PHYSIOLOGY 1 LECTURE 21 SKELETAL MUSCLE MECHANICS - MODEL

Skeletal Muscle Mechanical Model n A skeletal muscle fiber is connected to elastic tendons and covered with a coating of connective tissue called endomysium. Therefore, as the muscle contracts it must expend energy in stretching the elastic tendons to the point where the tension developed in the tendon meets the load being placed on the muscle before that load will begin to move.

Skeletal Muscle Mechanical Model n A simple way of illustrating this is to consider a rubber band attached to a can of soda pop and then pulled. The rubber band will stretch until the tension it has developed overcomes the force of friction holding the can in place. When the can starts to move the rubber band stays the same length. The same thing happens with the skeletal muscle tendons.

Skeletal Muscle Mechanical Model n We will consider two different types of muscle contraction. n Isometric - The muscle fiber does not change length but muscle tension is changing. n Isotonic - The muscle fiber changes length but muscle tension remains constant (i.e. the muscle is lifting the load).

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. 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.