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If no movement of a joint takes place but a muscle is activated and developing force (i.e., the force produced by the muscle equals the resistance), the action is called an isometricor staticmuscle action. This can occur when a weight is held stationary or the weight is too heavy to lift any higher. Some muscles, such as the postural muscles of the spine, primarily perform isometric muscle actions; they act to stabilize the upper body during most lifts.The maximal force that a muscle can produce changes throughout the range of motion and is a function of the architecture of the muscle and the joint angle. This relationship is described by the strength curvefor the muscle. The “sticking point” of an exercise is often related to a low point on the strength curve.
Muscle Anatomy 101 25The ascending curve is characterized by the ability to produce greater and greater force over the range of motion. This is the most common curve for exercise movements. Exercises such as the bench press, squat, shoulder press, and leg press follow this strength curve. Rubber band resistance also matches this curve best.Descending curves are less common but are seen in movements that are per-formed at unusual angles. The leg curl (page 223) is one example of an exercise where the most force that can be exerted is at the beginning of the range of motion.A bell curve represents exercises where the highest force that can be produced over the range of motion occurs somewhere in the middle of the curve. The biceps curl is a great example of an exercise that follows this strength curve.Let’s briefly cover how some different types of resistance training use these muscle actions and strength curves. More information on each of these types of lifting can be found in chapter 6.Free weightssuch as barbells and dumbbells require the use of both concentric and eccentric muscle actions. This type of training is also considered “free-form” exercise because there is no set path for the bar movement and this must be controlled by the user. Thus, assistance and support provided by the prime mover or core muscles are vital when using this type of equipment.Stack plate machinessuch as Universal or Nautilus weight machines are considered “fixed-form” exercise because the movement pattern is for the most part dictated by the machine itself. Both concentric and eccentric muscle actions are part of the repetition. Because there is less need for balance and stability, more weight can often be lifted when using these machines than in a free weight exercise. However, the stabilizing muscles get less training than during free weight lifts because of the machines guiding the movement.Brachialis(agonist)100°Biceps brachii(agonist)Concentric: muscle shortensTricepsbrachii(antagonist)Eccentric: muscle lengthensStatic: muscle lengthis unchangedBrachioradialis(synergist)100Figure 1.11 Concentric, eccentric, and isometric muscle action. The arrow indicates the direction of the movement.