Muscle Actions

Muscle forces are the main internal motors and brakes for human movement. While gravity and other external forces can be used to help us move, it is the torques created by skeletal muscles that are coordinat ed with the torques from external forces to obtain the human motion of interest. While some biomechanists are interested in the forces and motions created by smooth (visceral) or cardiac (heart) muscle, this text will focus on the actions of skeletal muscle that create human movement.

The activation of skeletal muscle has traditionally been called contraction. I will avoid this term because there are several good reasons why it is often inappropriate for describing what muscles actually do during movement (Cavanagh, 1988; Faulkner, 2003). Contraction implies shortening, which may only be accurate in describing the general interaction of actin and myosin in activated muscle. Contraction also conflicts with the many actions of muscles beyond shortening to overcome a resistance. Saying "eccentric contraction" is essentially saying "lengthening shortening"! Cavanagh suggests that the term "action" is most appropriate, and this book adopts this terminology. Muscle action is the neuro-muscular activation of muscles that contributes to movement or stabilization of the musculoskeletal system. We will see that muscles have three major actions (eccentric, isometric, concentric) resulting from both active and passive components of muscle tension. It could also be said that a fourth action of muscle is inaction, not being activated because their activation at that time would be inefficient or counterproductive to the task at hand.

Mechanically, the three kinds of actions are based on the balance of the forces and torques present at any given instant (Figure 3.9). If the torque the activated muscles creates is exactly equal to the torque of the resistance, an isometric action results. A bodybuilder's pose is a good example of isometric muscle actions of opposing muscle groups. Recall that isometric literally means "same length."

A concentric action occurs when the torque the muscle group makes is larger

Figure 3.9. The three kinds of muscle action are determined by the balance of torques (moments of force: M). In concentric action the torque of the abductors (Mm) is greater than the torque of the resistance (MR), so the arm rises. In isometric conditions the joint angle does not change because MM and MR are equal. In eccentric action MM

Figure 3.9. The three kinds of muscle action are determined by the balance of torques (moments of force: M). In concentric action the torque of the abductors (Mm) is greater than the torque of the resistance (MR), so the arm rises. In isometric conditions the joint angle does not change because MM and MR are equal. In eccentric action MM

is less than MR, so the arm is lowered.

than the torque of a resistance, resulting in muscle shortening. The upward lift of a dumbbell in an arm curl is the concentric phase of the exercise. In essence a concentric action occurs when a muscle activation results in shortening of the muscle-tendon unit. When the lifter gradually lowers the weight in an arm curl, the torque the muscle group makes is less than the torque of the resistance. This lowering of the dumbbell is an eccentric muscle action or the lengthening of an activated muscle. In eccentric actions muscles are used as brakes on external forces or motion like the brakes of your car.

The importance of these different muscle actions cannot be overemphasized. Functional anatomical analysis and most people tend to focus primarily on the concentric actions of muscles. This overemphasis of what is usually in the minority of muscle actions for most movements gives a false impression of how muscles create human movement. The following section on the limits of functional anatomy will expand on this idea by showing that muscles create movement in a variety of ways using all three muscle actions, not just concentric action.

Application: Eccentric Actions and Muscle Injury

Eccentric actions are common to all muscles and virtually every human movement. Eccentric actions of high intensity, repetitive nature, or during fatigue are associated with muscle injury.When eccentrically active muscles are rapidly overcome by external forces, a muscle strain injury can occur.When people perform physical activity beyond typical levels, especially eccentric muscle actions, the result is usually delayed-onset muscle soreness.This is why it is important in conditioning to include both eccentric and concentric phases of exercises. Some athletic events would benefit from emphasis on eccentric training. For example, long jumpers and javelin throwers need strong eccentric strength in the takeoff and plant leg.

Active and Passive Tension of Muscle

Activated muscles create forces by pulling about equally on all their attachments. This tensile force really has two sources: active and passive tension.

Active tension refers to the forces created between actin and myosin fibers in the sarcomeres of activated motor units. So active tension is the force created by the contractile proteins (actin and myosin) using chemical energy stored in ATP. This ability of muscles to create active tensile forces is unique compared to the connective tissue components (ligaments, tendons, bone) of the musculoskeletal system. The shape of this active tension potential of skeletal muscle is called the force-velocity relationship of muscle and is summarized in chapter 4.

Passive tension is the force that comes from an elongation of the connective tissue components of the muscletendon unit. When a person does a stretching exercise, the tension she feels in the muscles is the internal resistance of the muscletendon unit to the elongation of the stretch. This passive tension in stretching exercises can be quite large and may be responsible for the muscular weakness seen in muscles following stretching (Knudson, McHugh, & Magnus-son, 2000). In the midranges of joint motion, passive tension does not significantly contribute to muscle forces in normal movement (Siegler & Moskowitz, 1984); however, it is more a factor in low-force movements (Muraoka et al., 2005) and in various neuromuscular disorders (Lamontagne, Malouin, & Richards, 2000). Muscle passive tension is a significant factor affecting movement at the extremities of joint range of motion. The increase in passive tension limiting range of joint motion is quite apparent in multiarticular muscles and is called passive insufficiency. We will see in the following chapter that passive tension is an important component of the force-length relationship of muscle. The passive insufficiency of poor hamstring flexibility could lead to poor performance or risk of injury in activities that require combined hip flexion and knee extension, such as in a karate front kick (Figure 3.10). The passive tension in the hamstring muscles is high in Figure 3.10 because the muscle is simultaneously stretched across the hip and knee joint. We will learn later on in this chapter that the concept of range of motion is a complicated phenomenon that involves several mechanical variables.

Hill Muscle Model

One of the most widely used mechanical models of muscle that takes into account

Figure 3.10. The combined hip flexion and knee extension of a karate front kick may be limited by the passive insufficiency of the hamstring muscles. This technique requires excellent static and dynamic hamstring flexibility. Image courtesy of Master Steven J. Frey, 4th-Degree Black Belt.

Activity: Passive Tension

The effect of passive tension on joint motions can be felt easily in multi-joint muscles when the muscles are stretched across multiple joints. This phenomenon is called passive insufficiency. Lie down in a supine (face upwards) position and note the difference in hip flexion range of motion when the knee is flexed and extend-ed.The hamstring muscle group limits hip flexion when the knee is extended because these muscles cross both the hip and the knee joints. Clinical tests like the straight-leg raise (Eksstrand,Wiktorsson, Oberg, & Gillquist, 1982), active knee extension (Gajdosik & Lusin, 1983), and the sit-and-reach (Wells & Dillon, 1952) all use passive insufficiency to evaluate hamstring static flexibility. Careful body positioning is required in flexibility tests because of passive insufficiency and other mechanical factors across several joints. some aspects of this issue are explored in Lab Activity 3.

both the active and passive components of muscle tension is the three-component model developed by A. V. Hill in 1938 (Hill, 1970). Hill was an English physiologist who made substantial contributions to the understanding of the energetics (heat and force production) of isolated muscle actions. Hill was also interested in muscular work in athletics, and some of his experimental techniques represent ingenious early work in biomechanics (Hill, 1926, 1927). The Hill muscle model has two elements in series and one element in parallel (Figure 3.11). The contractile component (CC) represents the active tension of skeletal muscle, while the parallel elastic component (PEC) and series elastic component (SEC) represent two key sources of passive tension in muscle. The Hill muscle model has been the dominant theoretical model for understanding muscle mechanics and is usually used in biomechanical computer models employed to simulate human movement.

We can make several functional generalizations about the mechanical behavior of muscle based on Figure 3.11. First, there is elasticity (connective tissue) in the production of active muscle tension modeled by the series elastic component. The source of this series elasticity is likely a mixture of the actin/myosin filaments, cross bridge stiffness, sarcomere nonuniformity, and other sarcomere connective tissue components. Second, the passive tension of relaxed muscle that is easily felt in stretching exercises or in passive insufficiency affects motion at the extremes of joint range of motion. The "p" in the parallel elastic component is a key for students to remember this as the primary source of passive tension in the Hill muscle model. Third, muscle tension results from a complex interaction of active and passive sources of tension. This third point can be generalized beyond the simple Hill muscle model as a result of recent research that has focused on the complex transmission of force within the connective tissue components of muscle (Patel & Lieber, 1997). Muscles may not create equal forces at their attachments because of force transmitted to extramuscular connective tissues (Huijing & Baan, 2001).

The separation of the passive tension into series and parallel components in the Hill model and the exact equations used to represent the elastic (springs) and contractile components are controversial issues. Whatever the eventual source and complexity of elastic tension, it is important to remember that the stretch and recoil of elastic structures are an integral part of all muscle actions. It is likely that future research will increase our understanding of the interaction of active and passive components

Figure 3.11. The Hill model of muscle describes the active and passive tension created by the MTU. Active tension is modeled by the contractile component, while passive tension is modeled by the series and parallel elastic components.

of muscle tension in creating human movement. There are many other complexities in how muscles create movement. The next section will briefly review the logic of functional anatomical analysis and how biome-chanics must be combined with anatomy to understand how muscles create movement.

pists from subjective observation of movement are not correct (Bartlett, 1999; Herbert, Moore, Moseley, Schurr, & Wales, 1993). Kinesiology professionals can only determine the true actions of muscle by examining several kinds of biomechanical studies that build on anatomical information.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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