Term Test 2: Muscle Mechanics Flashcards
Muscle fibres
- Single muscle cells
- Multi-nucleated, long thread-like cells
- Each muscle fibre is encased in a connective tissue
sheath called the endomysium
Fascicles
- Bundles of muscle fibres (each containing 10-100
muscle fibres) - Encased in a connective tissue sheath called the
perimysium
Whole muscles
- Are bundles of muscle fascicles (can vary widely).
- Encased in a connective tissue sheath called the
epimysium
Tendons
Cord-like and aponeuroses (sheet-like) are woven connective tissues that extend beyond the muscle and connect to bone
Myofibrils
- Threadlike structures that lie parallel to each other
and run the full length of the muscle fibre. - Transverse light and dark bands appear across each
myofibril and align with the same bands on adjacent
myofibrils - Bands of light and dark repeat every 2.5 um, giving
skeletal muscle its striated appearance
Sarcomere
Is the basic contractile unit of the muscle. It is the repeating unit of the myofibril between the stripes
Myofilaments
Are protein filaments that overlap within the sarcomere
Thin filaments contain
- Actin
- Troponin C
- Tropomyosin proteins
Thick filaments contain
Myosin proteins
I-band
Region that contains only actin and Z-line (no overlapping myosin), appears as light band
A-band
- Region that contains myosin
- Overlapping actin, appears as dark band
H-zone
- Region of the A band
- Contains only myosin and M-line (no overlapping
actin)
M-line
Transverse band that anchors myosin to each other
Z-line
Transverse band that anchors actin to each other (Z-line to Z-line defines sarcomere)
Sliding filament theory
Proposes that muscle force arises from cyclic binding between thin actin and thick myosin filaments of the sarcomere
- Absence of calcium, tropomyosin prevents
myosin from attaching to actin
- During AP, calcium is released from the
sarcoplasmic reticulum and binds to troponin
C
- This induces a conformal change in
tropomyosin, giving myosin head access to
actin
- Myosin head binds actin and a power stroke
causes muscle contraction
- ATP binds allowing the release of actin
Electron micrographs
- Confirms the existence of thick myosin and thin
actin filaments - Stretching the muscle does not cause a change
in the length of the thick and thin filaments,
but only a change in the length of the I-band
and H-zone
Factors affecting muscle force development
- Length of muscle
- Velocity
- Physiological cross-sectional areas of the
muscle - Muscle geometry
- Level of activation
If you have ever done a hamstring curl on a flat bench, you may have noticed that your hamstrings feel very weak when in a fully flexed position. Why do you think that is? Why might assuming some hip flexion may make the exercise easier?
- Flexing the hip makes the length of the
hamstring longer - Making exercise easier
- You are on the ascending limb
- You can generate more force
Contractile element
Represents force development via cross-bridge attachments in the sarcomeres
Series elastic element
Represents force-deflection properties of tendon
Parallel elastic element
Represents force-deflection properties of the sarcolemma, epimysium, perimysium and endomysium
Shortening does not equal short
- The force-length relationship tells us that the
maximum force a muscle can generate
depends on whether it is short or long - The force-velocity relationship tells us that the
maximum force a muscle can generate
depends on whether it is shortening or
lengthening
Short and long
- Short does not equal shortening
- Long does not equal lengthening
- Short and long refer to the muscle length
- Short muscles can be shortening or lengthening
Shortening and lengthening
- Short does not equal shortening
- Long does not equal lengthening
- Shortening and lengthening refers to the muscle velocity
Muscle force-velocity relationship
- The greater the shortening velocity of the
muscle, the smaller the force that can be
produced (we cannot lift heavy objects quickly) - A muscle contracting eccentrically or
isometrically is capable of producing more
force than a muscle contracting concentrically
Muscle force-length relationship
In a whole muscle, Total = active (muscle contraction) + passive (connective tissue) tension
Activation
Number (or proportion of) muscle fibres that are stimulated to contract at any given time
Motor unit
Composed of a single motor neuron and all of the muscle fibres with which it synapses
- Electrical stimulation causes muscle to contract through the release of Ca2+
How do we increase activation?
- Increase the firing frequency of any given
motor unit - Recruit more motor units to produce greater
force (tension), you also recruit more motor
units to increase the proportion of fibres
running
Heneman’s size principal
Motor units are recruited from smallest (slow twitch) to largest (fast twitch)
Physiological cross-sectional area I
- Increasing the number of sarcomeres lined up
end to end = increasing number of cross-
bridges = increasing the length of the myofibril - If adding the sarcomeres in parallel by
increasing the number of myofibrils, the
number of possible attachments of cross
bridges have increased
Physiological cross-sectional area II
Adding sarcomeres in parallel makes a muscle stronger, but not faster. Strength is thus proportional to muscle cross-sectional area because it is proportional to the number of parallel-acting sarcomeres
