PHYSIOLOGY 1 LECTURE 18 SKELETAL MUSCLE MECHANICS - MODEL

Skeletal Muscle Mechanical Model n Objectives: The student should; n 1. Learn the basic model of skeletal muscle contraction. n 2. Learn about isometric and isotonic tension development n 3. Learn that opposing muscle groups imply that both muscles must develop tension. n 4. Learn the relationship of velocity of shorting to load.

Skeletal Muscle Mechanical Model n A skeletal muscle contraction may be considered in terms of a mechanical model. The elements of the model consists of contractile elements (sarcomere), series elastic elements (Tendons), and parallel elastic elements (Connective tissue - endomysium).

Skeletal Muscle Mechanical Model n Based on this model any muscle contraction can be considered to consists of four phases an isometric contraction phase (muscle tension development), an isotonic contraction phase (muscle contraction), an isotonic relaxation phase (muscle elongation), and finally an isometric relaxation phase (developed muscle tension is dissipated).

Skeletal Muscle Mechanical Model n A. Definition of terms n Isometric - (SAME LENGTH) - Muscle contractile machinery contracts but the series elastic element stretches an equal amount, therefore, the muscle fiber itself does not change length. This is the tension development or dissipation phases of muscle contraction.

Skeletal Muscle Mechanical Model n Isotonic - (SAME TENSION) - The tension developed by the muscle does not change as the muscle fiber contracts - Basically this is the movement phases of muscle contraction.

Skeletal Muscle Mechanical Model n Isometric - (SAME LENGTH) - n Muscle contractile machinery contracts but the tendons stretch an equal amount, therefore, the muscle fiber does not change length.

Skeletal Muscle Mechanical Model n Isotonic - (SAME TENSION) - n The tension in the muscle fiber remains constant but the fiber shortens as the contractile machinery continues to contract.

Skeletal Muscle Mechanical Model n B. Tension development does not necessarily imply muscle contraction, but may involve muscle lengthening (Eccentric Contraction) or no movement at all (Isometric Contraction). n C. Velocity of contraction is dependent on the load. The greater the load the slower the contraction.

Skeletal Muscle Mechanical Model n Mechanical Model - n SEE - Series Elastic Element (Tendons) n PEE - Parallel Elastic Element (Endomysium) n CE - Contractile Element (Sarcomere)

Skeletal Muscle Mechanical Model Phases n Any muscle contraction consists of four phases. n 1. Isometric Contraction Phase n 2. Isotonic Contraction Phase n 3. Isotonic Relaxation Phase n 4. Isometric Relaxation Phase

Mechanical Model Phases Isometric Contraction Phase n During the isometric contraction phase n SEE - Lengthens (Stretching tendons) n CE - Shortens (Sarcomere shortens) n PEE - Shortens (Endomysium is compressed) n Whole Muscle - Stays the same length

Mechanical Model Phases Isometric Contraction Phase

Mechanical Model Phases Isotonic Contraction Phase n During the isotonic contraction phase n SEE - Stays the same length n PEE - Shortens n CE - Shortens n Whole muscle shortens

Mechanical Model Phases Isotonic Contraction Phase

Mechanical Model Phases Isotonic Relaxation Phase n During the isotonic relaxation phase n SEE - Stays the same length n PEE - Lengthens n CE - Lengthens n Whole muscle lengthens

Mechanical Model Phases Isotonic Relaxation Phase

Mechanical Model Phases Isometric Relaxation Phase n During the isometric relaxation phase n SEE - Shortens n PEE - Lengthens n CE - Lengthens n Whole muscle stays the same length

Mechanical Model Phases Isometric Relaxation Phase

Sarcomere Length - Tension Relationship (Active Tension Curve) n III. The sarcomere length tension relationship or active tension curve is generated by placing a muscle in an isometric condition, attached to a strain gage then applying stretch to the muscle fiber. By stimulation a muscle contraction the tension developed for a given degree of sarcomere stretch can be measured.

Sarcomere Length - Tension Relationship (Active Tension Curve) n These experiments result in a bell shaped curve where the greatest tension developed for skeletal muscle occurs at the peak of the tension curve. The drops in tension development on either the left or the right hand limb of the curve has to do with the numbers of cross-bridges being formed, if the sarcomere is too long no cross bridges etc.

Velocity of Shortening - Load Relationship n IV. For any active muscle group it is clear that at zero load the muscle is capable of generating it”s greatest velocity of shortening then it is also clear that at some point the load will become so great that the muscle will not be able to lift it. The velocity of shortening to load relationship is not linear due to pathway the weight must follow, changes in the angle of the muscle to the bone, and the elastic properties of the muscle.

Mechanical Model n Summary n 1. What are the four phases of the mechanical model, describe each phase? n 2. What is an isotonic, isometric contraction? n 3. What is an active tension curve, velocity of shortening to load curve?