Biomechanics of Skeletal Muscle Yi-Ching Tsai Institute of Biomedical Engineering, National Yang-Ming University. Basic Biomechanics of the Musculoskeletal System

Biomechanics of Skeletal Muscle Composition and Structure Molecular Basis of Muscle Contraction Mechanics of Muscle Contraction Force Production in Muscle Muscle Fiber Differentiation Muscle Injury Muscle Remodeling

Properties of Muscle Tissue  Excitability: the ability to respond to stimulation  Contractility: the ability to shorten actively and exert a pull, or tension  Extensibility: the ability to continue to contract over a range of resting lengths  Elasticity: the ability of a muscle to rebound toward its original length after a contraction

Functions of Skeletal Muscle 1. Produce a movement 2. Maintain posture and body position 3. Support soft tissues 4. Maintain body temperature

Composition and Structure

Microanatomy of Skeletal Muscle  Skeletal muscle fibers are quite different from the “typical” cell. One obvious difference is their enormous size. For example, a skeletal muscle fiber from a leg muscle could have a diameter of 100 μm and a length equal to that of the entire muscle (30-40 cm). In addition, each skeletal muscle fiber is multinucleate.

Molecular Basis of Muscle Contraction  Sliding filament theory  All-or-none principle (twitch)  Cross-bridges: generate force and displacement  Ca 2+ : on and off  Production of the electric signal Method of measure: EMG

The Motor Unit  Includes: a single motor neuron and all of the muscle fibers innervated

Mechanics of Muscle Contraction  All-or-none principle (twitch)  The amount of tension produced in the skeletal muscle as a whole is determined by (1) the frequency of stimulation and (2) the number of muscle fibers activated  A twitch is a single stimulus-contraction-relaxation sequence in a muscle fiber

Summation and Tetanus (a) summation (b) incomplete tetanus (c) complete tetanus

Types of Muscle Work and Contraction 1. Concentric Contraction 2. Eccentric Contraction 3. Isokinetic Contraction 4. Isoinertial Contraction 5. Isotonic Contraction 6. Isometric Contraction

1. Concentric Contraction  When muscles develop sufficient tension to overcome the resistance of the body segment, the muscles shorten and cause joint movement. The net muscle moment generated by the muscles is in the same direction as the change in joint angle. An example of a concentric contraction is the action of the quadriceps in extending the knee when ascending stairs.

2. Eccentric Contraction  When a muscle cannot develop sufficient tension and is overcome by the external load, it progressively lengthens instead of shortening. The net muscle moment is in the opposite direction from the change in joint angle. One purpose of eccentric contraction is to decelerate the motion of a joint. For example, when one descents stairs, the quadriceps works eccentrically to decelerate flexion of the knee, thus decelerating the limb. The tension that it applies is less than the force of gravity pulling the body downward, but it is sufficient to allow controlled lowering of the body.

3. Isokinetic Contraction  This a type of dynamic muscle work in which movement of the joint is kept at a constant velocity, and hence the velocity of shortening or lengthening of the muscle is constant. Because velocity is held constant, muscle energy cannot be dissipated through acceleration of the body part and is entirely converted to a resisting moment. the muscle force varies with changes in its lever arm throughout the range of joint motion. The muscle contraction concentrically and eccentrically with different directions of joint motion. For examples, the flxor muscle of a joint contract concentrically during flexor and eccentrically during extension, acting as decelerators during the latter.

4. Isoinertial Contraction  This a type of dynamic muscle work when the resistance against which the muscle must contract remains constant. If the moment produced by the muscle is equal to or less than the resistance to be overcome, the muscle length remains unchanged and the muscle contracts isometrically. If the moment is greater than the resistance, the muscle shortens (contracts concentrically) and causes acceleration of the body part. Isoinertial contraction occurs, for example, when a constant external load is lifted. At the extremes of motion, the inertia of the load must be overcome, the involved muscles contract isometrically and muscle torque is maximal. In the midrange contract concentrically and the torque is submaximal.

5. Isotonic Contraction  This term is commonly used to define muscle contraction in which the tension is constant throughout a range of joint motion. Lifting an object of a desk, walking, running, and so forth involve isotonic contraction of this kind. The term does not take into account the leverage effects at the joint. However, because the muscle force moment arm changes throughout the range of joint motion, the muscle tension must also change. Thus, isotonic muscle contraction in the truest sense does not exist in the production of joint motion.

6. Isometric Contraction  Muscles are not always directly involved in the production of joint movements. The may exercise either a restraining or a holding action, such as that needed to maintain the body in an upright position in opposing the force of gravity. In this case the muscle attempts to shorten (i.e., the myofibrils shorten and in doing so stretch the series elastic component, thereby producing tension), but it does not overcome the load and cause movement, instead, it produces a moment the supports the load in a fixed position (e.g., maintains posture) because no change takes place in the distance between the muscle’s points of attachment. Examples of isometric contractions are pushing against a wall and trying to pick up a large car.

Force Production in Muscle  Length-Tension Relationship  Active and Passive Tension of Muscle  Load-Velocity Relationship  Force-Time Relationship  Effect of Skeletal Muscle Architecture  Effect of Temperature  Muscle Fatigue

Length-Tension Relationship

Active and Passive Tension of Muscle

Load-Velocity Relationship

Force-Time Relationship

Effect of Skeletal Muscle Architecture  The forces that the muscle can produce is proportional to the cross-section of the myofibril.  The velocity and the excursion (working range) that the muscle can produce are proportional to the length of the myofibril. EX: The quadriceps muscle contains shorter myofibrils and appears to be specialized for force production. The sartorius muscle has longer fibers and a smaller crosssectional area and is better suited for high excursion.

Effect of Temperature  Muscle temperature increases by means of two mechanisms: (1) Increase in blood flow, which occurs when an athlete “warms up” his or her muscles (2) Production of the heat of reaction generated by metabolism, by the release of the energy of contraction, and by friction as the contractile components slide over each other

Muscle Fatigue  A skeletal muscle fiber is said to be fatigued when it can no longer contract despite continued neural stimulation.

3-class lever in muscle activity

Muscle Fiber Differentiation red red white

Muscle Injury result from trauma or contractions against resistance  contusion, laceration, ruptures  ischemia, compartment syndromes, denervation

Muscle Remodeling  Effects of disuse and immobilization -loss of endurance and strength -muscle atrophies  Effects of physical training -increase the cross-sectional area of fibers -increase in muscle strength

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