Biomechanics of Muscles

Question 1

Hill muscle model

Answer 1

the combination of tendons and other connective tissues with the muscle can be modelled as a spring-damper system
1. CC - contractile component
2. SEC - series elastic component
3. PEC - parallel elastic component

Question 2

muscle contractions (one term, 3 responses)

Answer 2

1. twitch: mechanical response of muscle to a stimulus
2. latency period: time between stimulus and beginning of twitch
3. contraction time: time from start of tension development to peak tension (10-100ms)
4. relaxation time: time from peak tension to zero tension

Question 3

dynamic muscle contractions - concentric vs eccentric

Answer 3

dynamic contraction: mechanical work is performed, joint motion produced
1. concentric: enough tension to overcome resistance of body segment - muscles shorten
2. eccentric: cannot develop enough tension and is overcome by external load - muscle progressively lengthens and decelerates joint motion

Question 4

static muscle contraction - isometric

Answer 4

no mechanical work performed, and joint position is maintained
1. isometric: maintains muscle length and a certain position

Question 5

other types of muscle contractions

Answer 5

1. isokinetic: muscle changes length at constant velocity
2. isoinertial: the muscle is loaded with a constant resistance while it is contracting
3. isotonic: muscle tension is constant throughout joint motion

Question 6

force-length and force-velocity relationships of muscle

Answer 6

see slides

Question 7

pennation angle (q)

Answer 7

angle between a muscle’s fibers and its force-generating axis –> most human muscles have q= 0-30 degrees
1. smaller angle = allows maximal force production per fiber
2. larger angle = shortening of fibers, which can increase overall force production through force-velocity relationship

Question 8

effect of muscle length and angle on force

Answer 8

length: longer muscle fibers = greater velocity and range of muscle
cross-section: affects the force the muscle can produce (shorter fibers but stacked to give a large cross-sectional area produces more force)

Question 9

type I fibers

Answer 9

• slow-twitch oxidative (SO) red fibers
• slow contraction time due to low activity of myosin ATPase
• low anaerobic activity, high aerobic activity (more activity in presence of oxygen)
• small diameter fibers, produce little tension
• difficult to fatigue, better for longer and low intensity work
• red muscle due to high myoglobin content (binding to O2)

Question 10

type II A fibers

Answer 10

fast-twitch oxidative-glycolytic (FOG) red fibers
- middle between type I and type II B
- fast contraction time, but well developed for both aerobic and anaerobic activity
- maintains tension for faily long, but will eventually fatigue at higher rates of activity
- red due to high myoglobin content

Question 11

type II B fibers

Answer 11

fast-twitch glycolytic (FG) white fibers
- primarily rely on anaerobic activity (high intensity, lower time - breaks down glucose for energy without using oxygen)
- fibers have large diameter, produce great tension (due to large cross sectional area), but only for short periods before fatigue
- white due to low myoglobin content

Question 12

effects of disuse and immobilization

Answer 12

• loss of endurance and force production
• affects muscle composition, mainly type I fibers

Question 13

effects of muscle tears

Answer 13

• direct injury to sarcomeres from eccentric contractions
• due to loss of actin and myosin interdigitation
• 25% strain - damage at muscle tendon junctions in passive muscle
• strains between 73-225%: complete rupture of passive muscle
• increasing muscle architecture complexity increases strain required for rupture
• active muscle requires more force to rupture than passive

Question 14

blunt trauma

Answer 14

• tensed muscle distributes the impact force more broadly than relaxed muscle
• protects bone underneath from impact
• may cause deep bleeding within muscle (bruise)