Frolich, Human Anatomy, Mechanics of Movement

Mechanics of Movement I: Muscle Force and Action Across Joints

Review muscle force generationMuscle Mechanics

--force versus cross section--length versus strain

Lever mechanics and agonist/antagonistsStabilizing the joint—isometric and eccentric contraction

Frolich, Human Anatomy, Mechanics of Movement

Muscle Structure Review

Muscle fiber = muscle cell Fibers lined up = direction of pull Tendon attaches to bone Muscle pulls on bone

Frolich, Human Anatomy, Mechanics of Movement

Muscle Origin and Insertion

Origin Proximal Fixed

Insertion Distal Moves

(usually!!)

Frolich, Human Anatomy, Mechanics of Movement

Mechanics of Contraction Muscle fiber is one cell

made up of myofibrils, each filled with myofilament proteins actin and myosin, all lined up along length of muscle cell.

Action potential or depolarization of membrane releases calcium

Calcium changes shape of actin so myosin cross-bridges form and “row” or pull in to center of sarcomere.

Frolich, Human Anatomy, Mechanics of Movement

Visualizing muscle contraction

How actin-myosin complex (sarcomere)shortens muscle

Frolich, Human Anatomy, Mechanics of Movement

Summary of Muscle Organization/Function

Frolich, Human Anatomy, Mechanics of Movement

Summary of Muscle Organization/Function

Frolich, Human Anatomy, Mechanics of Movement

Summary of Muscle Organization/Function

Frolich, Human Anatomy, Mechanics of Movement

Levels of Muscle Organization

Frolich, Human Anatomy, Mechanics of Movement

Muscle Mechanics: Cross section versus force

Cross sectional area is proportional to Force of muscle

Frolich, Human Anatomy, Mechanics of Movement

Muscle Mechanics: length versus force

Force generation depends on current length of muscle or overlap in actin/myosin of sarcomeres

Muscle force strongest between 80-120% of normal resting length—WHY? (don’t forget role of cross-bridges)

Most muscles arranged to work in this range

Frolich, Human Anatomy, Mechanics of Movement

Muscle Mechanics: length versus total shortening

Length of muscle is proportional to ability to shorten (strain) Number of sarcomeres in series gives

shortening ability

Short, fat muscles Lots of force Less shortening range

Long, skinny muscles Less force More shortening range

Frolich, Human Anatomy, Mechanics of Movement

Types of fascicle arrangements

Affects length and cross section of muscle

Thus affects force and shortening properties of muscle

See Muscle Mechanics if this doesn’t make sense

Frolich, Human Anatomy, Mechanics of Movement

Long thin straight muscle versus short fat pinnate muscle

Gastrocnemius (calf muscle) Short and bulky Pinnate fibers Great force, low shortening distance Pushes off each step—”spring-

loaded”

Sartorius Tailor’s or hackey-sac

muscle Longest muscle in

body’ Thin and straight fibers Low force, great

shortening distance

Frolich, Human Anatomy, Mechanics of Movement

Muscle movement across joints is like lever system

Frolich, Human Anatomy, Mechanics of Movement

Agonist/Antagonist muscles

Frolich, Human Anatomy, Mechanics of Movement

Stabilization and Control Around Joint

Agonist Main Mover E.g. biceps

Antagonist Opposite motion

E.g. triceps

Synergist Aids agonist E.g. brachialis

Antagonist often “fires” or contracts or is stimulated simultaneously with agonist to stabilize around joint during movement

NOTE: Muscle “contraction” or stimulus to “fire” does not always result in muscle shortening

Frolich, Human Anatomy, Mechanics of Movement

Relation between muscle contraction (or “firing”) and shortening

Concentric contraction—muscle contracts and shortens to cause movement across joint

Isometric contraction—muscle contracts but stays same length to hold joint or body in same position

Eccentric contraction—muscle contracts while lengthening to stabilize joint during movement (most common in antagonist to slow movement caused by agonist)