Unit 3: Muscle Structure and Function

Describe the general structure of muscle from tendon to sarcomere

• Tendon:

  • attaches muscle to bone

• Myotendinous junction: interdigitation btw collagen fibers + muscle fibers

  • increases SA → decreases stress → less likely for injury

• Muscle → Muscle fascicle (bundle of fibers) → Muscle fiber (cell) → Myofibril (contractile structure + sarcomeres)

• Sarcomere: thick filament (myosin) and thin filament (actin)

Describe how fiber length and cross-sectional area affect excursion and force of contraction.

• Fiber LENGTH: # of sarcomeres in series

  • more sarcomeres in series → longer

  • Longer → increased velocity + increased excursion (stretch distance from start → end)

• Cross-sectional area

  • more sarcomeres in parallel → thicker

  • Thicker → increased force of contract + increased excursion (stretch distance from start → end)

Describe the effect of pennation angle on muscle force production.

Pennation angle: angle of insertion of muscle fibers into tendon

• Pennate angle → greater contraction force b/c force goes in diff directions

• Not all of total muscle fiber tension is in direction of muscle shortening/lengthening

• Higher PCSA than fusiform

• Result is a force muscle with a relatively limited range of
  lengthening/shortening

Define Muscle tension including active and passive tension

• Passive tension:

  • if you stretch muscle, it’ll bounce back (elastic forces and NO active contraction)

  • ex: structural proteins, tendon, sarcomere (have EF)

• Active tension:

  • force made by contractile parts (actin-myosin cross bridge)

  • depends on how much is muscle is activated + its length

Define Concentric, Eccentric, and Isometric muscle action.

Concentric:

• active shortening

• smaller force potential

• POS power and POS work

• brings origin + insertion close to each other

Eccentric

• active & lengthening

• NEG power and NEG work

• resists the other movement

Isometric

• active, same length / joint angle

• Power = 0 and Mechanical work = 0

• larger force potential ake max force available

• muscles + sarcomeres contracts meaning it still makes force, but NOT enough to change joint angle (NO movement)

Factors that affect muscle force

• Passive tension: when a muscle is stretched past its slack length, passive tension rises.

• Fiber type: different fiber types have different force/speed/fatigue properties.

• Fiber size: more sarcomeres in parallel increases force capacity.

• Fiber length: more sarcomeres in series increases excursion and contraction velocity more than peak force.

• Muscle size / cross-sectional area: larger muscles generally produce more force.

• Force-velocity relationship: fast shortening produces less force; isometric produces more; eccentric produces the most.

• Type of contraction: concentric, isometric, eccentric.

• Length-tension relationship: force changes with muscle/sarcomere length because actin-myosin overlap changes; total force = active + passive.

• Muscle architecture:

  • Pennate muscles: greater PCSA, better for force.

  • Fusiform muscles: better for velocity and excursion.

• Neural factors: more motor unit activation and higher firing frequency increase force.

  • Unfused tetanus: multiple twitches that are added together

  • Fused tetanus: send so many signals quickly but NOT turning off

Factors affecting muscle torque

Muscle torque is mainly determined by:

• Muscle force, including both active and passive force

• Moment arm

Factors affecting angular velocity and torque

Angular velocity is influenced by:

• Force-velocity relationship: lighter loads allow faster movement; higher loads slow movement.

• Fiber length / sarcomeres in series: longer fibers increase contraction velocity and excursion.

• Muscle architecture: fusiform muscles are better for velocity; pennate muscles favor force.

• Neural activation: greater and faster activation helps produce movement more quickly.

Torque is influenced by:

• Muscle force

• Moment arm

• Joint angle, because both force capacity and moment arm can change across the range of motion.

Summary

• Force muscles = larger PCSA, pennation, better torque potential

• Speed muscles = longer fibers, fusiform design, better angular velocity

Differentiate Active from passive tension including where in the length curve they can occur

Active tension on the curve

• Occurs through the range where actin and myosin can overlap.

• It is highest near the optimal/resting length

  • (max overlap + all myosin link)

• It is lower on the ascending limb when the muscle is too short.

  • (too much overlap → No force generated)

• It is lower on the descending limb when the muscle is too long/too stretched and cross-bridge overlap decreases.

  • (little overlap → No force generated)

Passive tension on the curve

• At short muscle lengths, there is a slack region, so passive tension is basically absent.

• Once the muscle is stretched beyond the critical length, passive tension increases as length increases

  • active force decreases as length increases

Excursion + operating range in muscle level

excursion: length change

operating range: length over which muscle can generate force

Changing fiber size vs fiber length

Fiber size

• Refers to the number of sarcomeres in parallel.

• Increasing fiber size increases the muscle fiber’s force-producing capacity.

Fiber length

• Refers to the number of sarcomeres in series.

• Increasing fiber length increases shortening distance, excursion, and contraction velocity more than maximum force.

Comparison

• Bigger fiber size = better for force

• Longer fiber length = better for speed and range of shortening

Changing muscle size vs muscle length

Muscle size

• Usually refers to cross-sectional area.

• Larger muscles generally produce more force.

Muscle length

• Refers to the distance from origin to insertion.

• Longer muscles generally allow greater excursion and often greater contraction velocity across a larger range.

Comparison

• Larger muscle size = greater force potential

• Longer muscle = greater movement range / excursion

Fusiform vs pennate muscle architecture

Fusiform

• Fibers run parallel to the tendon.

• Physiological cross-sectional area is about equal to anatomical cross-sectional area.

• Built more for motion and speed than stability.

• Produces greater contraction velocity and excursion.

Pennate

• Fibers run diagonally into the tendon.

• Has a greater physiological cross-sectional area (PCSA).

• Better at developing tension/force.

• Has a relatively more limited range of shortening/lengthening.

Comparison

• Fusiform = speed, excursion, movement

• Pennate = force, tension production

Architecture of various muscles and how it influences function

• Large PCSA

  • increases maximum tension/force

  • increases velocity

• Long Fibers

  • wider operating range

  • generates more force

  • generates more velocity

Compare Tension development in eccentric vs concentric contractions

Concentric contraction

• Active shortening

• Produces less force as shortening velocity increases.

Eccentric contraction

• Active lengthening

• Produces more force than isometric or concentric contractions.

Overall force pattern

• Fast concentric → least force-gen capacity

• Slow concentric → more force-gen capacity

• Isometric → more force-gen capacity than concentric

• Eccentric → greatest force-gen capacity

Relationship between type of muscle action and power / work

Concentric

• Muscle shortens while producing force

• Usually associated with positive work and positive power

Isometric

• Muscle produces force but there is no joint movement

• Therefore mechanical work is zero, so power is zero

Eccentric

• Muscle is active while lengthening

• Usually associated with negative work and negative power, because the muscle is absorbing energy rather than adding it

The muscle action and power given the angular velocity and internal moment

To determine muscle action and power, combine:

• the direction of angular velocity

• the direction of the internal moment

A helpful rule:

• If angular velocity and internal moment are in the same direction, the muscle is acting concentrically

• If angular velocity is zero, the muscle action is isometric

• If angular velocity and internal moment are in opposite directions, the muscle is acting eccentrically

Then for power:

• Same direction → positive power

• Zero angular velocity → zero power

• Opposite directions → negative power

EXAMPLES

If the knee is extending and the internal knee extensor moment is also in the extension direction:

• the quadriceps are shortening

• this is concentric

• power is positive

If the knee is flexing while there is still an internal knee extensor moment:

• the quadriceps are resisting the motion while lengthening

• this is eccentric

• power is negative

If there is an internal moment but no angular velocity:

• the muscle is isometric

• power is zero