triglycerides (which store fat). On the reverse side is catecholamine activity, which activates glycogenolysis in the muscle as well as lipolysis.
NEURAL CONTROL OF MUSCLES IN EXERCISE In order to move a load, the sarcomeres need to shorten. This shortening force is called muscle tension. This leads to two types of skeletal muscle contractions, which are isotonic and isometric contractions. In isotonic contractions, the muscle shortens and stays constant. There are two types of isotonic contractions, which are concentric and eccentric. Concentric contractions involve the shortening of the muscle in response to the load. Eccentric contraction occurs when the muscle tension decreases and the muscle lengthens. These are used for movement and balance in the body. Isometric contractions occur as the muscle produces tension without actually changing the angle of the skeletal joint. There is sarcomere shortening and muscle tension that cannot overcome the weight of the load. This happens when lifting a heavy load. Most actions of the body are a combination of isotonic and isometric contractions that together produce a wide range of possible outcomes. As you remember, every skeletal muscle fiber needs to have innervation by the axon terminal of a motor neuron in order to engage in the process of contraction. The group of muscle fibers that are innervated by a single motor neuron is referred to as a motor unit. The actual size of a motor unit depends on what the muscle is used for. Small motor units permit fine motor control. This is seen particularly in facial muscles. Extraocular muscles have six fibers per motor unit. The large motor units are seen when gross or large movements are necessary. The smaller motor units will have lower-threshold motor neurons that are more excitable and that fire to small numbers of muscle fibers. There will be the resultant small degree of tension on the muscle. When more strength is needed, the larger motor units, which have a higher threshold of activation, will become active, resulting in an increase in muscle contraction called recruitment. Recruitment increases the strength of the muscle. This is how the nervous system controls the strength of a given muscle.
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