Skeletal Muscle Mechanics

1
Skeletal Muscle Mechanics

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

2
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)

3
One twitch (fiber contraction) per excitation.
Subsequent twitches can increase force w/ limit
at tetanus
1. Increase action potential frequency.
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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
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3. maximize actin/myosin overlap
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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

7
Skeletal Muscle Mechanics

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

8
Levers - 3 components
Force
Load
Fulcrum
9
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.
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An example of a balanced 1st Class Lever
?B 5 x 0.2 1
?A 10 x 0.1 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.
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First class lever
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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
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Second class lever
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Third class lever
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• 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)