![]() A shortening contraction of a muscle is called a concentric contraction. Let’s first look at what happens when a muscle contracts and does shorten. For this reason, the tendency is to focus on the shortening contraction of a muscle and overlook the importance of its contraction when it does not shorten. When a muscle’s function is described, it is usually stated in terms of its joint actions, which are its shortening contractions. This pulling force can play an important role in musculoskeletal function. However, even if the brachialis contracts with insufficient strength to shorten, it is important to understand that it is still exerting a pulling force on its attachments. ![]() Thus the minimum force that the brachialis must generate if it is to contract and shorten is the weight of its lighter attachment-the forearm. Because the forearm and hand weigh less than the arm and upper body, when the brachialis contracts and shortens, the forearm usually moves, not the arm. Consequently, for the brachialis to contract and shorten, it must generate a force that is greater than the weight of the forearm (and hand) or the arm (and upper body). The resistance to moving the arm is the weight of the arm plus the weight of much of the upper part of the body that must move (go along for the ride) when the arm moves toward the forearm. The resistance to moving the forearm is the weight of the forearm plus the weight of the hand that must move (go along for the ride) with the forearm. For the brachialis to contract and shorten, it must move the forearm toward the arm or move the arm toward the forearm or both.įor the brachialis to contract and shorten, it must move the forearm toward the arm or move the arm toward the forearm or both. In fact, most muscle contractions do not result in the muscle shortening to examine the muscular system in this way is to overlook much of how the muscular system functions.įIGURE 3-1 The brachialis muscle attaches from the humerus in the arm to the ulna in the forearm. This is not necessarily true, and making this assumption can limit our ability to truly grasp how the muscular system functions. In English, the word “contract” means “to shorten.” This leads many students to assume that when a muscle contracts, it shortens. However, in this case, it can be counterproductive to try to understand muscle contraction by relating it to how the term contract is defined in English. When it comes to the study of muscle function, the operative word is contract because that is what muscles do. This helps us understand the new kinesiology term intuitively instead of having to memorize its meaning. The director of this symphony who coordinates these pulling forces is the nervous system.ĭetermining whether there is a cognate (i.e., a similar term in lay English) is usually extremely helpful when confronted with a new kinesiology term. However, when different aspects of various muscles are co-ordered to contract in concert with each other and in temporal sequence with one another, the sum of many “simple” pulling forces results in an amazingly fluid and complex array of movement patterns. Any one muscle is a simple machine that pulls. To call a muscle nothing more than a simple “pulling machine” does not lessen the amazing and awe-inspiring complexity of movement patterns that the muscular system produces. In effect, a muscle is nothing more than a simple “pulling machine.” When ordered to contract by the nervous system, it pulls on its attachments when not ordered to contract, it relaxes and does not pull ( Box 3-1). ![]() A muscle does not and cannot choose to pull on one of its attachments and not the other. Realizing that this pulling force is equal on both of its attachments is also important. ![]() If this pulling force is sufficiently strong, the muscle will succeed in shortening and will move one or both of the body parts to which it is attached. This action results in a pulling force being placed on its attachments. When a muscle contracts, it attempts to pull in toward its center. ![]() The essence of muscle function is that muscles create pulling forces. Visual and Kinesthetic Exercise for Learning a Muscle’s Actions, 98 Questions 2 and 3-Where Does the Muscle Cross the Joint? How Does the Muscle Cross the Joint? 97įunctional Group Approach to Learning Muscles, 97 Question 1-What Joint Does the Muscle Cross? 97 Naming a Muscle’s Attachments: Origin and Insertion versus Attachments, 91įive-Step Approach to Learning Muscles, 96įiguring Out a Muscle’s Actions (Step 3 in Detail), 96 ![]()
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