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)

One twitch (fiber contraction) per excitation. Subsequent twitches can increase force w/ limit at tetanus

1. Increase action potential frequency.

2. Increase number of motor units activated• THE MOTOR UNIT: One neuron and the fibers it innervates.

• Motor unit recruitment increases force.

Muscle

Muscle Fibers

Somatic Motor Neuron

Motor Unit

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

Load

Fulcrum

Force

When most skeletal muscles contract, the bones to which they are attached rotate around a joint. We call this ‘rotational’ force a torque.

Forcedistance

Torque = force x distance

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

An example of a balanced 1st Class Lever

F = 10 lb

F = 5 lb

0.2 m0.1 m

B = 5 x 0.2 = 1A = 10 x 0.1 = 1

€

τtotal=τA −τB =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 elbowdistance 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)