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
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
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)