Skeletal muscle mechanics
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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….

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Skeletal Muscle Mechanics

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Skeletal muscle mechanics

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

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)


Skeletal muscle mechanics

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

Muscle

Muscle Fibers

2. Increase number of motor units activated

  • THE MOTOR UNIT: One neuron and the fibers it innervates.

  • Motor unit recruitment increases force.

Somatic Motor Neuron

Motor Unit


Skeletal muscle mechanics

3. maximize actin/myosin overlap


The coding of contractile force

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 mechanics1

Skeletal Muscle Mechanics

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


Levers 3 components

Levers - 3 components

Force

Load

Fulcrum


Skeletal muscle mechanics

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

Force

distance

The perpendicular distance from the pivot point to the line of the force.


Skeletal muscle mechanics

An example of a balanced 1st Class Lever

A = 10 x 0.1 = 1

B = 5 x 0.2 = 1

0.1 m

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.


Skeletal muscle mechanics

First class lever


Skeletal muscle mechanics

Biomechanics problem:

Upon C1 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


Skeletal muscle mechanics

Second class lever


Skeletal muscle mechanics

Third class lever


Skeletal muscle mechanics

Biomechanics problems:

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)


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