Mechanics of whole muscle contraction

Question 1

Motor unit

Answer 1

Includes a motor neuron and all the muscle fibers it innervates

–>A functional contractile unit

Question 2

Motor nerve

Answer 2

• A bundle of many motor neurons (units)
• Innervates many motor units

Question 3

What muscles have small motor units?

Answer 3

Muscles that need control fine movements (fingers, eyes)

Question 4

What kind of muscles have large motor units?

Answer 4

Large weight-bearing muscles (thighs, hips)

Question 5

Series-elastic component

Answer 5

There are components involved in sarcomeres that have elastic properties

Includes non-contractile tendons, connective tissue, and structural proteins. These elastic components absorb tension as the muscle contracts

• Elastic components result in the time lag because they have to stretch out before they can actually lift anything/apply force

Question 6

Graded muscle responses

Answer 6

• Variations in the degree of muscle contraction which is required for proper control of skeletal movement
• Responses are graded by changing the frequency or strength of simulation

Eg. How much force your muscle decides to apply when lifting different weights

Question 7

Muscle twitch and its three periods

Answer 7

Response of a muscle to a single stimulus

Three periods:
1. Latent period- elastic components coupling

2. Period of contraction- cross bridges form and muscle shortens
3. Period of relaxation- Ca reabsorbed, muscle tension goes to zero

Question 8

Facilitation/wave summation of muscle response

Answer 8

Effects of multiple stimulus and muscle responses are summed

Steps:
1. Muscle response: a single stimulus leads to one muscle twitch

2. Wave summation: when stimuli are frequent, muscles cannot relax and the force of contraction increases
3. Incomplete tetanus: very rapid stimuli result in stronger but incomplete tetanus (no full summation yet)
4. Complete tetanus: sustained contraction occurs when twitches are fused together due to high-frequency stimuli

Question 9

Threshold during muscle twitch

Answer 9

Minimum stimulus needed to trigger a twitch

Question 10

Facilitation

Answer 10

happens when a new stimulus arrives after a twitch, but before the muscle has fully relaxed or its elastic components have fully recoiled

Question 11

Tetanic contraction

Answer 11

results from frequent stimuli leading to sustained muscle force. At this time, individual twitches are no longer evident

Question 12

Refractory period

Answer 12

duration when muscle cannot respond to a second stimulus

Question 13

Recruitment of motor units

Answer 13

Activation of additional motor units in response to stimulus increasing in size
- Needs a minimum stimulus for muscle contraction
- Muscle contracts with greater force as stimulus intensifies
- Maximum contraction is achieved when all motor units are recruited and fully activated

Question 14

Two types of muscle contraction

Answer 14

1. Isotonic contraction- generate tension by shortening the muscle (eg. Weight lifting)
2. Isometric contraction- muscle length remains constant (eg. yoga)

Question 15

Length-tension relationship

Answer 15

The relationship between muscle length and the force it can produce

• Optimal length: at a specific length, muscle fibers overlap perfectly for maximum force
• Stretched muscle: reduced overlap of actin-myosin; decreased force
• Compressed muscle: too much overlap of actin-myosin; decreased force

Question 16

Force-velocity relationship

Answer 16

The relationship between the force a muscle produces and the velocity of its contraction
- Inverse relationship: as load (or force) on a muscle increases, the velocity of contraction decreases
- Maximum velocity: achieved during no-load conditions
-Vmax is a property of
muscle fiber types
- Related to the rate of ATP
hydrolysis of the myosin
head
- Maximum force: seen at zero velocity (isometic contraction)

Question 17

What factors effect maximum velocity of a contraction?

Answer 17

Vmax is a property of muscle fiber type

Also related to rate of ATP hydrolysis of the myosin head