossforall, 2003).
Even though situations where isometric contraction is used occur all the time in our everyday lives, the discussion of concentric and eccentric contractions is much more relevant, especially in the study of physiology. Thus, these two would be compared in greater detail here, giving more attention to how they are used in an training and exercising, and how they can cause injuries if their consequences are not properly understood. As concentric contractions cause muscle shortening, they generally require greater initial acceleration to overcome inertia and facilitate movement of the resistance through the “sticking point” of the exercise. As a result, more effort is required to overcome strength disadvantages in the weaker angles of the range of motion (Harman, 1994; Lander, 1991; Lombardi, 1989). Since a muscle’s ability to generate torque--as when the bicep rotates the forearm around the elbow in a curl--changes considerably throughout the range of motion of an exercise, one must attempt to expend maximal effort at the beginning of a lift. This will provide the acceleration or momentum needed to move the resistance through its sticking region and through the full range of motion of an exercise, assuming one is training with as much resistance as fatigue allows--in other words, overloading the muscle, that is, allowing it to reach eccentric contraction (Downing and Lander, 2002).
A typical error relating to low concentric-contraction velocity occurs when an individual attempts to perform an exercise such as a bench press, but fails to move the barbell through the “sticking point” of the lift. Lowering the bar weight and training via overload principles will eventually allow this individual to increase the working load, but strength gains will be limited to the weakest angle in the range of motion of his or her strength curve. A less frequent error relative to concentric contraction and velocity occurs when an individual attempts a high-velocity concentric contraction with more resistance than that person can handle. This extremely dangerous practice is more often a problem for novice lifters. Nevertheless, it poses an invitation for serious joint, tendon, or ligament injury. Lifters who cannot control the bar put themselves at high risk for injury (Downing and Lander, 2002).
As eccentric or negative muscle contraction causes a muscle to lengthen, an eccentric contraction requires lower velocity due to a change in the muscle’s or muscle group’s leverage system. While concentric contractions simulate third-class levers, eccentric contractions simulate second-class levers. This leverage change causes the muscles that overcame the resistance of the bar during the concentric contraction to actually serve as the resistance during the eccentric contraction (Fleck & Kraemer, 1997). Individuals who do not understand this principle will often train at velocities that are too high during eccentric contractions. The most common error of this type occurs when an individual performs a high-velocity eccentric contraction (e.g., during a biceps curl) that results in the bar “dropping” down to the starting position with minimal contraction of the muscle or muscle group until the end of the contraction, at which time said muscle(s) “apply the brakes” to end the movement. This practice inhibits muscle contraction until the end of the movement, which will have an adverse affect on strength gains (Kreigbaum & Bartels, 1996), while predisposing the lifter to possible injury. Most injuries in weight-training activities occur at the end of the exercise (Stone & O’Bryant, 1987), when high eccentric velocities with heavy or even moderate resistance tend to place inordinate stress and shear on joints, ligaments, muscles, and muscle tendons as they attempt to “brake” at the end of the contraction.In addition, some misinformed weight trainers believe that they are training their fast-twitch muscle fibers by training at high speeds. Although fast-twitch muscles fibers are associated with strength training, they are a function of training intensity (resistance), not speed of contraction. Finally, since velocity errors are often closely related to attempting to train with too much resistance, the lifter should always be sure to accurately determine her or his initial workload and progress over normal resistance increments (Downing and Lander, 2002).
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