Muscle Mechanics 1

Question 1

Name the 2 major components of muscle tissue

Answer 1

• Contractile element (CE): Active

- Series elastic component (SEC): Passive

Question 2

A three component mechanical model of muscle consists of 1 active and 2 passive elements.
Name and describe each of these three components.

Answer 2

• ACTIVE: Contractile Element (CE)
  
  • Active component in myofibrils (cross bridging of actin & myosin filaments) -> shortening of mm fiber.
• PASSIVE: Series Elastic Component (SEC) – 85%
  
  • Passive elastic properties due to tendons (connective tissue ends).
  • Primary contributor to elastic recoil of stretched mm/ force development when stretched mm contracts:- For example during a vertical jump
• PASSIVE: Parallel Elastic Component (PEC) – 15%
  
  • Passive elastic properties due to mm membranes.
    Surround the groups of fibre, individual fibre which is within the belly of the muscle

Question 3

Tendons comprise which component of the musculo-tendinous unit?

Answer 3

Series Elastic Component (SEC)

Question 4

Describe the composition of tendons and explain how they achieve such strength.

Answer 4

• Consists of highly organised bundles of collagenous fibers
• Parallel fibers give structural rigidity to the tendon.
  
  • i.e. usually arranged parallel to direction of force

Question 5

How can the point of attachment of a tendon to the bone affect the mechanical force of muscle
contraction?

Answer 5

If length of lever is longer and with sufficient force production, a larger torque can be produced. (where bicep is attached to forearm)

Question 6

Explain each of the following muscle contractions

e. Concentric
f. Eccentric
g. Isometric
h. Isokinetic
i. Isotonic

Answer 6

Concentric – involves shortening of mm.
Eccentric – involves lengthening of mm.
Isometric – involves no change in length of mm.
Isokinetic – same speed (both con & ecc).
Isotonic – same force (both con & ecc).

Question 7

List all of the names given to the 3 categories of muscle fiber types

Answer 7

Type 1 (slow-twitch oxidative)

Type 2a (Fast-twitch oxidative glycolytic)

Type 2b (Fast twitch glycolytic)

Question 8

Describe the observable differences between these 3 muscle fiber types

Answer 8

ANSWER

Question 9

Which muscle fiber type reaches peak tension the fastest and why?

Answer 9

Fast twitch fibres (1/7 the time of ST)

• This is attributed to ↑ myosin [ATPase] in FT fibers.
• However, twitch times to achieve max. tension range widely across FT & ST fibers

Question 10

Which muscle fiber type generates the greatest isometric force? Explain your answer

Answer 10

Generally: FT & ST fibers generate approx. the same peak isometric force per cross-sectional area of mm. (if identical size)

* However, FT are larger in diameter than ST. 
* Consequently, individuals or mm with a higher % FT fibers can generate higher magnitudes of torque and power than those with more ST fibers

Question 11

From a muscle fiber type perspective, explain how one athlete could be more powerful than
another

Answer 11

Depending on the sport the athlete plays, the amount of different muscle fibres vary
For example:
- Marathon runners would have more slow-twitch muscle fibres than fast-twitch
- Weight lifter would have more fast twitch fibres than slow-twitch
Therefore athletes with fast-twitch would be more powerful

Question 12

Name the 2 umbrella types of muscle fiber arrangements.

Answer 12

Longitudinal/parallel

Pennate

Question 13

Explain the difference between these two types of muscle fiber arrangements (longitudinal & pennate) and how one of
these types can generate greater tension.

Answer 13

Longitudinal/parallel
- Fibers run parallel to line of pull of muscle.

Pennate

• Fibers not aligned with line of pull of muscle.
• A muscle with a pennate fiber composition can develop greater tension, however, the distance over which these muscles can shorten is compromised

Question 14

Explain the difference between the anatomical and physiological cross-sectional area of muscle
tissue.

Answer 14

Anatomical - the area of the cross-section of a muscle perpendicular to its longitudinal axis.

Physiological - the cross section of a muscle perpendicular to its fibers, generally at its largest point.

Question 15

What is a motor unit and what do they control?

Answer 15

• The combination of an individual motor neuron and all of the muscle fibers that it innervates
• Muscle fibers are organised into functional groups of different sizes.
• A motor unit consists of a synaptic junction in the ventral root of the spinal cord, a motor axon, a motor end plate in the muscle fibers, and the muscle fibers it innervates.
• Control muscle contraction

Question 16

Explain the process of motor unit activation

Hint: volley

Answer 16

• In humans, motor units are generally activated by a volley of nerve impulses.
  
  • Rapid successive impulses cause a summation effect → progressive elevation in tension until max. tension for that fiber is reached.
  • Repeated activation → maintenance of max. tension = tetanus.
  • Tension during tetanus = up to 4 x peak during single twitch.
  • Prolonged tetanus → fatigue causes gradual decline in tension.

Question 17

Explain how muscle force production is controlled or graded.

Answer 17

• Graded force production comes by either increasing the rate of stimulation or by the recruitment of additional motor units.

Question 18

Explain the Motor Unit Size Principle.

Answer 18

• Henneman (1974) reported that motor units were recruited according to the size principle.
• This principle states that the size of newly recruited motor units increases with the tension level at the time of recruitment.
• Smallest unit is recruited first and largest last.
• Allows tension to be achieved in graded steps.

Question 19

Explain in detail the muscle “length-tension relationship” and provide an example how this
relationship can advantage or disadvantage a performer

Answer 19

Length tension relationship

• The strength of a muscle contraction is a function of the number of cross-links made between the actin and myosin filaments within sarcomeres
• At mm resting length (approx 2.5μm) there are a maximum number of cross-bridges between filaments and thus a maximum tension is possible.
• As the muscle lengthens the filaments are pulled apart, reducing the number of cross-bridges and thus the achievable tension.
• As the muscle shortens to less than resting length there is an overlapping of cross-bridges and an interference takes place.
• This results in a reduction in tension until a full overlap occurs (approx 1.5μm) at which point tension is drastically reduced by the interfering elements (although it doesn’t drop to zero)
• A whole muscle can actively produce tension at lengths 60% to 160% of its resting length.
• Maximum tension can be developed in a whole muscle when approx. 120% of its resting length.

Question 20

Explain in detail the muscle “force-velocity relationship” and provide an example how this
relationship can advantage or disadvantage a performer

Answer 20

• The tension in a muscle decreases as it shortens under load (concentric contractions).
• The decrease of tension as the shortening velocity increases has been attributed to two main causes;
  1. Firstly, it appears there is a loss of tension as the cross bridges in the contractile element break and then reform in a shortened condition.
  2. Secondly, there appears to be fluid viscosity in the contractile element and the connective tissue

Question 21

Explain the term “electromechanical delay” and explain how this may be reduced.

Answer 21

Force-time relationship

• There is a delay between neural stimulation & development of tension within the mm, called electromechanical delay (EMD).
• Shorter max force development times are associated with a high % of FT fibres & in a trained state.
• Therefore, developing more fast twitch fibres reduces the delay

Question 22

Define the term “stretch shortening cycle (SSC)”

Answer 22

• Muscle action involving an eccentric (loading) contraction immediately followed by a concentric contraction → greater force.

Question 23

Discuss in detail the relationship between elastic energy and the SSC

Answer 23

• The shorter (time) the eccentric loading phase is, the more powerful the concentric muscle contraction.
• This more powerful action has been attributed to the recovery and utilisation of all the energy which has been stored in the elastic components of the muscle during the stretching action.
• Optimal stretch shortening movement would load the muscle eccentrically (stretch phase) as fast as possible to maximise the amount of energy stored by the elastic components

Question 24

Discuss in detail the relationship between neural augmentation and the SSC.

Answer 24

• Muscle spindles provide sensory info regarding changes in the length and tension of muscle fibers.
• Three main components to the stretch reflex:
  1. The muscle spindle and the sensory receptor (proprioceptors) within the muscle that responds to stretch.
  2. An afferent nerve fiber carries the sensory impulse from the spindle to the spinal cord.
  3. An efferent motor neuron in the spinal cord sends signals for the muscle to contract.
• If we eccentrically load the muscle faster we get a greater rate of firing of the muscle spindles which results in a more forceful concentric contraction of the muscle and a greater inhibition of antagonistic muscles.

Question 25

Provide several sporting examples of the SSC in use.

Answer 25

• Jumping in sport (subconscious eccentric loading)
• Walking - eccentric then concentric
• Kangaroo jumping