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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…
• 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
Force
distance
The perpendicular distance from the
pivot point to the line of the force.
An example of a balanced 1st 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
total  A  B 11  0
At balance, force X distance on one side of a lever
= force X distance on the other side.
First class lever
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
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)