Skeletal Muscle Mechanics Explain muscle properties during contractions

Skeletal Muscle Mechanics • Explain muscle properties during contractions of varying strengths. • Distinguish the three lever types and solve equations representing these relationships.

How to get more force in a muscle… • Increase action potential frequency in a motor unit (summation in fibers since twitch lasts longer than AP) • Increase number of motor units activated in the whole muscle • Maximize actin/myosin overlap • Increase number of myofibrils (body building; long term)

1. Increase action potential frequency. One twitch (fiber contraction) per excitation. Subsequent twitches can increase force w/ limit at tetanus

2. Increase number of motor units activated • THE MOTOR UNIT: One neuron and the fibers it innervates. • Motor unit recruitment increases force. Muscle Somatic Motor Neuron Motor Unit Muscle Fibers

3. maximize actin/myosin overlap

THE CODING OF CONTRACTILE FORCE • FREQUENCY CODING: HIGHER FREQUENCY OF INCOMING ACTION POTENTIALS GENERATES MORE FORCE • POPULATION CODING: MORE UNITS RECRUITED TO GET MORE FORCE • TOTAL FORCE POSSIBLE DETERMINED BY SIZE (CROSS/SXN) OF WHOLE MUSCLE

Skeletal Muscle Mechanics • Distinguish the three lever types and solve equations representing these relationships.

Levers - 3 components Force Load Fulcrum

When most skeletal muscles contract, the bones to which they are attached rotate around a joint. We call this ‘rotational’ force a torque. Torque = force x distance The perpendicular distance from the pivot point to the line of the force. Force

An example of a balanced 1 st Class Lever A = 10 x 0. 1 = 1 0. 1 m B = 5 x 0. 2 = 1 0. 2 m F = 5 lb F = 10 lb At balance, force X distance on one side of a lever = force X distance on the other side.

First class lever

Biomechanics problem: Upon C 1 vertebra, for example, the head weighs approx. 10 lbs. If you bend the head forward 3 inches, the weight that is placed anterior to the facets becomes 3 inches times 10 lbs which is equal to 30 inch pounds 3" x 10 lbs = 30 inch pounds The neck muscles which have to balance this load are positioned 2 inches behind the facets. How many pounds of force will these muscles will have to exert to balance the head? 3 " x 10 lbs = 2 " x 15 lbs 30 inch pounds = 30 inch pounds

Second class lever

Third class lever

Biomechanics problems: 1. Calculate the effort (muscle force) required to hold your arm bent and steady at your side. arm + hand = 10 lbs. distance from elbow to hand = 14 in. center of gravity for arm = 7 in. from elbow distance from elbow to biceps insertion = 2 in. 2. Calculate the effort (muscle force) required to curl a dumbell with your arm. Dumbell weight = 20 lbs (resistance arm is the full distance from elbow to hand in this case)