Week 7 - Muscle mechanics 1

Question 1

Name the 2 major components of muscle tissue.

Answer 1

1. Contractile element (CE) - active
   
   • ‘Red’ part of muscle, doesn’t actually connect to skeletal system, stretches and recoils
   • Tears easier under LESS load
2. Series elastic component (SEC) - passive
   
   • Outer coating, joins to tendons, ligaments, bones
   • Harder to tear, needs greater load

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

1. ACTIVE: Contractile Element (CE)
   
   • Active component in myofibrils (cross bridging of actin and myosin filaments) -> shortening of mm fibre
2. 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
3. PASSIVE: Parallel Elastic Component (PEC) – 15%
   
   • Passive elastic properties due to mm membranes.

Question 3

Tendons comprise which component of the musculo-tendinous unit?

Answer 3

Tendons comprise the Series Elastic Component (SEC) of the musculo-tendinous unit.

Question 4

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

Answer 4

• The larger the moment arm the larger the torque generated by muscular contraction.
• Depends on location of muscle attachment to bone and angle formed by the line of pull of the muscle and he limb that muscle attaches to – angle of application.
• Longer moment arm more shortening of muscle to produce same angular excursion as muscle with similar fiber length but smaller moment arm.

Question 5

Explain each of the following muscle contractions:
1. Concentric –
2. Eccentric –
3. Isometric –
4. Isokinetic –
5. Isotonic –

Answer 5

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

Question 6

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

Answer 6

• Slow twitch
  
  • Fast twitch
  • Fast glycolytic

Question 7

Describe the observable differences between these 3 muscle fiber types.

Answer 7

• ST: slow contraction, slow fatigue, small
• FT 2a (FOG): fast contraction, intermediate fatigue, medium size
• FT 2b (FG): fast contraction, fast fatigue, large in size
• FT fibres reach peak tension much quicker than ST, but twitch times to achieve max tension rane widely across ST and FT
• FT fibres reach greater force than ST, however if they were the same size, force would be equal

Question 8

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

Answer 8

Fast twitch muscle fibre - due to increased [ATPase]

Question 9

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

Answer 9

• Greater amount or utilisation of type 2 (FT) muscle fibres in working muscles
• Greater muscle cross sectional area/hypertrophy

Question 10

Name the 2 umbrella types of muscle fiber arrangements.

Answer 10

Longitudinal/Parallel
- Fibers run parallel to line of pull of muscle.
- E.g. biceps
- Can shorten and stretch a greater distance
Pennate
- Fibers not aligned with line of pull of muscle.
- E.g. rectus femoris
- Can get more muscle fibres per muscle = stronger, BUT length of shortening is reduced

Question 11

Explain the difference between these two types of muscle fiber arrangements and how one of these types can generate greater tension.

Answer 11

• Pennate - fibres not aligned with pull of muscle
• Longitudinal/parallel - fibres parallel to line of pull
• Pennate allows:
  
  • Greater number of muscle fibers per muscle.
  • This results in greater cross-sectional area of fibers per muscle.
  • The tension produced is proportional to the physiological cross-sectional area of the muscle = stronger, more fibres pulling BUT length of shortening is reduced
  • Speed often slower

Question 12

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

Answer 12

• Anatomical cross sectional area (mid line) VS physiological cross sectional area (fibres that don’t reach mid line of mm)

Question 13

What is a motor unit and what do they control?

Answer 13

• control skeletal muscle and their contraction
• 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.

Question 14

Explain the process of motor unit activation.

Answer 14

• 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 15

Explain how muscle force production is controlled or graded.

Answer 15

• Graded force production comes by either increasing the rate of stimulation or by the recruitment of additional motor units.
• Further tension increases requires a second motor unit.
• When the tension is reduced the process occurs in reverse, i.e. the firing rate of the second unit decreases until a minimum rate is reached and the motor unit drops out.

Question 16

Explain the Motor Unit Size Principle.

Answer 16

• 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 17

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

Answer 17

• The strength of a muscle contraction is a function of the number of cross-links made between the actin and myosin filaments within sarcomeres
• Too much overlap (thin filaments overlapping each other) = lower strength
• Not enough overlap = lower strength
• Medium overlap = highest strength

Question 18

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

Answer 18

the time lag between the (initial) stimulation of a muscle at the neuromuscular junction and a measurable change in force output (delay between neural stimulation and development of tension within muscle)

Question 19

Define the term “stretch shortening cycle (SSC)

Answer 19

refers to the muscle action when active muscle lengthening is immediately followed by active muscle shortening

Question 20

• Discuss in detail the relationship between elastic energy and the SSC.

Answer 20

• The faster we move through eccentric phase (like stretching rubber band or squatting down to jump) the more velocity the concentric phase will have
• Pre stretching stores elastic energy to be used in concentric phase, utilise as much as possible by moving fast/not pausing
• Prolonged stretch loses elastic energy
• BUT very fast eccentric movements predispose someone to injury

Question 21

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

Answer 21

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 22

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

Answer 22

• FT & ST fibers generate approx. the same peak isometric force per cross-sectional area of mm
  
  • FT fibres reach greater force than ST
  • However, FT are larger in diameter than ST.
    
    • If they were the same size = force production is equal

Question 23

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

Answer 23

• Parallel fibers give structural rigidity to the tendon.
  - i.e. usually arranged parallel to direction of force.
  - Tendons are usually covered in a sheath with a surrounding lubricating fluid.